Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2018	May 02, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	Booking Holdings Inc.
Entity Central Index Key	0001075531
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Common Stock, Shares Outstanding		48,174,965

UNAUDITED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 2,622,086	$ 2,541,604
Short-term investments in marketable securities	4,240,704	4,859,873
Accounts receivable, net of allowance for doubtful accounts of $43,620 and $39,282, respectively	1,530,321	1,217,801
Prepaid expenses and other current assets	1,111,753	415,527
Total current assets	9,504,864	9,034,805
Property and equipment, net	573,040	480,081
Intangible assets, net	2,140,098	2,176,823
Goodwill	2,754,747	2,737,671
Long-term investments in marketable securities	9,432,763	10,421,600
Other assets	613,448	600,283
Total assets	25,018,960	25,451,263
Current liabilities:
Accounts payable	841,274	667,523
Accrued expenses and other current liabilities	1,238,815	1,138,980
Deferred merchant bookings	1,425,649	980,455
Convertible debt	941,657	710,910
Total current liabilities	4,447,395	3,497,868
Deferred income taxes	481,220	481,139
Long-term U.S. transition tax liability	1,250,846	1,250,846
Other long-term liabilities	161,340	148,061
Long-term debt	7,990,418	8,809,788
Total liabilities	14,331,219	14,187,702
Convertible debt	51,398	2,963
Stockholders' equity:
Common stock, $0.008 par value; authorized 1,000,000,000 shares, 62,838,223 and 62,689,097 shares issued, respectively	488	487
Treasury stock, 14,589,938 and 14,216,819 shares, respectively	(9,430,421)	(8,698,829)
Additional paid-in capital	5,034,164	5,783,089
Retained earnings	14,975,706	13,938,869
Accumulated other comprehensive income	56,406	236,982
Total stockholders' equity	10,636,343	11,260,598
Total liabilities and stockholders' equity	$ 25,018,960	$ 25,451,263

UNAUDITED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 43,620	$ 39,282
Common stock, par value (in dollars per share)	$ 0.008	$ 0.008
Common stock, authorized shares (in shares)	1,000,000,000	1,000,000,000
Common stock, shares issued (in shares)	62,838,223	62,689,097
Treasury stock, shares (in shares)	14,589,938	14,216,819

UNAUDITED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Agency revenues	$ 2,113,339
Agency revenues		$ 1,785,313
Merchant revenues	526,467
Merchant revenues		442,045
Advertising and other revenues	288,395	192,046
Total revenues	2,928,201	2,419,404
Cost of revenues		80,401
Gross profit		2,339,003
Operating expenses:
Performance marketing	1,106,207	982,172
Brand marketing	101,402	80,818
Sales and other expenses	165,800	109,599
Personnel, including stock-based compensation of $71,405 and $58,948, respectively	498,887	351,030
General and administrative	162,139	135,547
Information technology	60,441	39,945
Depreciation and amortization	103,090	83,430
Total operating expenses	2,197,966	1,782,541
Operating income	730,235	556,462
Other income (expense):
Interest income	46,879	31,992
Interest expense	(70,235)	(55,717)
Net unrealized gains on marketable equity securities	54,514	0
Foreign currency transactions and other	(8,056)	(5,127)
Total other income (expense)	23,102	(28,852)
Earnings before income taxes	753,337	527,610
Income tax expense	146,127	71,987
Net income	$ 607,210	$ 455,623
Net income applicable to common stockholders per basic common share	$ 12.56	$ 9.26
Weighted-average number of basic common shares outstanding	48,349	49,192
Net income applicable to common stockholders per diluted common share	$ 12.34	$ 9.11
Weighted-average number of diluted common shares outstanding	49,205	50,025

UNAUDITED CONSOLIDATED STATEMENTS OF OPERATIONS (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Stock-based compensation expense	$ 71,405	$ 58,948

UNAUDITED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($)
$ in Thousands

3 Months Ended

Mar. 31, 2018

Mar. 31, 2017

Other comprehensive income, net of tax

Net income

$ 607,210

$ 455,623

Foreign currency translation adjustments, net of tax benefit of $16,023 and $19,733, respectively

^[1]

60,876

35,805

Reclassification of net unrealized gains on marketable equity securities to retained earnings, net of tax charge of $57,597

^[2]

(241,088)

Net unrealized (losses) gains on marketable securities, net of tax benefit of $1,428 and tax charge of $7,855, respectively

^[3]

(364)

345,934

Comprehensive income

$ 426,634

$ 837,362

[1]	(1) Foreign currency translation adjustments result from currency fluctuations on the translation of the Company's international non-U.S. Dollar denominated net assets. Foreign currency translation adjustments were favorable for the three months ended March 31, 2018 compared to the three months ended March 31, 2017 because the U.S. Dollar weakened against certain currencies in which the Company's net assets are denominated. During the three months ended March 31, 2018, the Company recorded deferred tax charges of $10.5 million to foreign currency translation adjustments related to its one-time deemed repatriation tax liability recorded at December 31, 2017 and current year foreign earnings subject to U.S. federal and state income tax, resulting from the introduction of the U.S. Tax Cuts and Jobs Act ("the Tax Act"). Prior to January 1, 2018, foreign currency translation adjustments excluded U.S. federal and state income taxes as a result of the Company's intention to indefinitely reinvest the earnings of its international subsidiaries outside of the United States. Foreign currency translation adjustments also include a tax benefit of $26.5 million and $19.7 million for the three months ended March 31, 2018 and 2017, respectively, associated with the Company's Euro-denominated debt, which is designated as a net investment hedge against the impact of currency fluctuations of the Company's Euro-denominated net assets (see Note 8).
[2]	(2) Changes in the fair value of the Company's investment in Ctrip.com International Ltd. ("Ctrip") equity securities for periods beginning after December 31, 2017 are recognized in net income pursuant to the adoption of the accounting update on financial instruments in the first quarter of 2018 (see Note 1). Net unrealized gains, net of tax, on marketable equity securities at December 31, 2017 have been reclassified to retained earnings.
[3]	(3) Net unrealized (losses) gains on marketable securities for the three months ended March 31, 2018 and 2017 includes net unrealized losses of $1.8 million and net unrealized gains of $353.8 million before tax on marketable securities, respectively, of which net unrealized gains of $32.4 million and $323.7 million, respectively, were exempt from tax in the Netherlands. Net unrealized losses of $34.2 million and net unrealized gains of $30.1 million for the three months ended March 31, 2018 and 2017 were taxable at a 25% tax rate in the Netherlands, resulting in tax benefits of $8.5 million and tax charges of $7.9 million, respectively. The Company also recorded U.S. tax charges of $7.1 million for the three months ended March 31, 2018 related to these investments.

UNAUDITED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Tax (benefit) associated with gain (loss) on components of foreign currency translation	$ (16,023)	$ (19,733)
Net unrealized (losses) gains on marketable securities, before tax	(1,800)	353,800
Tax exempt net unrealized (losses) gains on marketable securities, before tax, in the Netherlands	32,400	323,700
Taxable net unrealized (losses) gains on marketable securities, before tax	(34,200)	30,100
Tax (benefit) associated with gain (loss) on marketable securities	(1,428)	7,855
Reclassification of net unrealized gains on marketable equity securities to retained earnings, tax/(benefit)	57,597	0
Tax and Customs Administration, Netherlands
Tax (benefit) associated with gain (loss) on marketable securities	(8,500)	7,900
Domestic Tax Authority [Member]
Tax (benefit) associated with gain (loss) on marketable securities	7,100	0
Currency translation adjustment on deemed repatriation tax liability [Member]
Tax (benefit) associated with gain (loss) on components of foreign currency translation	10,500	0
Net Investment Hedging [Member]
Tax (benefit) associated with gain (loss) on components of foreign currency translation	$ (26,500)	$ (19,700)

UNAUDITED CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS' EQUITY - 3 months ended Mar. 31, 2018 - USD ($)
$ in Thousands

Total

Common Stock

Treasury Stock

Additional Paid-In Capital

Retained Earnings

Accumulated Other Comprehensive Income

Balance at Dec. 31, 2017

$ 11,260,598

$ 487

$ (8,698,829)

$ 5,783,089

$ 13,938,869

$ 236,982

Balance (in shares) at Dec. 31, 2017

62,689,000

(14,217,000)

Increase (Decrease) in Stockholders' Equity

Net income

607,210

Foreign currency translation adjustments, net of tax benefit of $16,023

60,876

^[1]

60,876

Net unrealized losses on marketable securities, net of tax benefit of $1,428

(364)

^[2]

(364)

Reclassification adjustment for convertible debt in mezzanine

(48,435)

Exercise of stock options and vesting of restricted stock units and/or performance share units

1,161

$ 1

1,160

Exercise of stock options and vesting of restricted stock units and/or performance share units (in shares)

149,000

Repurchase of common stock

$ (731,592)

Repurchase of common stock (in shares)

(373,119)

(373,000)

Stock-based compensation and other stock-based payments

$ 71,500

71,500

Conversion of debt

(773,150)

Balance (in shares) at Mar. 31, 2018

62,838,000

(14,590,000)

Balance at Mar. 31, 2018

10,636,343

$ 488

$ (9,430,421)

$ 5,034,164

14,975,706

56,406

Increase (Decrease) in Stockholders' Equity

Cumulative effect of adoption of accounting standard updates

$ 188,539

$ 429,627

$ (241,088)

[1]

(1) Foreign currency translation adjustments result from currency fluctuations on the translation of the Company's international non-U.S. Dollar denominated net assets. Foreign currency translation adjustments were favorable for the three months ended March 31, 2018 compared to the three months ended March 31, 2017 because the U.S. Dollar weakened against certain currencies in which the Company's net assets are denominated. During the three months ended March 31, 2018, the Company recorded deferred tax charges of $10.5 million to foreign currency translation adjustments related to its one-time deemed repatriation tax liability recorded at December 31, 2017 and current year foreign earnings subject to U.S. federal and state income tax, resulting from the introduction of the U.S. Tax Cuts and Jobs Act ("the Tax Act"). Prior to January 1, 2018, foreign currency translation adjustments excluded U.S. federal and state income taxes as a result of the Company's intention to indefinitely reinvest the earnings of its international subsidiaries outside of the United States. Foreign currency translation adjustments also include a tax benefit of $26.5 million and $19.7 million for the three months ended March 31, 2018 and 2017, respectively, associated with the Company's Euro-denominated debt, which is designated as a net investment hedge against the impact of currency fluctuations of the Company's Euro-denominated net assets (see Note 8).

[2]

(3) Net unrealized (losses) gains on marketable securities for the three months ended March 31, 2018 and 2017 includes net unrealized losses of $1.8 million and net unrealized gains of $353.8 million before tax on marketable securities, respectively, of which net unrealized gains of $32.4 million and $323.7 million, respectively, were exempt from tax in the Netherlands. Net unrealized losses of $34.2 million and net unrealized gains of $30.1 million for the three months ended March 31, 2018 and 2017 were taxable at a 25% tax rate in the Netherlands, resulting in tax benefits of $8.5 million and tax charges of $7.9 million, respectively. The Company also recorded U.S. tax charges of $7.1 million for the three months ended March 31, 2018 related to these investments.

UNAUDITED CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS' EQUITY (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Stockholders' Equity [Abstract]
Tax (benefit) associated with gain (loss) on components of foreign currency translation	$ (16,023)	$ (19,733)
Tax (benefit) associated with gain (loss) on marketable securities	$ (1,428)	$ 7,855

UNAUDITED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
OPERATING ACTIVITIES:
Net income	$ 607,210	$ 455,623
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	56,540	40,412
Amortization	46,550	43,018
Provision for uncollectible accounts	28,331	13,530
Deferred income tax expense (benefit)	8,654	(24,441)
Net unrealized gains on marketable equity securities	(54,514)	0
Stock-based compensation expense and other stock-based payments	71,500	59,059
Amortization of debt issuance costs	2,113	2,067
Amortization of debt discount	15,127	17,625
Changes in assets and liabilities:
Accounts receivable	(96,034)	(78,428)
Prepaid expenses and other current assets	(708,925)	(443,643)
Accounts payable, accrued expenses and other current liabilities	631,496	305,758
Other	32,327	(9,962)
Net cash provided by operating activities	640,375	380,618
INVESTING ACTIVITIES:
Purchase of investments	(713,748)	(1,498,723)
Proceeds from sale of investments	2,481,251	676,474
Additions to property and equipment	(131,987)	(70,559)
Acquisitions and other investments, net of cash acquired	0	(6)
Net cash provided by (used in) investing activities	1,635,516	(892,814)
FINANCING ACTIVITIES:
Proceeds from the issuance of long-term debt	0	1,051,722
Payments for conversion of senior notes	(1,487,109)	(4)
Payment of debt	(348)	0
Payments for repurchase of common stock	(718,941)	(209,797)
Proceeds from exercise of stock options	1,161	1,479
Net cash (used in) provided by financing activities	(2,205,237)	843,400
Effect of exchange rate changes on cash, cash equivalents and restricted cash and cash equivalents	10,599	21,737
Net increase in cash, cash equivalents and restricted cash and cash equivalents	81,253	352,941
Cash, cash equivalents and restricted cash and cash equivalents, beginning of period	2,563,341	2,082,007
Cash, cash equivalents and restricted cash and cash equivalents, end of period	2,644,594	2,434,948
SUPPLEMENTAL CASH FLOW INFORMATION:
Cash paid during the period for income taxes	783,974	536,192
Cash paid during the period for interest	74,275	38,496
Non-cash financing activity	$ 0	$ 1,000

BASIS OF PRESENTATION

3 Months Ended

Mar. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

BASIS OF PRESENTATION

Management of Booking Holdings Inc. (the "Company") is responsible for the Unaudited Consolidated Financial Statements included in this document. The Unaudited Consolidated Financial Statements have been prepared in accordance with accounting principles generally accepted in the United States of America ("GAAP") and include all normal and recurring adjustments that management of the Company considers necessary for a fair presentation of its financial position and operating results. The Company prepared the Unaudited Consolidated Financial Statements following the requirements of the Securities and Exchange Commission for interim reporting. As permitted under those rules, the Company condensed or omitted certain footnotes or other financial information that are normally required by GAAP for annual financial statements. These statements should be read in combination with the Consolidated Financial Statements in the Company's Annual Report on Form 10-K for the year ended December 31, 2017.

The Unaudited Consolidated Financial Statements include the accounts of the Company and its wholly-owned subsidiaries, including its primary brands of Booking.com, priceline, KAYAK, agoda.com, Rentalcars.com and OpenTable. All inter-company accounts and transactions have been eliminated in consolidation. The functional currency of the Company's foreign subsidiaries is generally the respective local currency. Assets and liabilities are translated into U.S. Dollars at the rate of exchange existing at the balance sheet date. Income statement amounts are translated at the average exchange rates for the period. Translation gains and losses are included as a component of "Accumulated other comprehensive income" in the accompanying Unaudited Consolidated Balance Sheets. Foreign currency transaction gains and losses are included in "Foreign currency transactions and other" in the Unaudited Consolidated Statements of Operations.

Revenues, expenses, assets and liabilities can vary during each quarter of the year. Therefore, the results and trends in these interim financial statements may not be the same as those for any subsequent quarter or the full year.

Change in Presentation

In the first quarter of 2018, the Company changed the presentation of "Performance advertising", "Brand advertising", and "Sales and marketing" to "Performance marketing", "Brand marketing" and "Sales and other expenses" in the Unaudited Consolidated Statements of Operations. The descriptions of these new lines are as follows:

"Performance marketing" expenses are marketing expenses generally measured by return on investment or an increase in bookings over a specified time period. These expenses consist primarily of the costs of: (1) search engine keyword purchases; (2) referrals from meta-search and travel research websites; (3) affiliate programs; and (4) other performance-based advertisements, including certain incentive programs.

"Brand marketing" expenses are marketing expenses to build brand awareness over a specified time period. These expenses consist primarily of television advertising, online video advertising (including the airing of our television advertising online) and online display advertising, as well as other marketing expenses such as public relations, trade shows and sponsorships.

"Sales and other expenses" are generally variable in nature and consist primarily of: (1) credit cards and other payment processing fees associated with merchant transactions; (2) fees paid to third parties that provide call center, website content translations and other services; (3) provisions for customer chargebacks associated with merchant transactions; (4) customer relations costs; (5) provisions for bad debt, primarily related to agency accommodation commission receivables; and (6) insurance claim costs.

Reclassification

In conjunction with the adoption of the current revenue standard effective January 1, 2018, the Company reclassified certain expenses from "Cost of revenues" to "Sales and other expenses" in its Unaudited Consolidated Statement of Operations for the three months ended March 31, 2017 to conform to the current period presentation.

The change in presentation and the reclassification for the three months ended March 31, 2017 had no impact on operating income or net income and are summarized below (in thousands):



Previously Reported		Three Months Ended March 31, 2017
	Cost of revenues	$	85,169
	Performance advertising	980,773
	Brand advertising	73,012
	Sales and marketing	114,036

Current Presentation		Three Months Ended March 31, 2017
	Cost of revenues	$	80,401
	Performance marketing	982,172
	Brand marketing	80,818
	Sales and other expenses	109,599

See Item 5 in this Quarterly Report for disclosure related to this change in presentation and the reclassification for the years ended December 31, 2017, 2016 and 2015.

Restricted Cash and Cash Equivalents: Restricted cash and cash equivalents at March 31, 2018 and December 31, 2017 principally relates to the minimum cash requirement for Rentalcars.com's insurance business established in the fourth quarter of 2017. The following table reconciles cash, cash equivalents and restricted cash and cash equivalents reported in the Unaudited Consolidated Balance Sheets to the total amount shown in the Unaudited Consolidated Statements of Cash Flows (in thousands):



	March 31, 2018		December 31, 2017
As included in the Unaudited Consolidated Balance Sheets:
Cash and cash equivalents	$	2,622,086	$	2,541,604
Restricted cash and cash equivalents included in prepaid expenses and other current assets	22,508		21,737
Total cash, cash equivalents and restricted cash and cash equivalents as shown in the Unaudited Consolidated Statements of Cash Flows	$	2,644,594	$	2,563,341

Recent Accounting Pronouncements Adopted

Recognition and Measurement of Financial Instruments

In January 2016, the Financial Accounting Standards Board (“FASB”) issued a new accounting update which amends the guidance on the recognition and measurement of financial instruments. The update (1) requires an entity to measure equity investments (except those accounted for under the equity method or those that result in consolidation of the investee) at fair value with changes in fair value recognized in net income rather than accumulated other comprehensive income, (2) allows an entity to elect to measure those equity investments that do not have a readily determinable fair value at cost less impairment, if any, plus or minus changes resulting from observable price changes in orderly transactions for the identical or a similar investment of the same issuer, (3) simplifies the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment, and (4) clarifies that an entity should evaluate the need for a valuation allowance on a deferred tax asset related to available-for-sale securities in combination with the entity’s evaluation of their other deferred tax assets.

This update is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. The Company adopted this update in the first quarter of 2018. The Company recorded an increase of $241.1 million to retained earnings for the net unrealized gain, net of tax, related to its investment in Ctrip equity securities, with an offsetting adjustment to accumulated other comprehensive income as of January 1, 2018. Changes in fair value of the Company's investment in Ctrip equity securities subsequent to January 1, 2018 are recognized in net income (see Note 5). In addition, the Company elected to continue to use the cost method of accounting for equity investments without a readily determinable fair value.

Revenue from Contracts with Customers

In May 2014, the FASB issued a new accounting standard on the recognition of revenue from contracts with customers that was designed to create greater comparability for financial statement users across industries and jurisdictions. The core principle of this new standard is that an "entity recognizes revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services." This new standard also requires enhanced disclosures on the nature, amount, timing and uncertainty of revenue from contracts with customers. Since May 2014, the FASB has issued several amendments to this new standard, including additional guidance, and deferred the effective date for public business entities to annual and interim periods beginning after December 15, 2017.

The Company adopted this new standard on January 1, 2018. The Company recorded a net increase to its retained earnings of $188.5 million, net of tax, as of January 1, 2018, due to the cumulative impact of adopting the new standard, with substantially all of the impact related to the Company’s travel reservation services. See Note 2 for more information on the effects of the adoption of this standard.

Other Recent Accounting Pronouncements

Premium Amortization on Purchased Callable Debt Securities

In March 2017, the FASB issued a new accounting update to shorten the premium amortization period of purchased callable debt securities with non-contingent call features that are callable at fixed prices and on preset dates from their contractual maturity to the earliest call date. For public business entities, this update is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption is permitted. Entities are required to apply this update on a modified retrospective basis with a cumulative-effect adjustment to retained earnings as of the beginning of the period of adoption. The Company is currently evaluating the impact to its Consolidated Financial Statements of adopting this update and does not expect there to be a material impact.

Simplifying the Test for Goodwill Impairment

In January 2017, the FASB issued a new accounting update to simplify the test for goodwill impairment by eliminating Step 2, which measures a goodwill impairment loss by comparing the implied fair value of a reporting unit's goodwill, which requires a hypothetical purchase price allocation, with the carrying amount of that reporting unit's goodwill. Under this update, an entity would perform its quantitative annual, or interim, goodwill impairment test using the current Step 1 test and recognize an impairment charge for the excess of the carrying value of a reporting unit over its fair value.

