Booking (BKNG) 10-Q 5 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 29, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	Booking Holdings Inc.
Entity Central Index Key	1,075,531
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Document Type	10-Q
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Common Stock, Shares Outstanding		46,329,586

UNAUDITED CONSOLIDATED BALANCE

UNAUDITED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 2,973,096	$ 2,541,604
Short-term investments in marketable securities	4,158,076	4,859,873
Accounts receivable, net of allowance for doubtful accounts of $55,683 and $39,282, respectively	1,719,059	1,217,801
Prepaid expenses and other current assets	617,616	415,527
Total current assets	9,467,847	9,034,805
Property and equipment, net	642,861	480,081
Intangible assets, net	2,117,275	2,176,823
Goodwill	2,845,129	2,737,671
Long-term Investments	9,114,218	10,872,527
Other assets	183,146	149,356
Total assets	24,370,476	25,451,263
Current liabilities:
Accounts payable	1,174,599	667,523
Accrued expenses and other current liabilities	1,654,370	1,138,980
Deferred merchant bookings	1,101,255