For public business entities, this update is effective for their annual or any interim goodwill impairment tests in fiscal years beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests occurring after January 1, 2017. The update will be applied prospectively. The Company has not early adopted this update.

Measurement of Credit Losses on Financial Instruments

In June 2016, the FASB issued a new accounting update on the measurement of credit losses for financial assets measured at amortized cost, which includes accounts receivable and available-for-sale debt securities. For financial assets measured at amortized cost, this update requires an entity to (1) estimate its lifetime expected credit losses upon recognition of the financial assets and establish an allowance to present the net amount expected to be collected, (2) recognize this allowance and changes in the allowance during subsequent periods through net income and (3) consider relevant information about past events, current conditions and reasonable and supportable forecasts in assessing the lifetime expected credit losses. For available-for-sale debt securities, this update made several targeted amendments to the existing other-than-temporary impairment model, including (1) requiring disclosure of the allowance for credit losses, (2) allowing reversals of the previously recognized credit losses until the entity has the intent to sell, is more-likely-than-not required to sell the securities or the maturity of the securities, (3) limiting impairment to the difference between the amortized cost basis and fair value and (4) not allowing entities to consider the length of time that fair value has been less than amortized cost as a factor in evaluating whether a credit loss exists.

This update is effective for public business entities for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption is permitted for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. Entities are required to apply this update on a modified retrospective basis with a cumulative-effect adjustment to retained earnings as of the beginning of the period of adoption. The Company is currently evaluating the impact to its Consolidated Financial Statements of adopting this update and does not expect there to be a material impact.

Leases

In February 2016, the FASB issued a new accounting standard intended to improve the financial reporting of lease transactions. The new accounting standard requires lessees to recognize an asset and a liability on the balance sheet for the right and obligation created by entering into a lease transaction for all leases with the exception of short-term leases. The new standard retains the dual-model concept by requiring entities to determine if a lease is an operating or financing lease and the current "bright line" percentages could be used as guidance in applying the new standard. The lessor accounting model remains largely unchanged. The new standard expands qualitative and quantitative disclosures for lessees.

The standard is effective for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted. The FASB allows two options to apply the standard beginning January 1, 2019, either on a prospective basis or by restating 2017 and 2018 financial statements.

The Company plans to adopt the new standard on January 1, 2019 and apply it on a prospective basis. The Company is currently evaluating the impact of adopting this new standard. The Company will recognize right-of-use assets and operating lease liabilities in its Consolidated Balance Sheet upon adoption.

REVENUE RECOGNITION

3 Months Ended

Mar. 31, 2018

Revenue from Contract with Customer [Abstract]

REVENUE RECOGNITION

Adoption of ASC Topic 606, Revenues from Contracts with Customers

On January 1, 2018, the Company adopted ASC 606, Revenue from Contracts with Customers, using a modified retrospective method applied to all contracts as of January 1, 2018. Therefore, for reporting periods beginning after December 31, 2017, the financial statements are prepared in accordance with the current revenue standard and the financial statements for 2016 and 2017 are presented under the previous revenue standard.

For periods beginning after December 31, 2017, the Company recognizes revenue for travel reservation services when the travel begins rather than when the travel is completed. For example, revenues for accommodation reservation services, which were principally recognized at check-out under the previous revenue standard, are now recognized at check-in under the current revenue standard. The Company currently expects this timing change will not have a significant impact to its annual revenues and net income, although the effects on quarterly revenues and net income are expected to be more significant.

In addition, revenues from priceline's Name Your Own Price® ("NYOP") transactions were previously presented on a gross basis with the amount remitted to the travel service providers reported as cost of revenues. Under the current revenue standard, NYOP revenues are reported on a net basis with the amount remitted to the travel service providers recorded as an offset in merchant revenues. Therefore, for periods beginning after December 31, 2017, the Company no longer presents "Cost of revenues" or "Gross profit" in its Consolidated Statements of Operations. Total revenues reported in 2018 are comparable to gross profit reported in previous years.

Billing and cash collections remain unchanged and, therefore, "Net cash provided by operating activities" as presented in the Consolidated Statements of Cash Flows is not impacted.

The Company recorded a net increase to its retained earnings of $188.5 million, net of tax, as of January 1, 2018, due to the cumulative impact of adopting the current revenue standard, with substantially all of the impact related to the Company’s travel reservation services. In addition, since the Company is using the modified retrospective method of adopting the current revenue standard, the Company is required to disclose the financial impacts to its Consolidated Balance Sheets and Consolidated Statements of Operations for all 2018 reporting periods (refer to the disclosures below for this additional information).

The cumulative effects of the revenue accounting changes on the Company's Unaudited Consolidated Balance Sheet as of January 1, 2018 were as follows (in thousands):



	Balance at December 31, 2017		Adjustments			Balance at January 1, 2018
ASSETS
Current assets:
Accounts receivable, net	$	1,217,801	$	205,324		$	1,423,125

LIABILITIES AND STOCKHOLDERS' EQUITY
Current liabilities:
Accounts payable	$	667,523	$	171,644		$	839,167
Accrued expenses and other current liabilities	1,138,980		44,374			1,183,354
Deferred merchant bookings	980,455		(201,647		)	778,808

Deferred income taxes	481,139		2,414			483,553

Stockholders' equity:
Retained earnings	13,938,869		188,539			14,127,408

The following tables summarize the impacts of adopting the current revenue standard (in thousands, except per share data):

Unaudited Consolidated Balance Sheets as of March 31, 2018:



	As reported (current revenue standard)		Current period adjustments			As adjusted (previous revenue standard)
ASSETS
Current assets:
Accounts receivable, net	$	1,530,321	$	(152,477	)	$	1,377,844
Prepaid expenses and other current assets	1,111,753		19,531			1,131,284

LIABILITIES AND STOCKHOLDERS' EQUITY
Current liabilities:
Accounts payable	$	841,274	$	(104,172	)	$	737,102
Accrued expenses and other current liabilities	1,238,815		(8,897		)	1,229,918
Deferred merchant bookings	1,425,649		117,367			1,543,016

Deferred income taxes	481,220		(984		)	480,236

Stockholders' equity:
Retained earnings	14,975,706		(133,224		)	14,842,482
Accumulated other comprehensive income	56,406		(3,310		)	53,096

Unaudited Consolidated Statements of Operations for the Three Months Ended March 31, 2018:



	As reported (current revenue standard)			Current period adjustments			As adjusted (previous revenue standard)

Agency revenues	$	2,113,339		$	55,987		$	2,169,326
Merchant revenues	526,467			61,540			588,007
Advertising and other revenues	288,395			91			288,486
Cost of revenues				45,660			45,660

Operating expenses:
Performance marketing	1,106,207			2,394			1,108,601
Sales and other expenses	165,800			205			166,005
General and administrative	162,139			801			162,940
Foreign currency transactions and other	(8,056		)	1,855			(6,201		)
Income tax expense	146,127			15,098			161,225
Net income	607,210			55,315		⁽¹⁾	662,525
Net income applicable to common stockholders per basic common share	12.56			1.14			13.70
Net income applicable to common stockholders per diluted common share	12.34			1.12			13.46

(1) The current period adjustment represents the net income recorded directly to retained earnings on January 1, 2018 of $188.5 million that would have been recognized in the first quarter of 2018 under the previous revenue standard, partially offset by $133.2 million that would have been recognized in the second quarter of 2018 under the previous revenue standard.

Revenue Recognition

Online travel reservation services

For periods beginning after December 31, 2017, the Company recognizes revenue for travel reservation services when the travel begins rather than when the travel is completed. Substantially all of the Company's revenues are generated by providing online travel reservation services, which principally allows travelers to book travel reservations (i.e., accommodation, rental car and airline ticket reservations) with travel service providers (i.e., a hotel or other accommodation, rental car company or airline) through the Company’s websites and mobile apps. While the Company generally refers to a consumer that books travel reservation services on the Company's platforms as its customer, for accounting purposes, the Company's "customers" are the travel service providers and the travelers in certain merchant transactions. The Company's contracts with the travel service providers give them the ability to market their reservation availability without transferring responsibility to deliver the travel service to the Company; therefore, the Company's revenues are presented on a net basis in the Consolidated Statements of Operations. These contracts include payment terms and establish the consideration to which the Company is entitled, which includes either a commission or a margin on the travel transaction. Revenue is measured based on the expected consideration specified in the contract with the travel service provider, considering the effects of sales incentives, "no show" cancellations (where the traveler has not cancelled the reservation and does not arrive on the scheduled reservation date) and "late" cancellations (where the travel service provider accepts a cancellation after its cancellation cut-off date). Estimates for cancellations are based on historical experience.

Online travel reservation services are recorded at a point in time when the Company has completed its post-booking services and the travelers begin using the arranged travel services. These services are classified into two categories:

•

Agency revenues are derived from travel-related transactions where the Company does not facilitate payments from travelers for the travel reservation services provided. The Company invoices the travel service providers for its commissions in the month that travel is completed. Agency revenues consist almost entirely of travel reservation commissions, as well as certain global distribution fees ("GDS"), reservation booking fees and certain travel insurance fees.

•

Merchant revenues are derived from services where the Company facilitates payments from travelers for the travel reservation services provided, generally at the time of booking. The Company records cash collected from travelers, which includes the amounts owed to the travel service providers and the Company’s commission or margin and fees, as deferred merchant bookings until the arranged travel service begins. Merchant revenues include net revenues (i.e., the amount charged to travelers less the amount owed to travel service providers) and travel reservation commissions in connection with our accommodation reservations and rental car services; ancillary fees, including damage excess waiver insurance and certain travel insurance fees and certain GDS reservation booking fees; and customer processing fees. Substantially all merchant revenues are for merchant services derived from transactions where travelers book accommodations reservations or rental car reservations from travel service providers at disclosed rates, which are subject to contractual arrangements.

Pursuant to the terms of the Company's merchant services, the travel service providers are permitted to bill the Company for the underlying cost of the services during a specified period of time. If the Company is not billed by the travel service providers within the specified period of time, the Company increases its revenue by the unbilled amounts.

Transaction-related taxes

For merchant transactions, the Company charges the traveler for taxes that the travel service provider will owe ("tax recovery charge"), which is either passed on to the travel service provider for payment to the taxing authority or paid directly by the Company to the taxing authority. Tax rate information for calculating the tax recovery charge is provided to the Company by the travel service providers. The Company has elected to include in revenues the tax recovery charge to the traveler offset by the payment of taxes to the tax authorities, which results in no overall impact to revenues.

Advertising and Other Revenues

Advertising and other revenues are primarily recognized by KAYAK and OpenTable and to a lesser extent by priceline for advertising placements on its website and Booking.com's BookingSuite branded accommodation marketing and business analytics services. KAYAK recognizes advertising revenue primarily by sending referrals to online travel companies ("OTCs") and travel service providers and from advertising placements on its websites and mobile apps. Revenue related to referrals is recognized when a customer clicks on a referral placement or upon completion of the travel. Revenue for advertising placements is recognized based upon when a customer clicks on an advertisement or when KAYAK displays an advertisement. OpenTable recognizes reservation fees when diners are seated through its online restaurant reservation service and subscription fees for restaurant management services on a straight-line basis over the contractual period in accordance with how the service is provided.

Loyalty Programs

The Company provides various loyalty programs, where participating travelers or diners are awarded loyalty incentives on current transactions that can be redeemed for future qualifying reservations booked with the travel service provider through the Company's websites or mobile apps or, in the case of OpenTable, at participating restaurants. The estimated fair value of the incentives that are expected to be redeemed is recognized as a reduction of revenues at the time the incentives are granted. In the first quarter of 2018, OpenTable introduced a three-year time-based expiration for points earned by diners, which resulted in a reduction of a portion of the loyalty liability of approximately $27 million. At March 31, 2018 and December 31, 2017, liabilities of $78.8 million and $104.7 million, respectively, for loyalty incentives were included in "Accrued expenses and other current liabilities" in the Unaudited Consolidated Balance Sheets.

The Company uses the portfolio approach to account for its loyalty points as the rewards programs share similar characteristics within each program in relation to the value provided to the traveler or diner and their breakage patterns. Using this portfolio approach is not expected to differ materially from applying the guidance to individual contracts.

Disaggregation of revenue

Geographic Information

The Company's international information consists of the results of Booking.com, Rentalcars.com (which began operating as part of Booking.com on January 1, 2018) and agoda.com and the results of the international businesses of KAYAK and OpenTable. This classification is independent of where the consumer resides, where the consumer is physically located while using the Company's services or the location of the travel service provider or restaurant. For example, a reservation made through Booking.com at a hotel in New York by a consumer in the United States is part of the Company's international results. The Company's geographic information is as follows (in thousands):



				International
Total revenues for the Three Months Ended March 31,	United States			The Netherlands		Other		Total
2018	$	378,691		$	2,122,724	$	426,786	$	2,928,201
2017	399,185		⁽¹⁾	1,687,303		332,916		2,419,404		⁽¹⁾

(1) Excludes $80.4 million cost of revenues for NYOP transactions, which is a reduction to merchant revenues in 2018.

Revenue by Type of Service

Approximately $2.5 billion or 85% of the Company's revenue relates to online accommodation reservation services. Revenue from all other sources of online travel reservation services or advertising and other revenues each represent less than 10% of the Company's total revenues.

Deferred Revenue

Cash payments received from travelers in advance of the Company completing its service obligations are included in "Deferred merchant bookings" in the Company's Unaudited Consolidated Balance Sheets and are comprised principally of amounts owed to the travel service providers as well as the Company's deferred revenue for its commission or margin and fees. As of March 31, 2018 and December 31, 2017, deferred merchant bookings includes deferred revenue of $219.9 million and $151.2 million, respectively. The Company expects to complete its service obligation within one year of booking. In the first quarter of 2018, the Company recognized revenue of $85.6 million related to the deferred revenue balance at December 31, 2017. In addition, the Company reduced the December 31, 2017 balance by $32.4 million for the January 1, 2018 impact of the adoption of the current revenue standard. The offsetting increase in the deferred revenue balance for the three months ended March 31, 2018 is principally driven by cash payments received from travelers, net of amounts payable to travel service providers, of $192.2 million for those online travel reservations in the current period.

STOCK-BASED EMPLOYEE COMPENSATION

3 Months Ended

Mar. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

STOCK-BASED EMPLOYEE COMPENSATION

Stock-based compensation expense included in personnel expenses in the Unaudited Consolidated Statements of Operations was approximately $71.4 million and $58.9 million for the three months ended March 31, 2018 and 2017, respectively.

Stock-based compensation expense is recognized in the financial statements based upon fair value. Fair value is recognized as an expense on a straight-line basis over the employee's requisite service period and forfeitures are accounted for when they occur. The fair value of performance share units and restricted stock units is determined based on the number of units granted and the quoted price of the Company's common stock as of the grant date. Stock-based compensation expense related to performance share units reflects the estimated probable outcome at the end of the performance period. The fair value of employee stock options assumed in acquisitions was determined using the Black-Scholes model and the market value of the Company's common stock at the respective acquisition dates.

Restricted Stock Units and Performance Share Units

The following table summarizes the activity of restricted stock units and performance share units ("share-based awards") during the three months ended March 31, 2018:



Share-Based Awards	Shares		Weighted-Average Grant Date Fair Value
Unvested at December 31, 2017	524,696			$	1,431.88
Granted	146,112			$	2,033.79
Vested	(146,766	)		$	1,271.70
Performance Shares Adjustment	6,487			$	1,654.15
Forfeited/Canceled	(6,718	)		$	1,594.90
Unvested at March 31, 2018	523,811			$	1,645.32

As of March 31, 2018, there was $574.0 million of total future compensation cost related to unvested share-based awards to be recognized over a weighted-average period of 2.3 years.

During the three months ended March 31, 2018, the Company made broad-based grants of 97,006 restricted stock units that generally have a three-year vesting period, subject to certain exceptions for terminations other than for "cause," for "good reason" or on account of death or disability. These share-based awards had a total grant date fair value of $197.3 million based on a grant date fair value per share of $2,033.79.

In addition, during the three months ended March 31, 2018, the Company granted 49,106 performance share units to executives and certain other employees. The performance share units had a total grant date fair value of $99.9 million based on a grant date fair value per share of $2,033.79. The performance share units are payable in shares of the Company's common stock upon vesting. Subject to certain exceptions for terminations other than for "cause," for "good reason" or on account of death or disability, recipients of these performance share units generally must continue their service through the requisite service period in order to receive any shares. Stock-based compensation related to performance share units reflects the estimated probable outcome at the end of the performance period. The actual number of shares to be issued on the vesting date will be determined upon completion of the performance period, which generally ends December 31, 2020, assuming there is no accelerated vesting for, among other things, a termination of employment under certain circumstances. As of March 31, 2018, the estimated number of probable shares to be issued is a total of 49,106 shares. If the maximum performance thresholds are met at the end of the performance period, a maximum number of 98,212 total shares could be issued. If the minimum performance thresholds are not met, 37,796 shares would be issued at the end of the performance period.

2017 Performance Share Units

During the year ended December 31, 2017, the Company granted 73,893 performance share units with a grant date fair value of $128.2 million, based on a grant date fair value per share of $1,735.10. The actual number of shares to be issued will be determined upon completion of the performance period which generally ends December 31, 2019, assuming there is no accelerated vesting for, among other things, a termination of employment under certain circumstances.

At March 31, 2018, there were 67,946 unvested 2017 performance share units outstanding, net of performance share units that were forfeited or vested since the grant date. As of March 31, 2018, the number of shares estimated to be issued pursuant to these performance share units at the end of the performance period is a total of 85,639 shares. If the maximum performance thresholds are met at the end of the performance period, a maximum of 135,892 total shares could be issued pursuant to these performance share units. If the minimum performance thresholds are not met, 54,689 shares would be issued at the end of the performance period.

2016 Performance Share Units

During the year ended December 31, 2016, the Company granted 85,735 performance share units with a grant date fair value of $111.7 million, based on a weighted-average grant date fair value per share of $1,302.25. The actual number of shares to be issued will be determined upon completion of the performance period which generally ends December 31, 2018.

At March 31, 2018, there were 70,750 unvested 2016 performance share units outstanding, net of performance share units that were forfeited or vested since the grant date. As of March 31, 2018, the number of shares estimated to be issued pursuant to these performance share units at the end of the performance period is a total of 113,660 shares. If the maximum thresholds are met at the end of the performance period, a maximum of 157,699 total shares could be issued pursuant to these performance share units. If the minimum performance thresholds are not met, 42,035 shares would be issued at the end of the performance period.

Stock Options

All outstanding employee stock options were assumed in acquisitions. The following table summarizes the activity for stock options during the three months ended March 31, 2018:



Employee Stock Options	Number of Shares		Weighted-Average Exercise Price			Aggregate Intrinsic Value (in thousands)			Weighted-Average Remaining Contractual Term (in years)
Balance, December 31, 2017	30,675			$	401.61		$	40,986	3.9
Exercised	(2,416	)		$	477.21
Forfeited	(11	)		$	66.98
Balance, March 31, 2018	28,248			$	395.35		$	47,599	3.6
Vested and exercisable as of March 31, 2018	28,160			$	403.59		$	47,509	3.6
Vested and exercisable as of March 31, 2018 and expected to vest thereafter	28,248			$	395.35		$	47,599	3.6

The aggregate intrinsic value of employee stock options exercised during the three months ended March 31, 2018 and 2017 was $3.9 million and $5.4 million, respectively. During the three months ended March 31, 2018 and 2017, stock options vested for 83 and 539 shares, respectively.

NET INCOME PER SHARE

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

NET INCOME PER SHARE

The Company computes basic net income per share by dividing net income applicable to common shareholders by the weighted-average number of common shares outstanding during the period. Diluted net income per share is based upon the weighted-average number of common and common equivalent shares outstanding during the period.

Common equivalent shares related to stock options, restricted stock units, and performance share units are calculated using the treasury stock method. Performance share units are included in the weighted-average common equivalent shares based on the number of shares that would be issued if the end of the reporting period were the end of the performance period, if the result would be dilutive.

The Company's convertible notes have net share settlement features requiring the Company upon conversion to settle the principal amount of the debt for cash and the conversion premium for cash or shares of the Company's common stock, at the Company's option. The convertible notes are included in the calculation of diluted net income per share if their inclusion is dilutive under the treasury stock method.

A reconciliation of the weighted-average number of shares outstanding used in calculating diluted earnings per share is as follows (in thousands):



	Three Months Ended March 31,
	2018	2017
Weighted-average number of basic common shares outstanding	48,349	49,192
Weighted-average dilutive stock options, restricted stock units and performance share units	282	227
Assumed conversion of Convertible Senior Notes	574	606
Weighted-average number of diluted common and common equivalent shares outstanding	49,205	50,025
Anti-dilutive potential common shares	1,432	2,256

Anti-dilutive potential common shares for the three months ended March 31, 2018 include approximately 1.0 million shares that could be issued under the Company's outstanding convertible notes. Under the treasury stock method, the convertible notes will generally have an anti-dilutive impact on net income per share if the conversion prices for the convertible notes exceed the Company's average stock price.

INVESTMENTS

3 Months Ended

Mar. 31, 2018

Investments, Debt and Equity Securities [Abstract]

INVESTMENTS

Short-term and Long-term Investments in Marketable Securities

The Company has classified its investments in marketable debt securities as available-for-sale securities. These securities are reported at estimated fair value with the aggregate unrealized gains and losses related to these investments, net of taxes, reflected as a part of "Accumulated other comprehensive income" in the Unaudited Consolidated Balance Sheets. Classification as short-term or long-term investment is based upon the maturity of the debt securities. As of March 31, 2018, the Company does not consider any of its investments to be other-than-temporarily impaired.

The Company's investments in marketable equity securities are reported at estimated fair value in the Unaudited Consolidated Balance Sheets. Pursuant to the adoption of the accounting update on financial instruments (see Note 1), for periods beginning after December 31, 2017, changes in fair value of the equity securities are recognized in net income rather than accumulated other comprehensive income. Therefore, the Company reclassified net unrealized gains of $298.7 million ($241.1 million net of tax) as of January 1, 2018 from accumulated other comprehensive income to retained earnings in the Unaudited Consolidated Balance Sheet. For the three months ended March 31, 2018, the Company recognized $54.5 million in "Net unrealized gains on marketable equity securities" in the Unaudited Consolidated Statement of Operations.

The following table summarizes, by major security type, the Company's investments as of March 31, 2018 (in thousands):



	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Short-term investments:
Debt securities:
International government securities	$	461,389	$	56	$	(555	)	$	460,890
U.S. government securities	794,489		—		(4,064		)	790,425
Corporate debt securities	2,952,709		256		(14,565		)	2,938,400
U.S government agency securities	4,435		—		(27		)	4,408
Commercial paper	46,581		—		—			46,581
Total short-term investments	$	4,259,603	$	312	$	(19,211	)	$	4,240,704

Long-term investments:
Debt securities:
International government securities	$	786,958	$	1,760	$	(584	)	$	788,134
U.S. government securities	555,413		—		(11,525		)	543,888
Corporate debt securities	5,744,951		7,272		(71,499		)	5,680,724
U.S. government agency securities	500		—		(6		)	494
Ctrip convertible debt securities	1,275,000		136,013		—			1,411,013
Equity securities:
Ctrip equity securities	655,311		353,199		—			1,008,510
Total long-term investments	$	9,018,133	$	498,244	$	(83,614	)	$	9,432,763

The Company's investment policy seeks to preserve capital and maintain sufficient liquidity to meet operational and other needs of the business. As of March 31, 2018, the weighted-average life of the Company’s fixed income investment portfolio, excluding the Company's investment in Ctrip convertible debt securities, was approximately 1.6 years with an average credit quality of A+/A1/A+.

The Company invests in international government securities with high credit quality. As of March 31, 2018, investments in international government securities principally included debt securities issued by the governments of the Netherlands, France, Belgium, Austria and Germany.

The following table summarizes, by major security type, the Company's investments as of December 31, 2017 (in thousands):



	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Short-term investments:
Debt securities:
International government securities	$	725,566	$	246	$	(436	)	$	725,376
U.S. government securities	996,112		5		(1,999		)	994,118
Corporate debt securities	3,067,703		449		(4,837		)	3,063,315
U.S. government agency securities	4,444		—		(30		)	4,414
Commercial paper	72,650		—		—			72,650
Total short-term investments	$	4,866,475	$	700	$	(7,302	)	$	4,859,873

Long-term investments:
Debt securities:
International government securities	$	607,000	$	1,588	$	(678	)	$	607,910
U.S. government securities	844,910		2		(10,636		)	834,276
Corporate debt securities	6,689,747		8,399		(41,722		)	6,656,424
U.S. government agency securities	500		—		(6		)	494
Ctrip convertible debt securities	1,275,000		103,100		(9,600		)	1,368,500
Equity securities:
Ctrip equity securities	655,311		299,697		(1,012		)	953,996
Total long-term investments	$	10,072,468	$	412,786	$	(63,654	)	$	10,421,600

Investments in Ctrip

On May 26, 2015 and August 7, 2014, the Company invested $250 million and $500 million, respectively, in five-year senior convertible notes issued at par value by Ctrip. On December 11, 2015, the Company invested $500 million in a Ctrip ten-year senior convertible note issued at par value, which included a put option allowing the Company, at its option, to require a prepayment in cash from Ctrip at the end of the sixth year of the note. On September 12, 2016, the Company invested $25 million in a Ctrip six-year senior convertible note issued at par value, which included a put option allowing the Company, at its option, to require prepayment in cash from Ctrip at the end of the third year of the note. The Company determined that the economic characteristics and risks of the put option are clearly and closely related to the note, and therefore were not embedded derivatives.

The Company evaluated the conversion features for all Ctrip senior convertible notes and only the conversion feature associated with the September 2016 investment met the definition of an embedded derivative (see Note 6). The Company monitors the conversion features of these notes to determine whether they meet the definition of an embedded derivative during each reporting period. The Ctrip convertible notes have been marked-to-market in accordance with the accounting guidance for available-for-sale securities. As of March 31, 2018, the Company had also invested $655.3 million in Ctrip American Depositary Shares ("ADSs").

In connection with the Company's investments in Ctrip's convertible notes, Ctrip granted the Company the right to appoint an observer to its board of directors and permission to acquire its shares (through the acquisition of Ctrip ADSs in the open market) so that combined with ADSs issuable upon conversion of the August 2014, May 2015 and September 2016 convertible notes, the Company could hold up to an aggregate of approximately 15% of Ctrip's outstanding equity plus any ADSs issuable upon the conversion of the December 2015 convertible notes. As of March 31, 2018, the Company did not have significant influence over Ctrip.

Equity Investments without Readily Determinable Fair Value

The Company held investments in equity securities of private companies, which are typically at an early stage of development, of $450.9 million at March 31, 2018 and December 31, 2017. These investments are measured at cost less impairment, if any, plus or minus changes resulting from observable price changes in orderly transactions for the identical or a similar investment of the same issuer and are included in "Other assets" in the Company's Unaudited Consolidated Balance Sheets. There were no identified events or changes in circumstances to indicate a potential impairment with these investments as of March 31, 2018.

FAIR VALUE MEASUREMENTS

3 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

FAIR VALUE MEASUREMENTS

Financial assets and liabilities carried at fair value as of March 31, 2018 are classified in the categories described in the tables below (in thousands):



	Level 1		Level 2		Total
ASSETS:
Cash and restricted cash equivalents:
Money market funds	$	1,930,983	$	—	$	1,930,983
International government securities	—		70,025		70,025
Time deposits	37,141		—		37,141
Short-term investments:
International government securities	—		460,890		460,890
U.S. government securities	—		790,425		790,425
Corporate debt securities	—		2,938,400		2,938,400
U.S government agency securities	—		4,408		4,408
Commercial paper	—		46,581		46,581
Long-term investments:
International government securities	—		788,134		788,134
U.S. government securities	—		543,888		543,888
Corporate debt securities	—		5,680,724		5,680,724
U.S. government agency securities	—		494		494
Ctrip convertible debt securities	—		1,411,013		1,411,013
Ctrip equity securities	1,008,510		—		1,008,510
Derivatives:
Currency exchange derivatives	—		440		440
Total assets at fair value	$	2,976,634	$	12,735,422	$	15,712,056



	Level 1		Level 2		Total
LIABILITIES:
Currency exchange derivatives	$	—	$	1,853	$	1,853

Financial assets and liabilities carried at fair value as of December 31, 2017 are classified in the categories described in the tables below (in thousands):



	Level 1		Level 2		Total
ASSETS:
Cash and restricted cash equivalents:
Money market funds	$	1,895,272	$	—	$	1,895,272
U.S. government securities	—		22,265		22,265
Corporate debt securities	—		6,674		6,674
Commercial paper	—		96,321		96,321
Time deposits	17,896		—		17,896
Short-term investments:
International government securities	—		725,376		725,376
U.S. government securities	—		994,118		994,118
Corporate debt securities	—		3,063,315		3,063,315
U.S. government agency securities	—		4,414		4,414
Commercial paper	—		72,650		72,650
Long-term investments:
International government securities	—		607,910		607,910
U.S. government securities	—		834,276		834,276
Corporate debt securities	—		6,656,424		6,656,424
U.S. government agency securities	—		494		494
Ctrip convertible debt securities	—		1,368,500		1,368,500
Ctrip equity securities	953,996		—		953,996
Derivatives:
Currency exchange derivatives	—		1,767		1,767
Total assets at fair value	$	2,867,164	$	14,454,504	$	17,321,668



	Level 1		Level 2		Total
LIABILITIES:
Currency exchange derivatives	$	—	$	127	$	127

There are three levels of inputs to measure fair value. The definition of each input is described below:


Level 1:	Quoted prices in active markets that are accessible by the Company at the measurement date for

identical assets and liabilities.


Level 2:	Inputs that are observable, either directly or indirectly. Such prices may be based upon quoted

prices for identical or comparable securities in active markets or inputs not quoted on active

markets, but corroborated by market data.


Level 3:	Unobservable inputs are used when little or no market data is available.

Investments in corporate debt securities, U.S. and international government securities, commercial paper, government agency securities and convertible debt securities are considered "Level 2" valuations because the Company has access to quoted prices, but does not have visibility into the volume and frequency of trading for all of these investments. For the Company's investments, a market approach is used for recurring fair value measurements and the valuation techniques use inputs that are observable, or can be corroborated by observable data, in an active marketplace.

The Company's derivative instruments are valued using pricing models. Pricing models take into account the contract terms as well as multiple inputs where applicable, such as interest rate yield curves, option volatility and currency rates. Derivatives are considered "Level 2" fair value measurements. The Company's derivative instruments are typically short-term in nature.

As of March 31, 2018 and December 31, 2017, the Company's cash consisted of bank deposits. Other financial assets and liabilities, including restricted cash and cash equivalents, accounts receivable, accounts payable, accrued expenses and deferred merchant bookings are carried at cost which approximates their fair value because of the short-term nature of these items. As of March 31, 2018 and December 31, 2017, the Company held investments in equity securities of private companies of $450.9 million and these investments are accounted for under the cost method of accounting (see Note 5). See Note 5 for information on the carrying value of the Company's investments in marketable securities, Note 8 for the estimated fair value of the Company's outstanding Senior Notes and Note 12 for the Company's contingent liabilities associated with business acquisitions.

In the normal course of business, the Company is exposed to the impact of foreign currency fluctuations. The Company limits these risks by following established risk management policies and procedures, including the use of derivatives. The Company does not use derivatives for trading or speculative purposes. All derivative instruments are recognized in the Unaudited Consolidated Balance Sheets at fair value. Gains and losses resulting from changes in the fair value of derivative instruments that are not designated as hedging instruments for accounting purposes are recognized in the Unaudited Consolidated Statements of Operations in the period that the changes occur. Changes in the fair value of derivatives designated as net investment hedges were recorded as foreign currency translation adjustments to offset a portion of the foreign currency translation adjustment from Euro-denominated net assets held by certain subsidiaries and were recognized in the Unaudited Consolidated Balance Sheets in "Accumulated other comprehensive income."

Derivatives Not Designated as Hedging Instruments — The Company is exposed to adverse movements in currency exchange rates as the operating results of its international operations are translated from local currency into U.S. Dollars upon consolidation. The Company enters into average-rate derivative contracts to hedge translation risks from short-term currency exchange rate fluctuations for the Euro, British Pound Sterling and certain other currencies versus the U.S. Dollar. As of March 31, 2018 and December 31, 2017, there were no outstanding derivative contracts related to foreign currency translation risks.

The Company also enters into foreign currency forward contracts to hedge its exposure to the impact of movements in currency exchange rates on its transactional balances denominated in currencies other than the functional currency. Derivative assets are included in "Prepaid expenses and other current assets" and derivative liabilities are included in "Accrued expenses and other current liabilities" in the Unaudited Consolidated Balance Sheets. Derivatives associated with these transaction risks resulted in foreign currency gains of $20.6 million and $6.8 million for the three months ended March 31, 2018 and 2017, respectively. These mark-to-market adjustments on the derivative contracts, offset by the effect of changes in currency exchange rates on transactions denominated in currencies other than the functional currency, resulted in net losses of $5.4 million and $5.9 million for the three months ended March 31, 2018 and 2017. These net impacts related to these derivatives are reported in "Foreign currency transactions and other" in the Unaudited Consolidated Statements of Operations.

The settlement of derivative contracts not designated as hedging instruments resulted in net cash inflows of $18.2 million and $2.7 million for the three months ended March 31, 2018 and 2017, respectively, and are reported within "Net cash provided by operating activities" in the Unaudited Consolidated Statements of Cash Flows.

Embedded Derivative — In September 2016, the Company invested $25 million in a Ctrip convertible note (see Note 5). The Company determined that the conversion option for this note met the definition of an embedded derivative. At March 31, 2018 and December 31, 2017, the embedded derivative had an estimated fair value of $2.1 million and $1.8 million, respectively, and is reported in the Unaudited Consolidated Balance Sheets with its host contract in "Long-term investments." The embedded derivative is bifurcated for measurement purposes only. The mark-to-market adjustments are included in "Foreign currency transactions and other" in the Company's Unaudited Consolidated Statement of Operations.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

INTANGIBLE ASSETS AND GOODWILL

3 Months Ended

Mar. 31, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

INTANGIBLE ASSETS AND GOODWILL

The Company's intangible assets at March 31, 2018 and December 31, 2017 consisted of the following (in thousands):



	March 31, 2018							December 31, 2017
	Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount		Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount		Amortization Period	Weighted- Average Useful Life
Supply and distribution agreements	$	1,068,509	$	(377,529	)	$	690,980	$	1,056,660	$	(355,000	)	$	701,660	3 - 20 years	16 years

Technology	137,800		(110,620		)	27,180		137,288		(104,478		)	32,810		1 - 5 years	5 years

Patents	1,623		(1,623		)	—		1,623		(1,623		)	—		15 years	15 years

Internet domain names	43,272		(30,262		)	13,010		42,265		(28,802		)	13,463		5 - 20 years	8 years

Trade names	1,783,038		(374,316		)	1,408,722		1,779,076		(350,447		)	1,428,629		4-20 years	19 years

Non-compete agreements	21,900		(21,694		)	206		21,900		(21,639		)	261		3-4 years	3 years
Total intangible assets	$	3,056,142	$	(916,044	)	$	2,140,098	$	3,038,812	$	(861,989	)	$	2,176,823

Intangible assets are amortized on a straight-line basis. Amortization expense was approximately $46.6 million and $43.0 million for the three months ended March 31, 2018 and 2017, respectively.

The amortization expense for intangible assets for the remainder of 2018 and the annual expense for the next five years and thereafter is expected to be as follows (in thousands):



Remainder of 2018	$	127,026
2019	162,014
2020	153,821
2021	147,461
2022	144,560
2023	142,444
Thereafter	1,262,772
	$	2,140,098

A roll-forward of goodwill for the three months ended March 31, 2018 consisted of the following (in thousands):



Balance at December 31, 2017	$	2,737,671
Currency translation adjustments	17,076
Balance at March 31, 2018	$	2,754,747

A substantial portion of the Company's intangibles and goodwill relates to the acquisition of OpenTable in July 2014 and KAYAK in May 2013. There were no events or changes in circumstances to indicate a potential impairment to goodwill or intangible assets as of March 31, 2018.

- Definition

The entire disclosure for the aggregate amount of goodwill and a description of intangible assets, which may include (a) for amortizable intangible assets (also referred to as finite-lived intangible assets), the carrying amount, the amount of any significant residual value, and the weighted-average amortization period, (b) for intangible assets not subject to amortization (also referred to as indefinite-lived intangible assets), the carrying amount, and (c) the amount of research and development assets acquired and written off in the period, including the line item in the income statement in which the amounts written off are aggregated, if not readily apparent from the income statement. Also discloses (a) for amortizable intangibles assets in total and by major class, the gross carrying amount and accumulated amortization, the total amortization expense for the period, and the estimated aggregate amortization expense for each of the five succeeding fiscal years, (b) for intangible assets not subject to amortization the carrying amount in total and by major class, and (c) for goodwill, in total and for each reportable segment, the changes in the carrying amount of goodwill during the period (including the aggregate amount of goodwill acquired, the aggregate amount of impairment losses recognized, and the amount of goodwill included in the gain (loss) on disposal of a reporting unit). If any part of goodwill has not been allocated to a reportable segment, discloses the unallocated amount and the reasons for not allocating. For each impairment loss recognized related to an intangible asset (excluding goodwill), discloses: (a) a description of the impaired intangible asset and the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method for determining fair value, (c) the caption in the income statement or the statement of activities in which the impairment loss is aggregated, and (d) the segment in which the impaired intangible asset is reported. For each goodwill impairment loss recognized, discloses: (a) a description of the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method of determining the fair value of the associated reporting unit, and (c) if a recognized impairment loss is an estimate not finalized and the reasons why the estimate is not final. May also disclose the nature and amount of any significant adjustments made to a previous estimate of an impairment loss.

+ References + Details

DEBT

3 Months Ended

Mar. 31, 2018

Debt Disclosure [Abstract]

DEBT

Revolving Credit Facility

In June 2015, the Company entered into a $2.0 billion five-year unsecured revolving credit facility with a group of lenders. Borrowings under the revolving credit facility will bear interest, at the Company’s option, at a rate per annum equal to either (i) the adjusted LIBOR for the interest period in effect for such borrowing plus an applicable margin ranging from 0.875% to 1.50%; or (ii) the greatest of (a) Bank of America, N.A.'s prime lending rate, (b) the federal funds rate plus 0.50%, and (c) an adjusted LIBOR for an interest period of one month plus 1.00%, plus an applicable margin ranging from 0.00% to 0.50%. Undrawn balances available under the revolving credit facility are subject to commitment fees at the applicable rate ranging from 0.085% to 0.20%.

The revolving credit facility provides for the issuance of up to $70.0 million of letters of credit as well as borrowings of up to $50.0 million on same-day notice, referred to as swingline loans. Borrowings under the revolving credit facility may be made in U.S. Dollars, Euros, British Pounds Sterling and any other foreign currency agreed to by the lenders. The proceeds of loans made under the facility would be used for working capital and general corporate purposes, which could include acquisitions, share repurchases or debt repayments. There were no borrowings outstanding and $3.9 million and $3.8 million of letters of credit issued under the facility at March 31, 2018 and December 31, 2017, respectively.

Outstanding Debt

Outstanding debt as of March 31, 2018 consisted of the following (in thousands):



March 31, 2018	Outstanding Principal Amount		Unamortized Debt Discount and Debt Issuance Cost			Carrying Value
Short-term debt:
0.35% Convertible Senior Notes due June 2020	$	1,000,000	$	(58,343	)	$	941,657
Long-term debt:
0.9% Convertible Senior Notes due September 2021	$	1,000,000	$	(77,838	)	$	922,162
0.8% (€1 Billion) Senior Notes due March 2022	1,229,850		(5,916		)	1,223,934
2.15% (€750 Million) Senior Notes due November 2022	922,388		(4,493		)	917,895
2.75% Senior Notes due March 2023	500,000		(3,051		)	496,949
2.375% (€1 Billion) Senior Notes due September 2024	1,229,850		(11,992		)	1,217,858
3.65% Senior Notes due March 2025	500,000		(3,180		)	496,820
3.6% Senior Notes due June 2026	1,000,000		(6,644		)	993,356
1.8% (€1 Billion) Senior Notes due March 2027	1,229,850		(5,004		)	1,224,846
3.55% Senior Notes due March 2028	500,000		(3,402		)	496,598
Total long-term debt	$	8,111,938	$	(121,520	)	$	7,990,418

Outstanding debt as of December 31, 2017 consisted of the following (in thousands):



December 31, 2017	Outstanding Principal Amount		Unamortized Debt Discount and Debt Issuance Cost			Carrying Value
Short-term debt:
1.0% Convertible Senior Notes due March 2018	$	714,304	$	(3,394	)	$	710,910
Long-term debt:
0.35% Convertible Senior Notes due June 2020	$	1,000,000	$	(64,825	)	$	935,175
0.9% Convertible Senior Notes due September 2021	1,000,000		(83,272		)	916,728
0.8% (€1 Billion) Senior Notes due March 2022	1,200,800		(6,238		)	1,194,562
2.15% (€750 Million) Senior Notes due November 2022	900,600		(4,683		)	895,917
2.75% Senior Notes due March 2023	500,000		(3,203		)	496,797
2.375% (€1 Billion) Senior Notes due September 2024	1,200,800		(12,240		)	1,188,560
3.65% Senior Notes due March 2025	500,000		(3,290		)	496,710
3.6% Senior Notes due June 2026	1,000,000		(6,840		)	993,160
1.8% (€1 Billion) Senior Notes due March 2027	1,200,800		(5,136		)	1,195,664
3.55% Senior Notes due March 2028	500,000		(3,485		)	496,515
Total long-term debt	$	9,003,000	$	(193,212	)	$	8,809,788

Based upon the closing price of the Company's common stock for the prescribed measurement period for the three months ended March 31, 2018, the contingent conversion threshold on the 2020 Notes (as defined below) was exceeded. Therefore, the 2020 Notes are currently convertible at the option of the holders and, accordingly, the Company reported the carrying value of the 2020 Notes as a current liability in the Company's Unaudited Consolidated Balance Sheet as of March 31, 2018. Since these notes are convertible at the option of the holders and the principal amount is required to be paid in cash, the Company reclassified the unamortized debt discount for the 2020 Notes in the amount of $51.4 million before tax as of March 31, 2018, from additional paid-in-capital to convertible debt in the mezzanine section in the Company's Unaudited Consolidated Balance Sheet. The contingent conversion threshold on the 2020 Notes was not exceeded at December 31, 2017, therefore, the 2020 Notes were reported as non-current liabilities in the Consolidated Balance Sheet as of that date. The determination of whether or not the 2020 Notes are convertible is performed on a quarterly basis. Consequently, the 2020 Notes may not be convertible in future quarters, and therefore, may again be classified as long-term debt, if the contingent conversion threshold is not met in such quarters.

The 2018 Notes (as defined below) became convertible on December 15, 2017, at the option of the holders, and remained convertible until the scheduled trading day immediately preceding the maturity date of March 15, 2018. Therefore, as of December 31, 2017, the Company reported the carrying value of the 2018 Notes as a current liability and reclassified the unamortized debt discount for the 2018 Notes in the amount of $3.0 million before tax from additional paid-in-capital to convertible debt in the mezzanine section in the Consolidated Balance Sheet as of that date.

The contingent conversion thresholds on the 2021 Notes (as defined below) were not exceeded at March 31, 2018 or December 31, 2017, and therefore these notes were reported as a non-current liability in the Unaudited Consolidated Balance Sheets.

Fair Value of Debt

As of March 31, 2018 and December 31, 2017, the estimated fair value of the outstanding Senior Notes was approximately $10.1 billion and $11.1 billion, respectively, and was considered a "Level 2" fair value measurement (see Note 6). Fair value was estimated based upon actual trades at the end of the reporting period or the most recent trade available as well as the Company's stock price at the end of the reporting period. A substantial portion of the market value of the Company's debt in excess of the outstanding principal amount relates to the conversion premium on the Convertible Senior Notes.

Convertible Debt

If the note holders exercise their option to convert, the Company delivers cash to repay the principal amount of the notes and delivers shares of common stock or cash, at its option, to satisfy the conversion value in excess of the principal amount. If the Company's convertible debt is redeemed or converted prior to maturity, a gain or loss on extinguishment is recognized. The gain or loss is the difference between the fair value of the debt component immediately prior to extinguishment and its carrying value. To estimate the fair value of the debt at the conversion date, the Company estimated its straight debt borrowing rate, considering its credit rating and straight debt of comparable corporate issuers.

Description of Senior Convertible Notes

In August 2014, the Company issued in a private placement $1.0 billion aggregate principal amount of Convertible Senior Notes due September 15, 2021, with an interest rate of 0.9% (the "2021 Notes"). The Company paid $11.0 million in debt issuance costs during the year ended December 31, 2014 related to this offering. The 2021 Notes are convertible, subject to certain conditions, into the Company's common stock at a conversion price of approximately $2,055.50 per share. The 2021 Notes are convertible, at the option of the holder, prior to September 15, 2021, upon the occurrence of specific events, including but not limited to a change in control, or if the closing sales price of the Company's common stock for at least 20 trading days in the period of 30 consecutive trading days ending on the last trading day of the immediately preceding calendar quarter is more than 150% of the conversion price in effect for the notes on the last trading day of the immediately preceding quarter. In the event that all or substantially all of the Company's common stock is acquired on or prior to the maturity of the 2021 Notes in a transaction in which the consideration paid to holders of the Company's common stock consists of all or substantially all cash, the Company would be required to make additional payments in the form of additional shares of common stock to the holders of the 2021 Notes in an aggregate value ranging from $0 to approximately $375 million depending upon the date of the transaction and the then current stock price of the Company. As of June 15, 2021, holders will have the right to convert all or any portion of the 2021 Notes, regardless of the Company's stock price. The 2021 Notes may not be redeemed by the Company prior to maturity. The holders may require the Company to repurchase the 2021 Notes for cash in certain circumstances. Interest on the 2021 Notes is payable on March 15 and September 15 of each year.

In May 2013, the Company issued in a private placement $1.0 billion aggregate principal amount of Convertible Senior Notes due June 15, 2020, with an interest rate of 0.35% (the "2020 Notes"). The 2020 Notes were issued with an initial discount of $20.0 million. The Company paid $1.0 million in debt issuance costs during the year ended December 31, 2013 related to this offering. The 2020 Notes are convertible, subject to certain conditions, into the Company's common stock at a conversion price of approximately $1,315.10 per share. The 2020 Notes are convertible, at the option of the holder, prior to June 15, 2020, upon the occurrence of specific events, including but not limited to a change in control, or if the closing sales price of the Company's common stock for at least 20 trading days in the period of 30 consecutive trading days ending on the last trading day of the immediately preceding calendar quarter is more than 150% of the conversion price in effect for the notes on the last trading day of the immediately preceding quarter. In the event that all or substantially all of the Company's common stock is acquired on or prior to the maturity of the 2020 Notes in a transaction in which the consideration paid to holders of the Company's common stock consists of all or substantially all cash, the Company would be required to make additional payments in the form of additional shares of common stock to the holders of the 2020 Notes in an aggregate value ranging from $0 to approximately $397 million depending upon the date of the transaction and the then current stock price of the Company. As of March 15, 2020, holders will have the right to convert all or any portion of the 2020 Notes, regardless of the Company's stock price. The 2020 Notes may not be redeemed by the Company prior to maturity. The holders may require the Company to repurchase the 2020 Notes for cash in certain circumstances. Interest on the 2020 Notes is payable on June 15 and December 15 of each year.

In March 2012, the Company issued in a private placement $1.0 billion aggregate principal amount of Convertible Senior Notes due March 15, 2018, with an interest rate of 1.0% (the "2018 Notes"). The 2018 Notes were convertible, subject to certain conditions, into the Company's common stock at a conversion price of approximately $944.61 per share. For the three months ended March 31, 2018, in connection with the maturity of the 2018 Notes, the Company paid $714.3 million to satisfy the aggregate principal amount due and paid an additional $773.2 million in satisfaction of the conversion value in excess of the principal amount.

Cash-settled convertible debt, such as the Company's Convertible Senior Notes, is separated into debt and equity components at issuance and each component is assigned a value. The value assigned to the debt component is the estimated fair value, as of the issuance date, of a similar bond without the conversion feature. The difference between the bond cash proceeds and this estimated fair value, representing the value assigned to the equity component, is recorded as a debt discount. Debt discount is amortized using the effective interest rate method over the period from the origination date through the stated maturity date. The Company estimated the straight debt borrowing rates at debt origination to be 3.18% for the 2021 Notes, 3.13% for the 2020 Notes, and 3.50% for the 2018 Notes. The yield to maturity was estimated at an at-market coupon priced at par.

Debt discount after tax of $82.5 million ($142.9 million before tax) less financing costs associated with the equity component of convertible debt of $1.6 million after tax was recorded in additional paid-in capital related to the 2021 Notes at December 31, 2014. Debt discount after tax of $92.4 million ($154.3 million before tax) less financing costs associated with the equity component of convertible debt of $0.1 million after tax was recorded in additional paid-in capital related to the 2020 Notes at June 30, 2013. Debt discount after tax of $80.9 million ($135.2 million before tax) less financing costs associated with the equity component of convertible debt of $2.8 million after tax was recorded in additional paid-in capital related to the 2018 Notes at March 31, 2012.

For the three months ended March 31, 2018 and 2017, the Company recognized interest expense of $19.8 million and $24.0 million, respectively, related to convertible notes, which was comprised of $4.5 million and $5.6 million, respectively, related to the contractual coupon interest, $14.5 million and $17.2 million, respectively, related to the amortization of debt discount and $0.8 million and $1.2 million respectively, related to the amortization of debt issuance costs. For the three months ended March 31, 2018 and 2017, included in the amortization of debt discount mentioned above was $0.7 million of original issuance discount related to the 2020 Notes. The remaining period for amortization of debt discount and debt issuance costs is the period until the stated maturity date for the respective debt. The weighted-average effective interest rates for the three months ended March 31, 2018 and 2017 are 3.3% and 3.4%, respectively.

Other Long-term Debt

In August 2017, the Company issued Senior Notes due March 15, 2023, with an interest rate of 2.75% (the "2023 Notes") for an aggregate principal amount of $500 million. The 2023 Notes were issued with an initial discount of $0.7 million. In addition, the Company paid $2.7 million in debt issuance costs during the year ended December 31, 2017. Interest on the 2023 Notes is payable semi-annually on March 15 and September 15.

In August 2017, the Company issued Senior Notes due March 15, 2028, with an interest rate of 3.55% (the "2028 Notes") for an aggregate principal amount of $500 million. The 2028 Notes were issued with an initial discount of $0.4 million. In addition, the Company paid $3.2 million in debt issuance costs during the year ended December 31, 2017. Interest on the 2028 Notes is payable semi-annually on March 15 and September 15.

In March 2017, the Company issued Senior Notes due March 10, 2022, with an interest rate of 0.8% (the "March 2022 Notes") for an aggregate principal amount of 1.0 billion Euros. The March 2022 Notes were issued with an initial discount of 2.1 million Euros. In addition, the Company paid $5.0 million in debt issuance costs during the year ended December 31, 2017. Interest on the March 2022 Notes is payable annually on March 10. Subject to certain limited exceptions, all payments of interest and principal for the March 2022 Notes will be made in Euros.

In May 2016, the Company issued Senior Notes due June 1, 2026, with an interest rate of 3.6% (the "2026 Notes") for an aggregate principal amount of $1.0 billion. The 2026 Notes were issued with an initial discount of $1.9 million. In addition, the Company paid $6.2 million in debt issuance costs during the year ended December 31, 2016. Interest on the 2026 Notes is payable semi-annually on June 1 and December 1.

In November 2015, the Company issued Senior Notes due November 25, 2022, with an interest rate of 2.15% (the "November 2022 Notes") for an aggregate principal amount of 750 million Euros. The November 2022 Notes were issued with an initial discount of 2.2 million Euros. In addition, the Company paid $3.7 million in debt issuance costs during the year ended December 31, 2015. Interest on the November 2022 Notes is payable annually on November 25. Subject to certain limited exceptions, all payments of interest and principal, including payments made upon any redemption of the November 2022 Notes will be made in Euros.

In March 2015, the Company issued Senior Notes due March 15, 2025, with an interest rate of 3.65% (the "2025 Notes") for an aggregate principal amount of $500 million. The 2025 Notes were issued with an initial discount of $1.3 million. In addition, the Company paid $3.2 million in debt issuance costs during the year ended December 31, 2015. Interest on the 2025 Notes is payable semi-annually on March 15 and September 15.

In March 2015, the Company issued Senior Notes due March 3, 2027, with an interest rate of 1.8% (the "2027 Notes") for an aggregate principal amount of 1.0 billion Euros. The 2027 Notes were issued with an initial discount of 0.3 million Euros. In addition, the Company paid $6.3 million in debt issuance costs during the year ended December 31, 2015. Interest on the 2027 Notes is payable annually on March 3. Subject to certain limited exceptions, all payments of interest and principal for the 2027 Notes will be made in Euros.

In September 2014, the Company issued Senior Notes due September 23, 2024, with an interest rate of 2.375% (the "2024 Notes") for an aggregate principal amount of 1.0 billion Euros. The 2024 Notes were issued with an initial discount of 9.4 million Euros. In addition, the Company paid $6.5 million in debt issuance costs during the year ended December 31, 2014. Interest on the 2024 Notes is payable annually on September 23. Subject to certain limited exceptions, all payments of interest and principal, including payments made upon any redemption of the 2024 Notes, will be made in Euros.

The aggregate principal value of the March 2022 Notes, November 2022 Notes, 2024 Notes and 2027 Notes and accrued interest thereon are designated as a hedge of the Company's net investment in certain Euro functional currency subsidiaries. The foreign currency transaction gains or losses on these liabilities are measured based upon changes in spot rates and are recorded in "Accumulated other comprehensive income" in the Unaudited Consolidated Balance Sheets. The Euro-denominated net assets of these subsidiaries are translated into U.S. Dollars at each balance sheet date, with effects of foreign currency changes also reported in "Accumulated other comprehensive income" in the Unaudited Consolidated Balance Sheets. Since the notional amount of the recorded Euro-denominated debt and related interest are not greater than the notional amount of the Company's net investment, the Company does not expect to incur any ineffectiveness on this hedge.

Debt discount is amortized using the effective interest rate method over the period from the origination date through the stated maturity date. The Company estimated the effective interest rates at debt origination to be 2.78% for the 2023 Notes, 3.56% for the 2028 Notes, 0.84% for the March 2022 Notes, 3.62% for the 2026 Notes, 2.20% for the November 2022 Notes, 3.68% for the 2025 Notes, 1.80% for the 2027 Notes and 2.48% for the 2024 Notes.

For the three months ended March 31, 2018 and 2017, the Company recognized interest expense of $43.3 million and $30.6 million, respectively, related to other long-term debt, which was principally comprised of $41.6 million and $29.5 million, respectively, related to the contractual coupon interest. The remaining interest expense relates to the amortization of debt discount and debt issuance costs. The remaining period for amortization of debt discount and debt issuance costs is the period until the stated maturity dates for the respective debt.

In March 2016, the Company received a ten-year loan from the State of Connecticut in the amount of $2.5 million with an interest rate of 1% in connection with the construction of office space in Connecticut. In 2017, $1.0 million of the loan was forgiven as a result of meeting certain employment and salary conditions. The remaining balance of the loan will be forgiven in 2019 if certain employment and salary conditions are met. As of March 31, 2018 and December 31, 2017, the loan in the amount of $1.5 million is reported in "Other long-term liabilities" in the Unaudited Consolidated Balance Sheets.

TREASURY STOCK	3 Months Ended
TREASURY STOCK	Mar. 31, 2018
Equity [Abstract]
TREASURY STOCK	TREASURY STOCK In the first quarter of 2016, the Company's Board of Directors authorized a program to repurchase up to $3.0 billion of the Company's common stock. In the first quarter of 2017, the Company's Board of Directors authorized an additional program to repurchase up to $2.0 billion of the Company's common stock. In the first quarter of 2018, the Company's Board of Directors authorized an additional program to repurchase up to $8.0 billion of the Company's common stock. As of March 31, 2018, the Company had a remaining authorization of $9.8 billion to repurchase its common stock. The Company may make repurchases of shares under its stock repurchase programs, depending on prevailing market conditions, alternate uses of capital and other factors. Whether and when to initiate and/or complete any repurchase of common stock and the amount of common stock repurchased will be determined at the Company's discretion. Additionally, the Board of Directors has given the Company the general authorization to repurchase shares of its common stock to satisfy employee withholding tax obligations related to stock-based compensation. In the three months ended March 31, 2018, the Company repurchased a total of 373,119 shares of its common stock in the open market for an aggregate cost of $731.6 million, which included 314,076 shares for $611.7 million acquired through its general repurchase programs and 59,043 shares for $119.9 million withheld to satisfy employee withholding tax obligations related to stock-based compensation. In the three months ended March 31, 2017, the Company repurchased a total of 124,915 shares of its common stock in the open market for an aggregate cost of $212.3 million, which included 80,027 shares for $135.0 million acquired through its general repurchase programs and 44,888 shares for $77.3 million withheld to satisfy employee withholding tax obligations related to stock-based compensation. In the three months ended March 31, 2018, stock repurchases in March 2018 of 13,578 shares for an aggregate cost of $28.0 million were settled in April 2018. In the three months ended December 31, 2017, stock repurchases in December 2017 of 18,217 shares for an aggregate cost of $32.0 million were settled in January 2018. For the three months ended March 31, 2018 and 2017, the Company remitted $103.2 million and $69.8 million of employee withholding taxes, respectively, to the tax authorities, which is different from the aggregate cost of the shares withheld for taxes for each period due to the timing in remitting the taxes. The cash remitted to the tax authorities is included in financing activities in the Unaudited Consolidated Statements of Cash Flows. As of March 31, 2018, there were 14,589,938 shares of the Company's common stock held in treasury.

INCOME TAXES	3 Months Ended
INCOME TAXES	Mar. 31, 2018
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES Income tax expense consists of U.S. and international income taxes, determined using an estimate of the Company's annual effective tax rate, which is based upon the applicable tax rates and tax laws of the countries in which the income is generated. A deferred tax liability is recognized for all taxable temporary differences, and a deferred tax asset is recognized for all deductible temporary differences and operating loss and tax credit carryforwards. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. The Company considers many factors when assessing the likelihood of future realization of the deferred tax assets, including its recent cumulative earnings experience by taxing jurisdiction, expectations of future income, tax planning strategies, the carryforward periods available for tax reporting purposes, and other relevant factors. The Company's effective tax rate for the three months ended March 31, 2018 was 19.4% compared to 13.6% for the three months ended March 31, 2017. The Company's 2018 effective tax rate differs from the U.S. federal statutory tax rate of 21%, primarily due to the benefit of the Netherlands Innovation Box Tax and current year excess tax benefits of $13.5 million recognized from the vesting of equity awards, partially offset by the effect of higher international tax rates and U.S. federal and state tax associated with the Company's current year international earnings, resulting from the introduction of the Tax Act. The Company's 2017 effective tax rate differs from the U.S. federal statutory tax rate of 35%, primarily as a result of lower international tax rates and current year excess tax benefits of $9.9 million recognized from the vesting of equity awards, partially offset by certain nondeductible expenses. The Company's effective tax rate was higher for the three months ended March 31, 2018, compared to the three months ended March 31, 2017, primarily as a result of U.S. federal and state tax associated with the Company's current year international earnings, resulting from the introduction of the Tax Act, and the increase in the Netherlands Innovation Box Tax rate from 5% to 7% as disclosed below. In addition, the Company recorded a tax benefit during the three months ended March 31, 2017 related to U.S. state tax law changes and other deductible expenses. During the three months ended March 31, 2018 and 2017, a substantial majority of the Company's income was generated in the Netherlands. According to Dutch corporate income tax law, income generated from qualifying innovative activities is taxed at a rate of 7% ("Innovation Box Tax") beginning on or after January 1, 2018 (at a rate of 5% previously) rather than the Dutch statutory rate of 25%. A portion of Booking.com's earnings during the three months ended March 31, 2018 and 2017 qualifies for Innovation Box Tax treatment, which had a significant beneficial impact on the Company's effective tax rate for those periods. U.S. Tax Reform On December 22, 2017, the Tax Act was enacted into law in the United States. The Tax Act made significant changes to U.S. federal tax law, including a reduction in the U.S. federal statutory tax rate from 35% to 21%, effective January 1, 2018. The Tax Act imposed a one-time deemed repatriation tax on accumulated unremitted international earnings, to be paid over eight years. The Company recorded provisional income tax expense of approximately $1.6 billion during the year ended December 31, 2017, which included U.S. state income taxes and international withholding taxes, related to the mandatory deemed repatriation of estimated accumulated international earnings of approximately $16.5 billion. The Company also recorded a provisional net income tax benefit of approximately $217.0 million during the year ended December 31, 2017 related to the remeasurement of the Company’s U.S. deferred tax assets and liabilities due to the reduction of the U.S. federal statutory rate from 35% to 21%. The Company currently expects to use approximately $204 million of deferred tax assets related to federal net operating loss carryforwards ("NOLs") and approximately $46 million of other tax credit carryforwards, and accordingly reduced the transition tax liability to approximately $1.3 billion, which is presented as "Long-term U.S. transition tax liability" in the Unaudited Consolidated Balance Sheets as of March 31, 2018 and December 31, 2017. At March 31, 2018, the Company had not completed its accounting for the tax effects of the Tax Act that were recorded as provisional during the year ended December 31, 2017 in accordance with Staff Accounting Bulletin No. 118 (“SAB 118”). The Company is continuing to analyze its accumulated unremitted international earnings subject to the U.S. federal deemed repatriation tax, and the Company is still evaluating whether and to what extent it will utilize its NOLs against the transition tax liability, which may impact the Company’s U.S. deferred tax assets and liabilities. As the Company refines its estimates and continues to evaluate the Tax Act, the Company will adjust its provision for income taxes in the period when a change in estimate occurs. The Company’s final accounting for the tax effects of the Tax Act may materially differ from the provisional amounts recorded during the year ended December 31, 2017 as a result of regulatory guidance that may be issued and changes in our assumptions and interpretations based on this guidance. The Company expects to complete its accounting within the measurement period. The Tax Act also introduced in 2018 a tax on 50% of Global Intangible Low-Taxed Income (“GILTI”), which is income determined to be in excess of a specified routine rate of return. Since the Company is still reviewing GILTI provisions and expects further guidance from the U.S. Treasury Department, Internal Revenue Service, state tax authorities and/or other authorities on the application of these provisions, the Company has not yet adopted an accounting policy as to whether the Company will treat taxes on GILTI as period costs or whether the Company will recognize deferred tax assets and liabilities when basis differences exist that are expected to affect the amount of GILTI inclusion upon reversal.

ACCUMULATED OTHER COMPREHENSIVE INCOME

3 Months Ended

Mar. 31, 2018

Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]

ACCUMULATED OTHER COMPREHENSIVE INCOME

The table below provides the balances for each classification of accumulated other comprehensive income as of March 31, 2018 and December 31, 2017 (in thousands):



	March 31, 2018		December 31, 2017
Foreign currency translation adjustments, net of tax benefit of $80,468 and $64,445, respectively ⁽¹⁾	$	45,176	$	(15,700	)

Net unrealized gains on marketable securities, net of tax:
Net unrealized gains on marketable equity securities, net of tax charge of $57,597 ⁽²⁾	—		241,088
Net unrealized gains on marketable debt securities, net of tax charge of $31,475 and $32,903, respectively ⁽³⁾	11,230		11,594

Accumulated other comprehensive income	$	56,406	$	236,982

(1) Foreign currency translation adjustments, net of tax, at March 31, 2018 and December 31, 2017, include accumulated net losses from fair value adjustments of $35.0 million after tax ($52.6 million before tax) associated with previously settled derivatives that were designated as net investment hedges.

Foreign currency translation adjustments, net of tax, include foreign currency transaction losses of $273.4 million after tax ($346.7 million before tax) and $190.4 million after tax ($237.2 million before tax) at March 31, 2018 and December 31, 2017, respectively, associated with the Company's Euro-denominated debt. The Company's Euro-denominated debt is designated as a hedge against the impact of currency fluctuations on its Euro-denominated net assets (see Note 8).

The remaining balance in foreign currency translation adjustments relates to the cumulative impacts of currency fluctuations on the Company's international non-U.S. Dollar denominated net assets. During the three months ended March 31, 2018, the Company recorded deferred tax charges of $10.5 million related to its one-time deemed repatriation tax liability recorded at December 31, 2017 and current year foreign earnings subject to U.S. federal and state income tax, resulting from the introduction of the Tax Act. Prior to January 1, 2018, foreign currency translation adjustments excluded U.S. federal and state income taxes as a result of the Company's intention to indefinitely reinvest the earnings of its international subsidiaries outside of the United States.

(2) Net unrealized gains on marketable equity securities, net of tax, at December 31, 2017 related to changes in the fair value of the Company's investment in Ctrip equity securities (see Note 5). Net unrealized gains before tax on equity securities at December 31, 2017 were $298.7 million, of which unrealized gains of $319.9 million were exempt from tax in the Netherlands. Unrealized losses of $21.2 million were taxable at a 25% tax rate in the Netherlands, which resulted in a tax benefit of $5.3 million at December 31, 2017. The Company also recorded U.S. tax charges of $62.9 million at December 31, 2017 related to these investments. Changes in fair value subsequent to January 1, 2018 are recognized in net income (see Note 1).

(3) Net unrealized gains on marketable debt securities, net of tax, relates to changes in the fair value of the Company's investments in debt securities (see Note 5). Net unrealized gains before tax on debt securities at March 31, 2018 and December 31, 2017 were $42.7 million and $44.5 million, respectively, of which unrealized losses of $52.9 million and $85.3 million, respectively, were exempt from tax in the Netherlands. Unrealized gains of $95.6 million and $129.8 million at March 31, 2018 and December 31, 2017, respectively, were taxable at a 25% tax rate in the Netherlands, resulting in tax charges of $23.9 million and $32.4 million, respectively. The Company also recorded U.S. tax charges of $7.6 million and $0.5 million at March 31, 2018 and December 31, 2017, respectively, related to these investments.

COMMITMENTS AND CONTINGENCIES	3 Months Ended
COMMITMENTS AND CONTINGENCIES	Mar. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES Competition Reviews The online travel industry has become the subject of investigations by various national competition authorities ("NCAs"), particularly in Europe. The Company is or has been involved in investigations predominantly related to whether Booking.com's contractual parity arrangements with accommodation providers, sometimes also referred to as "most favored nation" or "MFN" provisions, are anti-competitive because they require accommodation providers to provide Booking.com with room rates that are at least as low as those offered to other OTCs or through the accommodation provider's website. Some investigations relate to other issues such as reservation and cancellation clauses, commission payments and pricing behavior. For instance, on September 8, 2017, the Swiss Price Surveillance Office opened an investigation into the level of commissions of Booking.com in Switzerland. In Europe, investigations into Booking.com's price parity provisions were initiated in 2013 and 2014 by NCAs in France, Germany, Italy, Austria, Sweden, Ireland and Switzerland. A number of other NCAs have also looked at these issues. On April 21, 2015, the French, Italian and Swedish NCAs, working in close cooperation with the European Commission, announced that they had accepted "commitments" offered by Booking.com to resolve and close the investigations in France, Italy and Sweden. Under the commitments, Booking.com replaced its existing price parity agreements with accommodation providers with "narrow" price parity agreements. Under a narrow price parity agreement, subject to certain exceptions, an accommodation provider is still required to offer the same or better rates on Booking.com as it offers to a consumer directly online, but it is no longer required to offer the same or better rates on Booking.com as it offers to other OTCs. The commitments also allow an accommodation provider to, among other things, offer different terms and conditions (e.g., free WiFi) and availability to consumers that book with OTCs that offer lower rates of commission or other benefits, offer lower rates to consumers that book through offline channels and continue to discount through, among other things, accommodation loyalty programs, as long as those rates are not published or marketed online. The commitments apply to accommodations in France, Italy and Sweden and were effective on July 1, 2015. The foregoing description is a summary only and is qualified in its entirety by reference to the commitments published by the NCAs on April 21, 2015. On July 1, 2015, Booking.com voluntarily implemented the commitments given to the French, Italian and Swedish NCAs throughout the European Economic Area and Switzerland. Nearly all NCAs in the European Economic Area have now closed their investigations following Booking.com's implementation of the commitments in their jurisdictions. Booking.com has also agreed with the NCAs in Australia, New Zealand, Georgia and Brazil to implement the narrow price parity clause in these countries. However, the Australian NCA recently re-opened its investigation into Booking.com's use of price parity clauses in agreements with accommodation providers. In January 2017, the Turkish NCA imposed fines on Booking.com following an investigation into Booking.com's "wide" parity clauses. Following the Turkish NCA's decision, Booking.com implemented the narrow price parity clause in Turkey. Booking.com is in ongoing discussions with various NCAs in other countries regarding their concerns. The Company is currently unable to predict the long-term impact the implementation of these commitments will have on Booking.com's business, on investigations by other countries, or on industry practice more generally. On December 23, 2015, the German NCA issued a final decision prohibiting Booking.com's narrow price parity agreements with accommodations in Germany. The German NCA did not issue a fine, but has reserved its position regarding an order for disgorgement of profits. Booking.com is appealing the German NCA's decision. An Italian hotel association has appealed the Italian NCA's decision to accept the commitments by Booking.com. A working group of ten European NCAs (France, Germany, Belgium, Hungary, Ireland, Italy, the Netherlands, Czech Republic, the United Kingdom and Sweden) was established by the European Commission in December 2015 to monitor the effects of the narrow price parity clause in Europe. This working group (the "ECN Working Group") issued questionnaires during 2016 to OTCs, including Booking.com and Expedia, online price comparison sites (or "meta-search" sites) and hotels about the narrow price parity agreement. On April 6, 2017, the ECN Working Group published the results of this monitoring exercise. The report indicated that the replacement of the "wide" price parity agreement with the narrow price parity agreement generally improved conditions for competition. Although neither the European Commission nor any of the participating NCAs has opened a new investigation following the publication of the report, the ECN Working Group decided to keep the sector under review and re-assess the competitive situation in due course. The Company is unable to predict how these appeals and the remaining investigations in other countries will ultimately be resolved, or whether further action in Europe will be taken as a result of the ECN Working Group's ongoing review. Possible outcomes include requiring Booking.com to amend or remove its rate parity clause from its contracts with accommodation providers in those jurisdictions and/or the imposition of fines. The Company is unable to predict the impact these possible outcomes might have on its business. A number of European countries have adopted legislation making price parity agreements illegal, and it is possible other countries may adopt similar legislation in the future. For example, in August 2015, French legislation known as the "Macron Law" became effective. Among other things, the Macron Law makes price parity agreements illegal, including the narrow price parity agreements agreed to by the French NCA in April 2015. Legislation prohibiting narrow price parity agreements became effective in Austria on December 31, 2016 and in Italy on August 29, 2017. A motion calling on the Swiss government to introduce legislation prohibiting the narrow price parity clause was approved by the Swiss Parliament on September 18, 2017. In July 2017, a Belgian government minister announced plans to put forward a similar proposal before the Belgian Parliament. It is not yet clear how the Macron Law, the Austrian or Italian legislation or the proposed Swiss or Belgian legislation may affect the Company's business in the long term. European NCAs are continuing to review the activities of online platforms, including through the use of consumer protection powers. On October 27, 2017 the United Kingdom's NCA launched a consumer law investigation into the clarity, accuracy and presentation of information on hotel booking sites with a specific focus on the display of search results, claims regarding discounts, methods of "pressure selling" (such as creating false impressions regarding room availability) and failure to disclose hidden charges. The consumer protection department of the German NCA announced the opening of a sector inquiry into online price comparison sites in various sectors including travel and hotels on October 24, 2017. Further, on March 9, 2018, the Danish NCA announced that it had begun a review of the competitive conditions of the online hotel booking market. Outside Europe, the Singaporean NCA announced on April 9, 2018 the launch of a market review into the online travel sector, with a focus on agreements between booking platforms and flight and hotel service providers. The Australian House of Representatives Standing Committee on Industry, Innovation, Science and Resources has also commenced an inquiry into the impacts of global online companies on local businesses in Australia. We are unable to predict what, if any, effect such actions will have on our business, industry practices or online commerce more generally. Competition-related investigations, legislation or issues could also give rise to private litigation. For example, Booking.com is involved in private litigation in Sweden related to its narrow price parity provisions. We are unable to predict how this litigation will be resolved, or whether it will impact Booking.com's business in Sweden. French Tax Matter French tax authorities conducted an audit of Booking.com of the years 2003 through 2012. They are asserting that Booking.com has a permanent establishment in France and are seeking to recover what they claim are unpaid income taxes and value-added taxes. In December 2015, the French tax authorities issued Booking.com assessments related to those tax years for approximately 356 million Euros, the majority of which would represent penalties and interest. The Company believes that Booking.com has been, and continues to be, in compliance with French tax law, and the Company is contesting the assessments. The Company's objection to the assessments was denied by the French tax authorities. If the Company is unable to resolve the matter with the French tax authorities, it would expect to challenge the assessments in the French courts. In order to contest the assessments in court, the Company may be required to pay, upfront, the full amount or a significant part of any such assessments, though such payment would not constitute an admission by the Company that it owes the taxes. Alternatively, any resolution or settlement of the matter with the French tax authorities may also require a payment as part of such resolution or settlement. French tax authorities have begun a similar audit of the tax years 2013 through 2015, which could result in additional assessments. Turkish Matter From time to time the Company has been subject to legal proceedings and claims regarding whether it is subject to local registration requirements, such as requirements to register as a travel agent. In March 2017, in connection with a lawsuit begun in 2015 by the Association of Turkish Travel Agencies claiming that Booking.com is required to meet certain registration requirements in Turkey, a Turkish court ordered Booking.com to suspend offering Turkish hotels and accommodations to Turkish residents. Although Booking.com is appealing the order and believes it to be without basis, this order has had a negative impact on the Company's growth and results of operations, and is expected to continue to negatively impact the Company's results of operations. Other The Company accrues for certain legal contingencies where it is probable that a loss has been incurred and the amount can be reasonably estimated. Such accrued amounts are not material to the Company's balance sheets and provisions recorded have not been material to the Company's results of operations or cash flows. An estimate of a reasonably possible loss or range of loss cannot be reasonably made. From time to time, the Company has been, and expects to continue to be, subject to legal proceedings and claims in the ordinary course of business, including claims of alleged infringement of third-party intellectual property rights. The Company is also subject to legal proceedings in the United States related to travel transaction taxes (e.g., hotel occupancy taxes, sales taxes, etc.). Such claims, even if not meritorious, could result in the expenditure of significant financial and managerial resources, divert management's attention from the Company's business objectives and adversely affect the Company's business, results of operations, financial condition and cash flows. Contingent Consideration for Business Acquisitions As of March 31, 2018 and December 31, 2017, the Company's Unaudited Consolidated Balance Sheets included a long-term liability of approximately $9 million for estimated contingent payments, which was considered a "Level 3" fair value measurement (see Note 6). The estimated acquisition-date contingent liability is based upon the probability-weighted average payments for specific performance factors from the acquisition date through the performance period which ends at March 31, 2019. The range of undiscounted outcomes for the estimated contingent payments is approximately $0 to $90 million. Building Construction In September 2016, the Company signed a turnkey agreement to construct an office building in the Netherlands, which will be the future headquarters of the Booking.com business. The turnkey agreement provided for payments by Booking.com of approximately 270 million Euros and consists of two components, land-use rights and the building to be constructed. Upon signing this agreement, Booking.com paid approximately 48 million Euros to the developer, which included approximately 43 million Euros for the acquired land-use rights and approximately 5 million Euros for the building construction. The land-use rights are included in "Other assets" and the building construction-in-progress is included in "Property and equipment, net" in the Unaudited Consolidated Balance Sheets at March 31, 2018 and December 31, 2017. The land-use rights asset and required future lease payments to the Municipality in Amsterdam of approximately 60 million Euros are recognized as rent expense on a straight-line basis over the remaining 49-year term of the lease and are recorded in general and administrative expense in the Unaudited Consolidated Statements of Operations. Booking.com paid approximately 34 million Euros related to the building construction in the first quarter of 2018, with the remaining amount being paid periodically from the second quarter of 2018 until the expected completion of the building in early 2021. The Company will also make additional capital expenditures to fit out and furnish the office space.

BASIS OF PRESENTATION (Policies)

3 Months Ended

Mar. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Basis of Accounting, Policy

Reclassification, Policy

Change in Presentation

Reclassification

The change in presentation and the reclassification for the three months ended March 31, 2017 had no impact on operating income or net income and are summarized below (in thousands):



Previously Reported		Three Months Ended March 31, 2017
	Cost of revenues	$	85,169
	Performance advertising	980,773
	Brand advertising	73,012
	Sales and marketing	114,036

Current Presentation		Three Months Ended March 31, 2017
	Cost of revenues	$	80,401
	Performance marketing	982,172
	Brand marketing	80,818
	Sales and other expenses	109,599

See Item 5 in this Quarterly Report for disclosure related to this change in presentation and the reclassification for the years ended December 31, 2017, 2016 and 2015.

Cash and Cash Equivalents, Restricted Cash and Cash Equivalents, Policy



	March 31, 2018		December 31, 2017
As included in the Unaudited Consolidated Balance Sheets:
Cash and cash equivalents	$	2,622,086	$	2,541,604
Restricted cash and cash equivalents included in prepaid expenses and other current assets	22,508		21,737
Total cash, cash equivalents and restricted cash and cash equivalents as shown in the Unaudited Consolidated Statements of Cash Flows	$	2,644,594	$	2,563,341

Recent Accounting Pronouncements Adopted

Recognition and Measurement of Financial Instruments

Revenue from Contracts with Customers

Other Recent Accounting Pronouncements

Premium Amortization on Purchased Callable Debt Securities

Simplifying the Test for Goodwill Impairment

Measurement of Credit Losses on Financial Instruments

Leases

REVENUE RECOGNITION Revenue Recognition (Policies)

3 Months Ended

Mar. 31, 2018

Revenue from Contract with Customer [Abstract]

Revenue Recognition, Policy

Revenue Recognition

Online travel reservation services

•

Transaction-related taxes

Advertising and Other Revenues

Loyalty Programs

BASIS OF PRESENTATION (Tables)

3 Months Ended

Mar. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Reclassifications

Reclassification

The change in presentation and the reclassification for the three months ended March 31, 2017 had no impact on operating income or net income and are summarized below (in thousands):



Previously Reported		Three Months Ended March 31, 2017
	Cost of revenues	$	85,169
	Performance advertising	980,773
	Brand advertising	73,012
	Sales and marketing	114,036

Current Presentation		Three Months Ended March 31, 2017
	Cost of revenues	$	80,401
	Performance marketing	982,172
	Brand marketing	80,818
	Sales and other expenses	109,599

See Item 5 in this Quarterly Report for disclosure related to this change in presentation and the reclassification for the years ended December 31, 2017, 2016 and 2015.

Restrictions on Cash and Cash Equivalents



	March 31, 2018		December 31, 2017
As included in the Unaudited Consolidated Balance Sheets:
Cash and cash equivalents	$	2,622,086	$	2,541,604
Restricted cash and cash equivalents included in prepaid expenses and other current assets	22,508		21,737
Total cash, cash equivalents and restricted cash and cash equivalents as shown in the Unaudited Consolidated Statements of Cash Flows	$	2,644,594	$	2,563,341

REVENUE RECOGNITION (Tables)

3 Months Ended

Mar. 31, 2018

Revenue from Contract with Customer [Abstract]

Cumulative Effect of and Impact of Revenue Accounting Changes

The cumulative effects of the revenue accounting changes on the Company's Unaudited Consolidated Balance Sheet as of January 1, 2018 were as follows (in thousands):



	Balance at December 31, 2017		Adjustments			Balance at January 1, 2018
ASSETS
Current assets:
Accounts receivable, net	$	1,217,801	$	205,324		$	1,423,125

LIABILITIES AND STOCKHOLDERS' EQUITY
Current liabilities:
Accounts payable	$	667,523	$	171,644		$	839,167
Accrued expenses and other current liabilities	1,138,980		44,374			1,183,354
Deferred merchant bookings	980,455		(201,647		)	778,808

Deferred income taxes	481,139		2,414			483,553

Stockholders' equity:
Retained earnings	13,938,869		188,539			14,127,408

The following tables summarize the impacts of adopting the current revenue standard (in thousands, except per share data):

Unaudited Consolidated Balance Sheets as of March 31, 2018:



	As reported (current revenue standard)		Current period adjustments			As adjusted (previous revenue standard)
ASSETS
Current assets:
Accounts receivable, net	$	1,530,321	$	(152,477	)	$	1,377,844
Prepaid expenses and other current assets	1,111,753		19,531			1,131,284

LIABILITIES AND STOCKHOLDERS' EQUITY
Current liabilities:
Accounts payable	$	841,274	$	(104,172	)	$	737,102
Accrued expenses and other current liabilities	1,238,815		(8,897		)	1,229,918
Deferred merchant bookings	1,425,649		117,367			1,543,016

Deferred income taxes	481,220		(984		)	480,236

Stockholders' equity:
Retained earnings	14,975,706		(133,224		)	14,842,482
Accumulated other comprehensive income	56,406		(3,310		)	53,096

Unaudited Consolidated Statements of Operations for the Three Months Ended March 31, 2018:



	As reported (current revenue standard)			Current period adjustments			As adjusted (previous revenue standard)

Agency revenues	$	2,113,339		$	55,987		$	2,169,326
Merchant revenues	526,467			61,540			588,007
Advertising and other revenues	288,395			91			288,486
Cost of revenues				45,660			45,660

Operating expenses:
Performance marketing	1,106,207			2,394			1,108,601
Sales and other expenses	165,800			205			166,005
General and administrative	162,139			801			162,940
Foreign currency transactions and other	(8,056		)	1,855			(6,201		)
Income tax expense	146,127			15,098			161,225
Net income	607,210			55,315		⁽¹⁾	662,525
Net income applicable to common stockholders per basic common share	12.56			1.14			13.70
Net income applicable to common stockholders per diluted common share	12.34			1.12			13.46

Geographic Information

The Company's geographic information is as follows (in thousands):



				International
Total revenues for the Three Months Ended March 31,	United States			The Netherlands		Other		Total
2018	$	378,691		$	2,122,724	$	426,786	$	2,928,201
2017	399,185		⁽¹⁾	1,687,303		332,916		2,419,404		⁽¹⁾

(1) Excludes $80.4 million cost of revenues for NYOP transactions, which is a reduction to merchant revenues in 2018

STOCK-BASED EMPLOYEE COMPENSATION (Tables)

3 Months Ended

Mar. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Activity of unvested restricted stock units and performance share units

Restricted Stock Units and Performance Share Units

The following table summarizes the activity of restricted stock units and performance share units ("share-based awards") during the three months ended March 31, 2018:



Share-Based Awards	Shares		Weighted-Average Grant Date Fair Value
Unvested at December 31, 2017	524,696			$	1,431.88
Granted	146,112			$	2,033.79
Vested	(146,766	)		$	1,271.70
Performance Shares Adjustment	6,487			$	1,654.15
Forfeited/Canceled	(6,718	)		$	1,594.90
Unvested at March 31, 2018	523,811			$	1,645.32

Schedule of share-based compensation, stock options, activity

Stock Options

All outstanding employee stock options were assumed in acquisitions. The following table summarizes the activity for stock options during the three months ended March 31, 2018:



Employee Stock Options	Number of Shares		Weighted-Average Exercise Price			Aggregate Intrinsic Value (in thousands)			Weighted-Average Remaining Contractual Term (in years)
Balance, December 31, 2017	30,675			$	401.61		$	40,986	3.9
Exercised	(2,416	)		$	477.21
Forfeited	(11	)		$	66.98
Balance, March 31, 2018	28,248			$	395.35		$	47,599	3.6
Vested and exercisable as of March 31, 2018	28,160			$	403.59		$	47,509	3.6
Vested and exercisable as of March 31, 2018 and expected to vest thereafter	28,248			$	395.35		$	47,599	3.6

NET INCOME PER SHARE (Tables)

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

Reconciliation of the weighted average number of shares outstanding used in calculating diluted earnings per share

A reconciliation of the weighted-average number of shares outstanding used in calculating diluted earnings per share is as follows (in thousands):



	Three Months Ended March 31,
	2018	2017
Weighted-average number of basic common shares outstanding	48,349	49,192
Weighted-average dilutive stock options, restricted stock units and performance share units	282	227
Assumed conversion of Convertible Senior Notes	574	606
Weighted-average number of diluted common and common equivalent shares outstanding	49,205	50,025
Anti-dilutive potential common shares	1,432	2,256

INVESTMENTS (Tables)

3 Months Ended

Mar. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Investments

The following table summarizes, by major security type, the Company's investments as of March 31, 2018 (in thousands):



	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Short-term investments:
Debt securities:
International government securities	$	461,389	$	56	$	(555	)	$	460,890
U.S. government securities	794,489		—		(4,064		)	790,425
Corporate debt securities	2,952,709		256		(14,565		)	2,938,400
U.S government agency securities	4,435		—		(27		)	4,408
Commercial paper	46,581		—		—			46,581
Total short-term investments	$	4,259,603	$	312	$	(19,211	)	$	4,240,704

Long-term investments:
Debt securities:
International government securities	$	786,958	$	1,760	$	(584	)	$	788,134
U.S. government securities	555,413		—		(11,525		)	543,888
Corporate debt securities	5,744,951		7,272		(71,499		)	5,680,724
U.S. government agency securities	500		—		(6		)	494
Ctrip convertible debt securities	1,275,000		136,013		—			1,411,013
Equity securities:
Ctrip equity securities	655,311		353,199		—			1,008,510
Total long-term investments	$	9,018,133	$	498,244	$	(83,614	)	$	9,432,763

The following table summarizes, by major security type, the Company's investments as of December 31, 2017 (in thousands):



	Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Short-term investments:
Debt securities:
International government securities	$	725,566	$	246	$	(436	)	$	725,376
U.S. government securities	996,112		5		(1,999		)	994,118
Corporate debt securities	3,067,703		449		(4,837		)	3,063,315
U.S. government agency securities	4,444		—		(30		)	4,414
Commercial paper	72,650		—		—			72,650
Total short-term investments	$	4,866,475	$	700	$	(7,302	)	$	4,859,873

Long-term investments:
Debt securities:
International government securities	$	607,000	$	1,588	$	(678	)	$	607,910
U.S. government securities	844,910		2		(10,636		)	834,276
Corporate debt securities	6,689,747		8,399		(41,722		)	6,656,424
U.S. government agency securities	500		—		(6		)	494
Ctrip convertible debt securities	1,275,000		103,100		(9,600		)	1,368,500
Equity securities:
Ctrip equity securities	655,311		299,697		(1,012		)	953,996
Total long-term investments	$	10,072,468	$	412,786	$	(63,654	)	$	10,421,600

FAIR VALUE MEASUREMENTS (Tables)

3 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

Financial instruments carried at fair value

Financial assets and liabilities carried at fair value as of March 31, 2018 are classified in the categories described in the tables below (in thousands):



	Level 1		Level 2		Total
ASSETS:
Cash and restricted cash equivalents:
Money market funds	$	1,930,983	$	—	$	1,930,983
International government securities	—		70,025		70,025
Time deposits	37,141		—		37,141
Short-term investments:
International government securities	—		460,890		460,890
U.S. government securities	—		790,425		790,425
Corporate debt securities	—		2,938,400		2,938,400
U.S government agency securities	—		4,408		4,408
Commercial paper	—		46,581		46,581
Long-term investments:
International government securities	—		788,134		788,134
U.S. government securities	—		543,888		543,888
Corporate debt securities	—		5,680,724		5,680,724
U.S. government agency securities	—		494		494
Ctrip convertible debt securities	—		1,411,013		1,411,013
Ctrip equity securities	1,008,510		—		1,008,510
Derivatives:
Currency exchange derivatives	—		440		440
Total assets at fair value	$	2,976,634	$	12,735,422	$	15,712,056



	Level 1		Level 2		Total
LIABILITIES:
Currency exchange derivatives	$	—	$	1,853	$	1,853

Financial assets and liabilities carried at fair value as of December 31, 2017 are classified in the categories described in the tables below (in thousands):



	Level 1		Level 2		Total
ASSETS:
Cash and restricted cash equivalents:
Money market funds	$	1,895,272	$	—	$	1,895,272
U.S. government securities	—		22,265		22,265
Corporate debt securities	—		6,674		6,674
Commercial paper	—		96,321		96,321
Time deposits	17,896		—		17,896
Short-term investments:
International government securities	—		725,376		725,376
U.S. government securities	—		994,118		994,118
Corporate debt securities	—		3,063,315		3,063,315
U.S. government agency securities	—		4,414		4,414
Commercial paper	—		72,650		72,650
Long-term investments:
International government securities	—		607,910		607,910
U.S. government securities	—		834,276		834,276
Corporate debt securities	—		6,656,424		6,656,424
U.S. government agency securities	—		494		494
Ctrip convertible debt securities	—		1,368,500		1,368,500
Ctrip equity securities	953,996		—		953,996
Derivatives:
Currency exchange derivatives	—		1,767		1,767
Total assets at fair value	$	2,867,164	$	14,454,504	$	17,321,668



	Level 1		Level 2		Total
LIABILITIES:
Currency exchange derivatives	$	—	$	127	$	127

INTANGIBLE ASSETS AND GOODWILL (Tables)

3 Months Ended

Mar. 31, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

Intangible assets

The Company's intangible assets at March 31, 2018 and December 31, 2017 consisted of the following (in thousands):



	March 31, 2018							December 31, 2017
	Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount		Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount		Amortization Period	Weighted- Average Useful Life
Supply and distribution agreements	$	1,068,509	$	(377,529	)	$	690,980	$	1,056,660	$	(355,000	)	$	701,660	3 - 20 years	16 years

Technology	137,800		(110,620		)	27,180		137,288		(104,478		)	32,810		1 - 5 years	5 years

Patents	1,623		(1,623		)	—		1,623		(1,623		)	—		15 years	15 years

Internet domain names	43,272		(30,262		)	13,010		42,265		(28,802		)	13,463		5 - 20 years	8 years

Trade names	1,783,038		(374,316		)	1,408,722		1,779,076		(350,447		)	1,428,629		4-20 years	19 years

Non-compete agreements	21,900		(21,694		)	206		21,900		(21,639		)	261		3-4 years	3 years
Total intangible assets	$	3,056,142	$	(916,044	)	$	2,140,098	$	3,038,812	$	(861,989	)	$	2,176,823

Annual estimated amortization expense for intangible assets for the remainder of 2018, the next five years and thereafter

The amortization expense for intangible assets for the remainder of 2018 and the annual expense for the next five years and thereafter is expected to be as follows (in thousands):



Remainder of 2018	$	127,026
2019	162,014
2020	153,821
2021	147,461
2022	144,560
2023	142,444
Thereafter	1,262,772
	$	2,140,098

Goodwill

goodwill for the three months ended March 31, 2018 consisted of the following (in thousands):



Balance at December 31, 2017	$	2,737,671
Currency translation adjustments	17,076
Balance at March 31, 2018	$	2,754,747

DEBT (Tables)

3 Months Ended

Mar. 31, 2018

Debt Disclosure [Abstract]

Schedule of Debt

Outstanding Debt

Outstanding debt as of March 31, 2018 consisted of the following (in thousands):



March 31, 2018	Outstanding Principal Amount		Unamortized Debt Discount and Debt Issuance Cost			Carrying Value
Short-term debt:
0.35% Convertible Senior Notes due June 2020	$	1,000,000	$	(58,343	)	$	941,657
Long-term debt:
0.9% Convertible Senior Notes due September 2021	$	1,000,000	$	(77,838	)	$	922,162
0.8% (€1 Billion) Senior Notes due March 2022	1,229,850		(5,916		)	1,223,934
2.15% (€750 Million) Senior Notes due November 2022	922,388		(4,493		)	917,895
2.75% Senior Notes due March 2023	500,000		(3,051		)	496,949
2.375% (€1 Billion) Senior Notes due September 2024	1,229,850		(11,992		)	1,217,858
3.65% Senior Notes due March 2025	500,000		(3,180		)	496,820
3.6% Senior Notes due June 2026	1,000,000		(6,644		)	993,356
1.8% (€1 Billion) Senior Notes due March 2027	1,229,850		(5,004		)	1,224,846
3.55% Senior Notes due March 2028	500,000		(3,402		)	496,598
Total long-term debt	$	8,111,938	$	(121,520	)	$	7,990,418

Outstanding debt as of December 31, 2017 consisted of the following (in thousands):



December 31, 2017	Outstanding Principal Amount		Unamortized Debt Discount and Debt Issuance Cost			Carrying Value
Short-term debt:
1.0% Convertible Senior Notes due March 2018	$	714,304	$	(3,394	)	$	710,910
Long-term debt:
0.35% Convertible Senior Notes due June 2020	$	1,000,000	$	(64,825	)	$	935,175
0.9% Convertible Senior Notes due September 2021	1,000,000		(83,272		)	916,728
0.8% (€1 Billion) Senior Notes due March 2022	1,200,800		(6,238		)	1,194,562
2.15% (€750 Million) Senior Notes due November 2022	900,600		(4,683		)	895,917
2.75% Senior Notes due March 2023	500,000		(3,203		)	496,797
2.375% (€1 Billion) Senior Notes due September 2024	1,200,800		(12,240		)	1,188,560
3.65% Senior Notes due March 2025	500,000		(3,290		)	496,710
3.6% Senior Notes due June 2026	1,000,000		(6,840		)	993,160
1.8% (€1 Billion) Senior Notes due March 2027	1,200,800		(5,136		)	1,195,664
3.55% Senior Notes due March 2028	500,000		(3,485		)	496,515
Total long-term debt	$	9,003,000	$	(193,212	)	$	8,809,788

ACCUMULATED OTHER COMPREHENSIVE INCOME (Tables)

3 Months Ended

Mar. 31, 2018

Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]

Balances for each classification of accumulated other comprehensive income (loss)

The table below provides the balances for each classification of accumulated other comprehensive income as of March 31, 2018 and December 31, 2017 (in thousands):



	March 31, 2018		December 31, 2017
Foreign currency translation adjustments, net of tax benefit of $80,468 and $64,445, respectively ⁽¹⁾	$	45,176	$	(15,700	)

Net unrealized gains on marketable securities, net of tax:
Net unrealized gains on marketable equity securities, net of tax charge of $57,597 ⁽²⁾	—		241,088
Net unrealized gains on marketable debt securities, net of tax charge of $31,475 and $32,903, respectively ⁽³⁾	11,230		11,594

Accumulated other comprehensive income	$	56,406	$	236,982

BASIS OF PRESENTATION Basis of Presentation (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Cost of revenues		$ 80,401
Performance marketing	$ 1,106,207	982,172
Brand marketing	101,402	80,818
Sales and other expenses	165,800	109,599
Cash and cash equivalents	2,622,086			$ 2,541,604
Restricted cash included in prepaid expenses and other current assets	22,508			21,737
Total cash, cash equivalents and restricted cash as shown in the Unaudited Consolidated Statements of Cash Flows	2,644,594	2,434,948		2,563,341	$ 2,082,007
Cumulative Effect of New Accounting Principle in Period of Adoption	188,539
Retained earnings	14,975,706		$ 14,127,408	$ 13,938,869
Retained Earnings
Cumulative Effect of New Accounting Principle in Period of Adoption	429,627
Accumulated Other Comprehensive Income
Cumulative Effect of New Accounting Principle in Period of Adoption	(241,088)
Accounting Standards Update 2016-01 \| Retained Earnings
Cumulative Effect of New Accounting Principle in Period of Adoption			241,100
Accounting Standards Update 2016-01 \| Accumulated Other Comprehensive Income
Cumulative Effect of New Accounting Principle in Period of Adoption			(241,088)
Adjustments \| Accounting Standards Update 2014-09
Cost of revenues	45,660
Performance marketing	2,394
Sales and other expenses	205
Retained earnings	$ (133,224)		$ 188,539
Previously Reported
Cost of revenues		85,169
Performance advertising		980,773
Brand advertising		73,012
Sales and marketing		$ 114,036

REVENUE RECOGNITION - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative effect of adoption of accounting standard updates	$ 188,539
Retained earnings	14,975,706		$ 14,127,408	$ 13,938,869
Reduction in customer incentive liability	27,000
Customer incentive liability	78,800			104,700
Revenues	2,928,201	$ 2,419,404
Deferred revenue for merchant revenue transactions	219,900			$ 151,200
Revenue recognized related to the deferred revenue balance	85,600
Cash payments received from travelers reservations	192,200
Online Accommodation Reservation Services
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	2,500,000
Difference between Revenue Guidance in Effect before and after Topic 606 \| Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Retained earnings	$ (133,224)		188,539
Deferred revenue for merchant revenue transactions			$ (32,400)
Revenues \| Product Concentration Risk
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Concentration risk percentage	85.00%

REVENUE RECOGNITION - Cumulative Effect of Revenue Accounting Changes on the Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Current assets:
Accounts receivable, net	$ 1,530,321	$ 1,423,125	$ 1,217,801
Current liabilities:
Accounts payable	841,274	839,167	667,523
Accrued expenses and other current liabilities	1,238,815	1,183,354	1,138,980
Deferred merchant bookings	1,425,649	778,808	980,455
Deferred income taxes	481,220	483,553	481,139
Stockholders' equity:
Retained earnings	14,975,706	14,127,408	13,938,869
Cumulative effect of adoption of accounting standard updates	188,539
Adjustments \| Accounting Standards Update 2014-09
Current assets:
Accounts receivable, net	(152,477)	205,324
Current liabilities:
Accounts payable	(104,172)	171,644
Accrued expenses and other current liabilities	(8,897)	44,374
Deferred merchant bookings	117,367	(201,647)
Deferred income taxes	(984)	2,414
Stockholders' equity:
Retained earnings	(133,224)	$ 188,539
As Adjusted (Previous Revenue Standard) [Member]
Current assets:
Accounts receivable, net	1,377,844		1,217,801
Current liabilities:
Accounts payable	737,102		667,523
Accrued expenses and other current liabilities	1,229,918		1,138,980
Deferred merchant bookings	1,543,016		980,455
Deferred income taxes	480,236		481,139
Stockholders' equity:
Retained earnings	14,842,482		$ 13,938,869
Retained Earnings
Stockholders' equity:
Cumulative effect of adoption of accounting standard updates	$ 429,627

REVENUE RECOGNITION - Impacts of Adopting Current Revenue Standard (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative effect of adoption of accounting standard updates	$ 188,539
Current assets:
Accounts receivable, net	1,530,321		$ 1,423,125	$ 1,217,801
Prepaid expenses and other current assets	1,111,753			415,527
Current liabilities:
Accounts payable	841,274		839,167	667,523
Accrued expenses and other current liabilities	1,238,815		1,183,354	1,138,980
Deferred merchant bookings	1,425,649		778,808	980,455
Deferred income taxes	481,220		483,553	481,139
Stockholders' equity:
Retained earnings	14,975,706		14,127,408	13,938,869
Accumulated other comprehensive income	56,406			236,982
Consolidated Statements of Operations
Agency revenues	2,113,339
Agency revenues		$ 1,785,313
Merchant revenues	526,467
Merchant revenues		442,045
Advertising and other revenues	288,395	192,046
Cost of revenues		80,401
Operating expenses:
Performance marketing	1,106,207	982,172
Sales and other expenses	165,800	109,599
General and administrative	162,139	135,547
Foreign currency transactions and other	(8,056)	(5,127)
Income tax expense	146,127	71,987
Net income	$ 607,210	$ 455,623
Net income applicable to common stockholders per basic common share	$ 12.56	$ 9.26
Net income applicable to common stockholders per diluted common share	$ 12.34	$ 9.11
Adjustments \| Accounting Standards Update 2014-09
Current assets:
Accounts receivable, net	$ (152,477)		205,324
Prepaid expenses and other current assets	19,531
Current liabilities:
Accounts payable	(104,172)		171,644
Accrued expenses and other current liabilities	(8,897)		44,374
Deferred merchant bookings	117,367		(201,647)
Deferred income taxes	(984)		2,414
Stockholders' equity:
Retained earnings	(133,224)		$ 188,539
Accumulated other comprehensive income	(3,310)
Consolidated Statements of Operations
Agency revenues	55,987
Merchant revenues	61,540
Advertising and other revenues	91
Cost of revenues	45,660
Operating expenses:
Performance marketing	2,394
Sales and other expenses	205
General and administrative	801
Foreign currency transactions and other	1,855
Income tax expense	15,098
Net income	$ 55,315
Net income applicable to common stockholders per basic common share	$ 1.14
Net income applicable to common stockholders per diluted common share	$ 1.12
As Adjusted (Previous Revenue Standard) [Member]
Current assets:
Accounts receivable, net	$ 1,377,844			1,217,801
Prepaid expenses and other current assets	1,131,284
Current liabilities:
Accounts payable	737,102			667,523
Accrued expenses and other current liabilities	1,229,918			1,138,980
Deferred merchant bookings	1,543,016			980,455
Deferred income taxes	480,236			481,139
Stockholders' equity:
Retained earnings	14,842,482			$ 13,938,869
Accumulated other comprehensive income	53,096
Consolidated Statements of Operations
Agency revenues	2,169,326
Merchant revenues	588,007
Advertising and other revenues	288,486
Cost of revenues	45,660
Operating expenses:
Performance marketing	1,108,601
Sales and other expenses	166,005
General and administrative	162,940
Foreign currency transactions and other	(6,201)
Income tax expense	161,225
Net income	$ 662,525
Net income applicable to common stockholders per basic common share	$ 13.70
Net income applicable to common stockholders per diluted common share	$ 13.46

REVENUE RECOGNITION - Geographic Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 2,928,201	$ 2,419,404
Cost of revenues		80,401
United States
Disaggregation of Revenue [Line Items]
Revenues	378,691	399,185
The Netherlands
Disaggregation of Revenue [Line Items]
Revenues	2,122,724	1,687,303
Other
Disaggregation of Revenue [Line Items]
Revenues	$ 426,786	$ 332,916

STOCK-BASED EMPLOYEE COMPENSATION (Stock-based Employee Compensation) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-based compensation expense	$ 71,405	$ 58,948

STOCK-BASED EMPLOYEE COMPENSATION (Restricted Stock Units and Performance Share Units) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted Stock Units and Performance Share Units
Share-Based Awards - Shares
Unvested at December 31, 2017	524,696
Granted (in shares)	146,112
Vested (in shares)	(146,766)
Performance Share Units Adjustment (in shares)	6,487
Forfeited (in shares)	(6,718)
Unvested at March 31, 2018	523,811	524,696
Share-Based Awards - Weighted Average Grant Date Fair Value
Unvested at December 31, 2017	$ 1,431.88
Granted (in dollars per share)	2,033.79
Vested (in dollars per share)	1,271.70
Performance Share Units Adjustment (in dollars per share)	1,654.15
Forfeited (in shares)	1,594.90
Unvested at March 31, 2018	$ 1,645.32	$ 1,431.88
Total unrecognized estimated compensation expense, unvested share-based awards	$ 574.0
Total future compensation cost related to unvested share-based awards, expected period of recognition	2 years 4 months
Restricted Stock Units (RSUs)
Share-Based Awards - Shares
Granted (in shares)	97,006
Share-Based Awards - Weighted Average Grant Date Fair Value
Granted (in dollars per share)	$ 2,033.79
Vesting period (in years)	3 years
Grant date fair value	$ 197.3
Performance Share Units 2018 Grants
Share-Based Awards - Shares
Granted (in shares)	49,106
Share-Based Awards - Weighted Average Grant Date Fair Value
Granted (in dollars per share)	$ 2,033.79
Grant date fair value	$ 99.9
Estimated number of probable shares to be issued (in shares)	49,106
Maximum shares that could be issued (in shares)	98,212
Minimum shares that could be issued (in shares)	37,796
Performance Share Units 2017 Grants
Share-Based Awards - Shares
Granted (in shares)		73,893
Unvested at March 31, 2018	67,946
Share-Based Awards - Weighted Average Grant Date Fair Value
Granted (in dollars per share)		$ 1,735.10
Grant date fair value		$ 128.2
Estimated number of probable shares to be issued (in shares)	85,639
Maximum shares that could be issued (in shares)	135,892
Minimum shares that could be issued (in shares)	54,689
Performance Share Units 2016 Grants
Share-Based Awards - Shares
Granted (in shares)			85,735
Unvested at March 31, 2018	70,750
Share-Based Awards - Weighted Average Grant Date Fair Value
Granted (in dollars per share)			$ 1,302.25
Grant date fair value			$ 111.7
Estimated number of probable shares to be issued (in shares)	113,660
Maximum shares that could be issued (in shares)	157,699
Minimum shares that could be issued (in shares)	42,035

STOCK-BASED EMPLOYEE COMPENSATION (Stock Options) (Details) - Employee Stock Option [Member] - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Number of Shares
Beginning balance (in shares)	30,675
Exercised (in shares)	(2,416)
Forfeited (in shares)	(11)
Ending balance (in shares)	28,248.000		30,675
Weighted-Average Exercise Price
Beginning balance (in dollars per share)	$ 401.61
Exercised (in dollars per share)	477.21
Forfeited (in dollars per share)	66.98
Ending balance (in dollars per share)	$ 395.35		$ 401.61
Aggregate Intrinsic Value	$ 47,599		$ 40,986
Weighted Average Remaining Contractual Term	3 years 7 months		3 years 11 months
Vested and exercisable as of March 31, 2018
Number of Shares	28,160
Weighted Average Exercise Price (in dollars per share)	$ 403.59
Aggregate Intrinsic Value	$ 47,509
Weighted Average Remaining Contractual Term, Exercisable	3 years 7 months
Vested and exercisable as of March 31, 2018 and expected to vest thereafter
Number of Shares	28,248
Weighted Average Exercise Price (in dollars per share)	$ 395.35
Aggregate Intrinsic Value	$ 47,599
Weighted Average Remaining Contractual Term	3 years 7 months
Stock options exercised, total intrinsic value	$ 3,900	$ 5,400
Options, vested, number of shares (in shares)	83	539

NET INCOME PER SHARE (Details) - shares shares in Thousands	3 Months Ended
NET INCOME PER SHARE (Details) - shares shares in Thousands	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Weighted-average number of basic common shares outstanding	48,349	49,192
Weighted-average dilutive stock options, restricted stock units and performance share units (in shares)	282	227
Assumed conversion of Convertible Senior Notes (in shares)	574	606
Weighted-average number of diluted common and common equivalent shares outstanding	49,205	50,025
Unvested Stock Awards Outstanding and Convertible Debt Securities [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive potential common shares	1,432	2,256
Convertible Debt [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive potential common shares	1,000

INVESTMENTS (Details) - USD ($) $ in Thousands					3 Months Ended
INVESTMENTS (Details) - USD ($) $ in Thousands	Sep. 12, 2016	Dec. 11, 2015	May 26, 2015	Aug. 07, 2014	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Marketable securities:
Short-term investments in marketable securities					$ 4,240,704			$ 4,859,873
Long-term investments in marketable securities					9,432,763			10,421,600
Investments [Abstract]
Cumulative effect of adoption of accounting standard updates					188,539
Net unrealized gains on marketable equity securities					$ 54,514	$ 0
Weighted average life of all fixed income investments, excluding the Company's investment in Ctrip convertible debt securities					1 year 7 months
Equity Investments without Readily Determinable Fair Value					$ 450,900
Cost Method Investments								450,900
Ctrip.com International, Ltd.
Investments [Abstract]
Debt Investment, Term	6 years	10 years	5 years	5 years
Maximum Ownership Percentage in Ctrip			15.00%
Short-term Investments
Marketable securities:
Cost					4,259,603
Gross Unrealized Gains					312
Gross Unrealized Losses					(19,211)
Short-term investments in marketable securities					4,240,704
Available-for-sale securities:
Cost								4,866,475
Gross Unrealized Gains								700
Gross Unrealized Losses								(7,302)
Fair Value								4,859,873
Short-term Investments \| International government securities
Debt securities:
Cost					461,389
Gross Unrealized Gains					56
Gross Unrealized Losses					(555)
Fair Value					460,890
Available-for-sale securities:
Cost								725,566
Gross Unrealized Gains								246
Gross Unrealized Losses								(436)
Fair Value								725,376
Short-term Investments \| U.S. government securities
Debt securities:
Cost					794,489
Gross Unrealized Gains					0
Gross Unrealized Losses					(4,064)
Fair Value					790,425
Available-for-sale securities:
Cost								996,112
Gross Unrealized Gains								5
Gross Unrealized Losses								(1,999)
Fair Value								994,118
Short-term Investments \| Corporate debt securities
Debt securities:
Cost					2,952,709
Gross Unrealized Gains					256
Gross Unrealized Losses					(14,565)
Fair Value					2,938,400
Available-for-sale securities:
Cost								3,067,703
Gross Unrealized Gains								449
Gross Unrealized Losses								(4,837)
Fair Value								3,063,315
Short-term Investments \| U.S government agency securities
Debt securities:
Cost					4,435
Gross Unrealized Gains					0
Gross Unrealized Losses					(27)
Fair Value					4,408
Available-for-sale securities:
Cost								4,444
Gross Unrealized Gains								0
Gross Unrealized Losses								(30)
Fair Value								4,414
Short-term Investments \| Commercial paper
Debt securities:
Cost					46,581
Gross Unrealized Gains					0
Gross Unrealized Losses					0
Fair Value					46,581
Available-for-sale securities:
Cost								72,650
Gross Unrealized Gains								0
Gross Unrealized Losses								0
Fair Value								72,650
Long-term Investments
Marketable securities:
Cost					9,018,133
Gross Unrealized Gains					498,244
Gross Unrealized Losses					(83,614)
Long-term investments in marketable securities					9,432,763
Available-for-sale securities:
Cost								10,072,468
Gross Unrealized Gains								412,786
Gross Unrealized Losses								(63,654)
Fair Value								10,421,600
Long-term Investments \| International government securities
Debt securities:
Cost					786,958
Gross Unrealized Gains					1,760
Gross Unrealized Losses					(584)
Fair Value					788,134
Available-for-sale securities:
Cost								607,000
Gross Unrealized Gains								1,588
Gross Unrealized Losses								(678)
Fair Value								607,910
Long-term Investments \| U.S. government securities
Debt securities:
Cost					555,413
Gross Unrealized Gains					0
Gross Unrealized Losses					(11,525)
Fair Value					543,888
Available-for-sale securities:
Cost								844,910
Gross Unrealized Gains								2
Gross Unrealized Losses								(10,636)
Fair Value								834,276
Long-term Investments \| Corporate debt securities
Debt securities:
Cost					5,744,951
Gross Unrealized Gains					7,272
Gross Unrealized Losses					(71,499)
Fair Value					5,680,724
Available-for-sale securities:
Cost								6,689,747
Gross Unrealized Gains								8,399
Gross Unrealized Losses								(41,722)
Fair Value								6,656,424
Long-term Investments \| U.S government agency securities
Debt securities:
Cost					500
Gross Unrealized Gains					0
Gross Unrealized Losses					(6)
Fair Value					494
Available-for-sale securities:
Cost								500
Gross Unrealized Gains								0
Gross Unrealized Losses								(6)
Fair Value								494
Long-term Investments \| Ctrip convertible debt securities \| Ctrip.com International, Ltd.
Debt securities:
Cost	$ 25,000	$ 500,000	$ 250,000	$ 500,000	1,275,000
Gross Unrealized Gains					136,013
Gross Unrealized Losses					0
Fair Value					1,411,013
Available-for-sale securities:
Cost								1,275,000
Gross Unrealized Gains								103,100
Gross Unrealized Losses								(9,600)
Fair Value								1,368,500
Long-term Investments \| Ctrip equity securities \| Ctrip.com International, Ltd.
Equity securities:
Cost					655,311
Gross Unrealized Gains					353,199
Gross Unrealized Losses					0
Fair Value					1,008,510
Available-for-sale securities:
Cost								655,311
Gross Unrealized Gains								299,697
Gross Unrealized Losses								(1,012)
Fair Value								$ 953,996
Retained Earnings
Investments [Abstract]
Cumulative effect of adoption of accounting standard updates					$ 429,627
Accounting Standards Update 2016-01 \| Retained Earnings
Investments [Abstract]
Cumulative Effect on Retained Earnings, before Tax							$ 298,700
Cumulative effect of adoption of accounting standard updates							$ 241,100

FAIR VALUE MEASUREMENTS Fair value measurements (Details) - Recurring Basis - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
ASSETS:
Total assets at fair value	$ 15,712,056	$ 17,321,668
Foreign Currency Contracts
ASSETS:
Total assets at fair value	440	1,767
LIABILITIES:
Total liabilities at fair value	1,853	127
Money market funds \| Cash Equivalents
ASSETS:
Total assets at fair value	1,930,983	1,895,272
International government securities \| Cash Equivalents
ASSETS:
Total assets at fair value	70,025
International government securities \| Short-term Investments
ASSETS:
Total assets at fair value	460,890	725,376
International government securities \| Long-term Investments
ASSETS:
Total assets at fair value	788,134	607,910
U.S. government securities \| Cash Equivalents
ASSETS:
Total assets at fair value		22,265
U.S. government securities \| Short-term Investments
ASSETS:
Total assets at fair value	790,425	994,118
U.S. government securities \| Long-term Investments
ASSETS:
Total assets at fair value	543,888	834,276
Commercial paper \| Cash Equivalents
ASSETS:
Total assets at fair value		96,321
Commercial paper \| Short-term Investments
ASSETS:
Total assets at fair value	46,581	72,650
Time deposits \| Cash Equivalents
ASSETS:
Total assets at fair value	37,141	17,896
Corporate debt securities \| Cash Equivalents
ASSETS:
Total assets at fair value		6,674
Corporate debt securities \| Short-term Investments
ASSETS:
Total assets at fair value	2,938,400	3,063,315
Corporate debt securities \| Long-term Investments
ASSETS:
Total assets at fair value	5,680,724	6,656,424
U.S government agency securities \| Short-term Investments
ASSETS:
Total assets at fair value	4,408	4,414
U.S government agency securities \| Long-term Investments
ASSETS:
Total assets at fair value	494	494
Level 1
ASSETS:
Total assets at fair value	2,976,634	2,867,164
Level 1 \| Foreign Currency Contracts
ASSETS:
Total assets at fair value	0	0
LIABILITIES:
Total liabilities at fair value	0	0
Level 1 \| Money market funds \| Cash Equivalents
ASSETS:
Total assets at fair value	1,930,983	1,895,272
Level 1 \| International government securities \| Cash Equivalents
ASSETS:
Total assets at fair value	0
Level 1 \| International government securities \| Short-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 \| International government securities \| Long-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 \| U.S. government securities \| Cash Equivalents
ASSETS:
Total assets at fair value		0
Level 1 \| U.S. government securities \| Short-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 \| U.S. government securities \| Long-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 \| Commercial paper \| Cash Equivalents
ASSETS:
Total assets at fair value		0
Level 1 \| Commercial paper \| Short-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 \| Time deposits \| Cash Equivalents
ASSETS:
Total assets at fair value	37,141	17,896
Level 1 \| Corporate debt securities \| Cash Equivalents
ASSETS:
Total assets at fair value		0
Level 1 \| Corporate debt securities \| Short-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 \| Corporate debt securities \| Long-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 \| U.S government agency securities \| Short-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 \| U.S government agency securities \| Long-term Investments
ASSETS:
Total assets at fair value	0	0
Level 2
ASSETS:
Total assets at fair value	12,735,422	14,454,504
Level 2 \| Foreign Currency Contracts
ASSETS:
Total assets at fair value	440	1,767
LIABILITIES:
Total liabilities at fair value	1,853	127
Level 2 \| Money market funds \| Cash Equivalents
ASSETS:
Total assets at fair value	0	0
Level 2 \| International government securities \| Cash Equivalents
ASSETS:
Total assets at fair value	70,025
Level 2 \| International government securities \| Short-term Investments
ASSETS:
Total assets at fair value	460,890	725,376
Level 2 \| International government securities \| Long-term Investments
ASSETS:
Total assets at fair value	788,134	607,910
Level 2 \| U.S. government securities \| Cash Equivalents
ASSETS:
Total assets at fair value		22,265
Level 2 \| U.S. government securities \| Short-term Investments
ASSETS:
Total assets at fair value	790,425	994,118
Level 2 \| U.S. government securities \| Long-term Investments
ASSETS:
Total assets at fair value	543,888	834,276
Level 2 \| Commercial paper \| Cash Equivalents
ASSETS:
Total assets at fair value		96,321
Level 2 \| Commercial paper \| Short-term Investments
ASSETS:
Total assets at fair value	46,581	72,650
Level 2 \| Time deposits \| Cash Equivalents
ASSETS:
Total assets at fair value	0	0
Level 2 \| Corporate debt securities \| Cash Equivalents
ASSETS:
Total assets at fair value		6,674
Level 2 \| Corporate debt securities \| Short-term Investments
ASSETS:
Total assets at fair value	2,938,400	3,063,315
Level 2 \| Corporate debt securities \| Long-term Investments
ASSETS:
Total assets at fair value	5,680,724	6,656,424
Level 2 \| U.S government agency securities \| Short-term Investments
ASSETS:
Total assets at fair value	4,408	4,414
Level 2 \| U.S government agency securities \| Long-term Investments
ASSETS:
Total assets at fair value	494	494
Ctrip.com International, Ltd. \| Ctrip convertible debt securities \| Long-term Investments
ASSETS:
Total assets at fair value	1,411,013	1,368,500
Ctrip.com International, Ltd. \| Ctrip equity securities \| Long-term Investments
ASSETS:
Total assets at fair value	1,008,510	953,996
Ctrip.com International, Ltd. \| Level 1 \| Ctrip convertible debt securities \| Long-term Investments
ASSETS:
Total assets at fair value	0	0
Ctrip.com International, Ltd. \| Level 1 \| Ctrip equity securities \| Long-term Investments
ASSETS:
Total assets at fair value	1,008,510	953,996
Ctrip.com International, Ltd. \| Level 2 \| Ctrip convertible debt securities \| Long-term Investments
ASSETS:
Total assets at fair value	1,411,013	1,368,500
Ctrip.com International, Ltd. \| Level 2 \| Ctrip equity securities \| Long-term Investments
ASSETS:
Total assets at fair value	$ 0	$ 0

FAIR VALUE MEASUREMENTS Equity investments without readily determinable fair values and derivatives (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Sep. 12, 2016	Dec. 11, 2015	May 26, 2015	Aug. 07, 2014
Fair Value Disclosures [Abstract]
Cost Method Investments			$ 450,900
Equity Investments without Readily Determinable Fair Value	$ 450,900
Derivatives Not Designated as Hedging Instruments
Foreign currency gains (losses), net of derivative activity	(5,400)	$ (5,900)
Foreign Currency Contracts, Translation Risk
Derivatives Not Designated as Hedging Instruments
Foreign currency derivative instruments not designated as hedging instruments at fair value, net	0		0
Foreign Currency Contracts, Transaction Risk
Derivatives Not Designated as Hedging Instruments
Foreign currency gains (losses) recorded in Foreign currency transactions and other	20,600	6,800
Foreign Currency Contracts
Derivatives Not Designated as Hedging Instruments
Net cash inflow from settlement of derivative contracts included in operating activities	18,200	$ 2,700
Ctrip.com International, Ltd. \| Ctrip convertible debt securities \| Long-term Investments
Derivatives Not Designated as Hedging Instruments
Cost of Ctrip convertible notes	1,275,000			$ 25,000	$ 500,000	$ 250,000	$ 500,000
Embedded derivative, fair value of embedded derivative	$ 2,100		$ 1,800

INTANGIBLE ASSETS AND GOODWILL (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Finite-lived intangible assets
Gross Carrying Amount	$ 3,056,142		$ 3,038,812
Accumulated Amortization	(916,044)		(861,989)
Net Carrying Amount	2,140,098		2,176,823
Amortization	46,550	$ 43,018
Annual estimated amortization expense for intangible assets
Remainder of 2018	127,026
2019	162,014
2020	153,821
2021	147,461
2022	144,560
2023	142,444
Thereafter	1,262,772
Total	2,140,098
Goodwill:
Balance at December 31, 2017	2,737,671
Currency translation adjustments	17,076
Balance at March 31, 2018	2,754,747
Supply and distribution agreements
Finite-lived intangible assets
Gross Carrying Amount	1,068,509		1,056,660
Accumulated Amortization	(377,529)		(355,000)
Net Carrying Amount	$ 690,980		701,660
Finite lived intangibles, weighted average useful life	16 years
Technology
Finite-lived intangible assets
Gross Carrying Amount	$ 137,800		137,288
Accumulated Amortization	(110,620)		(104,478)
Net Carrying Amount	$ 27,180		32,810
Finite lived intangibles, weighted average useful life	5 years
Patents
Finite-lived intangible assets
Finite lived intangibles, useful life	15 years
Gross Carrying Amount	$ 1,623		1,623
Accumulated Amortization	(1,623)		(1,623)
Net Carrying Amount	$ 0		0
Finite lived intangibles, weighted average useful life	15 years
Internet domain names
Finite-lived intangible assets
Gross Carrying Amount	$ 43,272		42,265
Accumulated Amortization	(30,262)		(28,802)
Net Carrying Amount	$ 13,010		13,463
Finite lived intangibles, weighted average useful life	8 years
Trade Names
Finite-lived intangible assets
Gross Carrying Amount	$ 1,783,038		1,779,076
Accumulated Amortization	(374,316)		(350,447)
Net Carrying Amount	$ 1,408,722		1,428,629
Finite lived intangibles, weighted average useful life	19 years
Noncompete Agreements
Finite-lived intangible assets
Gross Carrying Amount	$ 21,900		21,900
Accumulated Amortization	(21,694)		(21,639)
Net Carrying Amount	$ 206		$ 261
Finite lived intangibles, weighted average useful life	3 years
Minimum \| Supply and distribution agreements
Finite-lived intangible assets
Finite lived intangibles, useful life	3 years
Minimum \| Technology
Finite-lived intangible assets
Finite lived intangibles, useful life	1 year
Minimum \| Internet domain names
Finite-lived intangible assets
Finite lived intangibles, useful life	5 years
Minimum \| Trade Names
Finite-lived intangible assets
Finite lived intangibles, useful life	4 years
Minimum \| Noncompete Agreements
Finite-lived intangible assets
Finite lived intangibles, useful life	3 years
Maximum \| Supply and distribution agreements
Finite-lived intangible assets
Finite lived intangibles, useful life	20 years
Maximum \| Technology
Finite-lived intangible assets
Finite lived intangibles, useful life	5 years
Maximum \| Internet domain names
Finite-lived intangible assets
Finite lived intangibles, useful life	20 years
Maximum \| Trade Names
Finite-lived intangible assets
Finite lived intangibles, useful life	20 years
Maximum \| Noncompete Agreements
Finite-lived intangible assets
Finite lived intangibles, useful life	4 years

DEBT (Revolving Credit Facility) (Details) - USD ($)	Jun. 19, 2015	Mar. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Letters of credit issued		$ 3,900,000	$ 3,800,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity	$ 2,000,000,000
Line of credit facility, term	5 years
Line of credit, Current		$ 0	$ 0
Letter of Credit
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity	$ 70,000,000
Swingline Loans
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity	$ 50,000,000.0
Minimum \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Commitment fee rate	0.085%
Minimum \| Rate 2C \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.00%
Maximum \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Commitment fee rate	0.20%
Maximum \| Rate 2C \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
London Interbank Offered Rate (LIBOR) \| Minimum \| Rate 1 \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.875%
London Interbank Offered Rate (LIBOR) \| Maximum \| Rate 1 \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.50%
Federal Funds Purchased \| Rate 2B \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
One Month LIBOR \| Rate 2C \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.00%

DEBT (Outstanding Debt) (Details) $ / shares in Units, $ in Thousands, € in Millions			1 Months Ended	3 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($)	Aug. 20, 2014 USD ($) Days $ / shares	May 31, 2013 USD ($) Days $ / shares	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Mar. 31, 2018 EUR (€)	Dec. 31, 2017 EUR (€)	Aug. 15, 2017 USD ($)	Mar. 10, 2017 EUR (€)	May 23, 2016 USD ($)	Nov. 25, 2015 EUR (€)	Mar. 13, 2015 USD ($)	Mar. 03, 2015 EUR (€)	Sep. 23, 2014 EUR (€)	Jun. 30, 2013 USD ($)	Mar. 31, 2012 USD ($) $ / shares
Debt Instrument
Outstanding Principal Amount, Long-term debt				$ 8,111,938		$ 9,003,000
Unamortized Debt Discount and Debt Issuance Cost				(121,520)		(193,212)
Carrying Value, Long-term debt				7,990,418		8,809,788
Description of Senior Notes
Convertible debt				51,398		2,963
Payments Related to Conversion of Senior Notes				1,487,109	$ 4
Amortization of debt discount included in interest expense				15,127	17,625
0.35% Convertible Senior Notes due June 2020
Debt Instrument
Outstanding Principal Amount, Short-term debt				1,000,000
Outstanding Principal Amount, Long-term debt						1,000,000
Unamortized Debt Discount and Debt Issuance Cost				(58,343)		(64,825)
Carrying value, Short-term debt				941,657
Carrying Value, Long-term debt						$ 935,175
Description of Senior Notes
Convertible debt				$ 51,400
Aggregate principal amount			$ 1,000,000
Interest rate on Long-term Debt			0.35%	0.35%		0.35%					0.35%	0.35%
Debt financing costs paid										$ 1,000
Convertible debt conversion price (in dollars per share) \| $ / shares			$ 1,315.10
Ratio of closing share price to conversion price as a condition for conversion of the convertible notes, minimum			150.00%
Unamortized debt discount			$ 20,000
Effective interest rate at debt origination or modification			3.13%
Debt discount related to convertible notes, net of tax																				$ 92,400
Debt discount related to convertible notes, before tax																				154,300
Convertible carrying amount of equity component related to finance costs net of tax																				$ 100
Amortization of debt discount included in interest expense				$ 700	700
0.9% Convertible Senior Notes due September 2021
Debt Instrument
Outstanding Principal Amount, Long-term debt				1,000,000		$ 1,000,000
Unamortized Debt Discount and Debt Issuance Cost				(77,838)		(83,272)
Carrying Value, Long-term debt				$ 922,162		$ 916,728
Description of Senior Notes
Aggregate principal amount		$ 1,000,000
Interest rate on Long-term Debt		0.90%		0.90%		0.90%					0.90%	0.90%
Debt financing costs paid									$ 11,000
Convertible debt conversion price (in dollars per share) \| $ / shares		$ 2,055.50
Ratio of closing share price to conversion price as a condition for conversion of the convertible notes, minimum		150.00%
Effective interest rate at debt origination or modification		3.18%
Debt discount related to convertible notes, net of tax									82,500
Debt discount related to convertible notes, before tax									142,900
Convertible carrying amount of equity component related to finance costs net of tax									1,600
0.8% (€1 Billion) Senior Notes due March 2022
Debt Instrument
Outstanding Principal Amount, Long-term debt				$ 1,229,850		$ 1,200,800
Unamortized Debt Discount and Debt Issuance Cost				(5,916)		(6,238)
Carrying Value, Long-term debt				$ 1,223,934		$ 1,194,562
Description of Senior Notes
Aggregate principal amount \| €											€ 1,000.0	€ 1,000.0		€ 1,000.0
Interest rate on Long-term Debt				0.80%		0.80%					0.80%	0.80%		0.80%
Debt financing costs paid						$ 5,000
Unamortized debt discount \| €														€ 2.1
Effective interest rate at debt origination or modification														0.84%
2.15% (€750 Million) Senior Notes due November 2022
Debt Instrument
Outstanding Principal Amount, Long-term debt				$ 922,388		900,600
Unamortized Debt Discount and Debt Issuance Cost				(4,493)		(4,683)
Carrying Value, Long-term debt				$ 917,895		$ 895,917
Description of Senior Notes
Aggregate principal amount \| €											€ 750.0	€ 750.0				€ 750.0
Interest rate on Long-term Debt				2.15%		2.15%					2.15%	2.15%				2.15%
Debt financing costs paid								$ 3,700
Unamortized debt discount \| €																€ 2.2
Effective interest rate at debt origination or modification																2.20%
2.75% Senior Notes due March 2023
Debt Instrument
Outstanding Principal Amount, Long-term debt				$ 500,000		$ 500,000
Unamortized Debt Discount and Debt Issuance Cost				(3,051)		(3,203)
Carrying Value, Long-term debt				$ 496,949		$ 496,797
Description of Senior Notes
Aggregate principal amount													$ 500,000
Interest rate on Long-term Debt				2.75%		2.75%					2.75%	2.75%	2.75%
Debt financing costs paid						$ 2,700
Unamortized debt discount													$ 700
Effective interest rate at debt origination or modification													2.78%
2.375% (€1 Billion) Senior Notes due September 2024
Debt Instrument
Outstanding Principal Amount, Long-term debt				$ 1,229,850		1,200,800
Unamortized Debt Discount and Debt Issuance Cost				(11,992)		(12,240)
Carrying Value, Long-term debt				$ 1,217,858		$ 1,188,560
Description of Senior Notes
Aggregate principal amount \| €											€ 1,000.0	€ 1,000.0							€ 1,000.0
Interest rate on Long-term Debt				2.375%		2.375%					2.375%	2.375%							2.375%
Debt financing costs paid									$ 6,500
Unamortized debt discount \| €																			€ 9.4
Effective interest rate at debt origination or modification																			2.48%
3.65% Senior Notes due March 2025
Debt Instrument
Outstanding Principal Amount, Long-term debt				$ 500,000		$ 500,000
Unamortized Debt Discount and Debt Issuance Cost				(3,180)		(3,290)
Carrying Value, Long-term debt				$ 496,820		$ 496,710
Description of Senior Notes
Aggregate principal amount																	$ 500,000
Interest rate on Long-term Debt				3.65%		3.65%					3.65%	3.65%					3.65%
Debt financing costs paid								3,200
Unamortized debt discount																	$ 1,300
Effective interest rate at debt origination or modification																	3.68%
3.6% Senior Notes due June 2026
Debt Instrument
Outstanding Principal Amount, Long-term debt				$ 1,000,000		$ 1,000,000
Unamortized Debt Discount and Debt Issuance Cost				(6,644)		(6,840)
Carrying Value, Long-term debt				$ 993,356		$ 993,160
Description of Senior Notes
Aggregate principal amount															$ 1,000,000
Interest rate on Long-term Debt				3.60%		3.60%					3.60%	3.60%			3.60%
Debt financing costs paid							$ 6,200
Unamortized debt discount															$ 1,900
Effective interest rate at debt origination or modification															3.62%
1.8% (€1 Billion) Senior Notes due March 2027
Debt Instrument
Outstanding Principal Amount, Long-term debt				$ 1,229,850		$ 1,200,800
Unamortized Debt Discount and Debt Issuance Cost				(5,004)		(5,136)
Carrying Value, Long-term debt				$ 1,224,846		$ 1,195,664
Description of Senior Notes
Aggregate principal amount \| €											€ 1,000.0	€ 1,000.0						€ 1,000.0
Interest rate on Long-term Debt				1.80%		1.80%					1.80%	1.80%						1.80%
Debt financing costs paid								$ 6,300
Unamortized debt discount \| €																		€ 0.3
Effective interest rate at debt origination or modification																		1.80%
3.55% Senior Notes due March 2028
Debt Instrument
Outstanding Principal Amount, Long-term debt				$ 500,000		$ 500,000
Unamortized Debt Discount and Debt Issuance Cost				(3,402)		(3,485)
Carrying Value, Long-term debt				$ 496,598		$ 496,515
Description of Senior Notes
Aggregate principal amount													$ 500,000
Interest rate on Long-term Debt				3.55%		3.55%					3.55%	3.55%	3.55%
Debt financing costs paid						$ 3,200
Unamortized debt discount													$ 400
Effective interest rate at debt origination or modification													3.56%
Convertible Debt 1.00 Percent Due March 2018 [Member]
Debt Instrument
Outstanding Principal Amount, Short-term debt						714,304
Unamortized Debt Discount and Debt Issuance Cost						(3,394)
Carrying value, Short-term debt						710,910
Description of Senior Notes
Convertible debt						$ 3,000
Aggregate principal amount																					$ 1,000,000
Interest rate on Long-term Debt						1.00%						1.00%									1.00%
Convertible debt conversion price (in dollars per share) \| $ / shares																					$ 944.61
Payments Related to Conversion of Senior Notes				$ 714,300
Debt Conversion, Converted Instrument, Cash				773,200
Effective interest rate at debt origination or modification																					3.50%
Debt discount related to convertible notes, net of tax																					$ 80,900
Debt discount related to convertible notes, before tax																					135,200
Convertible carrying amount of equity component related to finance costs net of tax																					$ 2,800
Convertible Debt
Description of Senior Notes
Interest expense related to debt				19,800	24,000
Coupon Interest expense				4,500	5,600
Amortization of debt discount included in interest expense				14,500	17,200
Amortization of debt issuance costs				$ 800	$ 1,200
Debt, weighted average interest rate				3.30%	3.40%
Other Long-term Debt
Description of Senior Notes
Interest expense related to debt				$ 43,300	$ 30,600
Coupon Interest expense				41,600	29,500
1% CT Loan Due March 2016
Description of Senior Notes
Aggregate principal amount	$ 2,500			$ 1,500		$ 1,500
Interest rate on Long-term Debt	1.00%			1.00%		1.00%					1.00%	1.00%
Debt instrument, term	10 years
Debt instrument, decrease, forgiveness					$ 1,000
Minimum \| 0.35% Convertible Senior Notes due June 2020
Description of Senior Notes
Minimum and Maximum consecutive days the closing sales price of common stock must exceed a specified percentage of conversion price to trigger conversion feature of note (in days) \| Days			20
Additional payment to debt holder, settled In shares, aggregate value			$ 0
Minimum \| 0.9% Convertible Senior Notes due September 2021
Description of Senior Notes
Minimum and Maximum consecutive days the closing sales price of common stock must exceed a specified percentage of conversion price to trigger conversion feature of note (in days) \| Days		20
Additional payment to debt holder, settled In shares, aggregate value		$ 0
Maximum \| 0.35% Convertible Senior Notes due June 2020
Description of Senior Notes
Minimum and Maximum consecutive days the closing sales price of common stock must exceed a specified percentage of conversion price to trigger conversion feature of note (in days) \| Days			30
Additional payment to debt holder, settled In shares, aggregate value			$ 397,000
Maximum \| 0.9% Convertible Senior Notes due September 2021
Description of Senior Notes
Minimum and Maximum consecutive days the closing sales price of common stock must exceed a specified percentage of conversion price to trigger conversion feature of note (in days) \| Days		30
Additional payment to debt holder, settled In shares, aggregate value		$ 375,000
Level 2
Description of Senior Notes
Estimated market value of outstanding debt				$ 10,100,000		$ 11,100,000

TREASURY STOCK (Details) - USD ($) $ in Thousands	3 Months Ended
TREASURY STOCK (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Mar. 31, 2016
Equity, Class of Treasury Stock [Line Items]
Remaining authorization to repurchase common stock	$ 9,800,000
Treasury stock shares acquired	373,119		124,915
Treasury stock value acquired	$ 731,592		$ 212,300
Repurchase of common shares to satisfy employee withholding tax obligations related to stock-based compensation (in shares)	59,043		44,888
Repurchase of common shares to satisfy employee withholding tax obligations related to stock-based compensation	$ 119,900		$ 77,300
Payments Related to Tax Withholding for Share-based Compensation	$ 103,200		$ 69,800
Treasury stock, shares (in shares)	14,589,938	14,216,819
Repurchase Program (Q12016)
Equity, Class of Treasury Stock [Line Items]
Amount of common stock repurchases authorized				$ 3,000,000
Treasury stock shares acquired			80,027
Treasury stock value acquired			$ 135,000
Treasury Stock Repurchased but unsettled by period end		18,217
Treasury Stock Repurchased but unsettled by period end, amount		$ 32,000
Repurchase Program (Q12017)
Equity, Class of Treasury Stock [Line Items]
Amount of common stock repurchases authorized			$ 2,000,000
Treasury Stock Repurchased but unsettled by period end	13,578
Treasury Stock Repurchased but unsettled by period end, amount	$ 28,000
Repurchase Program (Q12018)
Equity, Class of Treasury Stock [Line Items]
Amount of common stock repurchases authorized	$ 8,000,000
Repurchase Program (Q12016) and (Q12017)
Equity, Class of Treasury Stock [Line Items]
Treasury stock shares acquired	314,076
Treasury stock value acquired	$ 611,700

INCOME TAXES (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
INCOME TAXES (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Tax Contingency [Line Items]
Effective income tax rate reconciliation, percent	19.40%	13.60%
Income tax (benefit) arising from excess tax benefits	$ (13.5)	$ (9.9)
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Transition Tax for Accumulated Foreign Earnings, Provisional Income Tax Expense (Benefit)			$ 1,600.0
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Provisional Undistributed Accumulated Earnings of Foreign Subsidiary			16,500.0
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax, Provisional Income Tax Expense (Benefit)			217.0
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Transition Tax for Accumulated Foreign Earnings, Provisional Usage of Net Operating Losses	204.0
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Transition Tax for Accumulated Foreign Earnings, Provisional Usage of Tax Credit Carryforwards	46.0
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Transition Tax For Accumulated Foreign Earnings, Provisional Liability	$ 1,300.0		$ 1,300.0
Internal Revenue Service (IRS)
Income Tax Contingency [Line Items]
Statutory federal rate	21.00%	35.00%
Tax and Customs Administration, Netherlands
Income Tax Contingency [Line Items]
Statutory federal rate	25.00%		25.00%
Effective income tax rate at innovation box tax rate	7.00%	5.00%

ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($)
$ in Thousands

3 Months Ended

12 Months Ended

Mar. 31, 2018

Mar. 31, 2017

Dec. 31, 2017

Accumulated Other Comprehensive Income (Loss) [Line Items]

Stockholders' equity

$ 10,636,343

$ 11,260,598

Tax (benefit) associated with gain (loss) on components of foreign currency translation

(16,023)

$ (19,733)

Foreign currency translation adjustments

Accumulated Other Comprehensive Income (Loss) [Line Items]

Stockholders' equity

^[1]

45,176

(15,700)

Accumulated other comprehensive income (loss)

Accumulated Other Comprehensive Income (Loss) [Line Items]

Stockholders' equity

56,406

236,982

Net Investment Hedging [Member] | Foreign currency translation adjustments

Accumulated Other Comprehensive Income (Loss) [Line Items]

AOCI Tax, Attributable to Parent

(80,468)

(64,445)

Net Investment Hedging [Member] | Foreign currency translation adjustments | Euro Senior Notes

Accumulated Other Comprehensive Income (Loss) [Line Items]

Stockholders' equity

273,400

190,400

Stockholders' equity, before tax

346,700

237,200

Net Investment Hedging [Member] | Foreign currency translation adjustments | Foreign Currency Forward

Accumulated Other Comprehensive Income (Loss) [Line Items]

Stockholders' equity

(35,000)

Stockholders' equity, before tax

(52,600)

Ctrip equity securities | Net unrealized gains/(losses) on marketable securities

Accumulated Other Comprehensive Income (Loss) [Line Items]

Stockholders' equity

^[2]

241,088

Stockholders' equity, before tax

298,700

AOCI Tax, Attributable to Parent

57,597

Debt Securities [Member] | Net unrealized gains/(losses) on marketable securities

Accumulated Other Comprehensive Income (Loss) [Line Items]

Stockholders' equity

^[3]

11,230

11,594

Stockholders' equity, before tax

42,700

44,500

AOCI Tax, Attributable to Parent

31,475

32,903

Currency translation adjustment on deemed repatriation tax liability [Member]

Accumulated Other Comprehensive Income (Loss) [Line Items]

Tax (benefit) associated with gain (loss) on components of foreign currency translation

$ 10,500

$ 0

Internal Revenue Service (IRS)

Accumulated Other Comprehensive Income (Loss) [Line Items]

Statutory federal rate

21.00%

35.00%

Internal Revenue Service (IRS) | Ctrip equity securities | Net unrealized gains/(losses) on marketable securities

Accumulated Other Comprehensive Income (Loss) [Line Items]

AOCI Tax, Attributable to Parent

62,900

Internal Revenue Service (IRS) | Debt Securities [Member] | Net unrealized gains/(losses) on marketable securities

Accumulated Other Comprehensive Income (Loss) [Line Items]

AOCI Tax, Attributable to Parent

$ 7,600

$ 500

Tax and Customs Administration, Netherlands

Accumulated Other Comprehensive Income (Loss) [Line Items]

Statutory federal rate

25.00%

Tax and Customs Administration, Netherlands | Ctrip equity securities | Net unrealized gains/(losses) on marketable securities

Accumulated Other Comprehensive Income (Loss) [Line Items]

Stockholders' equity, before tax, tax-exempt

$ 319,900

Stockholders' equity, before tax, taxable

(21,200)

AOCI Tax, Attributable to Parent

(5,300)

Tax and Customs Administration, Netherlands | Debt Securities [Member] | Net unrealized gains/(losses) on marketable securities

Accumulated Other Comprehensive Income (Loss) [Line Items]

Stockholders' equity, before tax, tax-exempt

$ (52,900)

(85,300)

Stockholders' equity, before tax, taxable

95,600

129,800

AOCI Tax, Attributable to Parent

$ 23,900

$ 32,400

[1]	(1) Foreign currency translation adjustments, net of tax, at March 31, 2018 and December 31, 2017, include accumulated net losses from fair value adjustments of $35.0 million after tax ($52.6 million before tax) associated with previously settled derivatives that were designated as net investment hedges. Foreign currency translation adjustments, net of tax, include foreign currency transaction losses of $273.4 million after tax ($346.7 million before tax) and $190.4 million after tax ($237.2 million before tax) at March 31, 2018 and December 31, 2017, respectively, associated with the Company's Euro-denominated debt. The Company's Euro-denominated debt is designated as a hedge against the impact of currency fluctuations on its Euro-denominated net assets (see Note 8). The remaining balance in foreign currency translation adjustments relates to the cumulative impacts of currency fluctuations on the Company's international non-U.S. Dollar denominated net assets. During the three months ended March 31, 2018, the Company recorded deferred tax charges of $10.5 million related to its one-time deemed repatriation tax liability recorded at December 31, 2017 and current year foreign earnings subject to U.S. federal and state income tax, resulting from the introduction of the Tax Act. Prior to January 1, 2018, foreign currency translation adjustments excluded U.S. federal and state income taxes as a result of the Company's intention to indefinitely reinvest the earnings of its international subsidiaries outside of the United States.
[2]	(2) Net unrealized gains on marketable equity securities, net of tax, at December 31, 2017 related to changes in the fair value of the Company's investment in Ctrip equity securities (see Note 5). Net unrealized gains before tax on equity securities at December 31, 2017 were $298.7 million, of which unrealized gains of $319.9 million were exempt from tax in the Netherlands. Unrealized losses of $21.2 million were taxable at a 25% tax rate in the Netherlands, which resulted in a tax benefit of $5.3 million at December 31, 2017. The Company also recorded U.S. tax charges of $62.9 million at December 31, 2017 related to these investments. Changes in fair value subsequent to January 1, 2018 are recognized in net income (see Note 1).
[3]	(3) Net unrealized gains on marketable debt securities, net of tax, relates to changes in the fair value of the Company's investments in debt securities (see Note 5). Net unrealized gains before tax on debt securities at March 31, 2018 and December 31, 2017 were $42.7 million and $44.5 million, respectively, of which unrealized losses of $52.9 million and $85.3 million, respectively, were exempt from tax in the Netherlands. Unrealized gains of $95.6 million and $129.8 million at March 31, 2018 and December 31, 2017, respectively, were taxable at a 25% tax rate in the Netherlands, resulting in tax charges of $23.9 million and $32.4 million, respectively. The Company also recorded U.S. tax charges of $7.6 million and $0.5 million at March 31, 2018 and December 31, 2017, respectively, related to these investments.

COMMITMENTS AND CONTINGENCIES (Details) € in Millions	3 Months Ended
COMMITMENTS AND CONTINGENCIES (Details) € in Millions	Mar. 31, 2018 USD ($) country	Mar. 31, 2018 EUR (€) country	Mar. 31, 2017 USD ($)	Sep. 30, 2016 EUR (€)	Dec. 31, 2017 USD ($)
Commitments and Contingencies
Pricing Parity Working Group \| country	10	10
Payments to Acquire Property, Plant, and Equipment \| $	$ 131,987,000		$ 70,559,000
French Tax Audit [Member]
Commitments and Contingencies
Assessed taxes including interest and penalties		€ 356
Series of individually immaterial business acquisitions [Member]
Commitments and Contingencies
Contingent consideration arrangements, range of outcomes, minimum value \| $	0
Contingent consideration arrangements, range of outcomes, maximum value \| $	90,000,000
Booking.com [Member] \| Headquarters [Member]
Commitments and Contingencies
Contractual Obligation for Building Construction				€ 270
Units of account				2
Payments to Acquire Productive Assets				€ 48
Acquisition of land use rights				43
Payments to Acquire Property, Plant, and Equipment		€ 34		5
Ground Lease [Member] \| Booking.com [Member] \| Headquarters [Member]
Commitments and Contingencies
Contractual Obligation for Building Construction				€ 60
Lessee, Operating Lease, Term of Contract			49 years
Fair Value, Inputs, Level 3 [Member] \| Series of individually immaterial business acquisitions [Member]
Commitments and Contingencies
Contingent consideration liability \| $	$ 9,000,000				$ 9,000,000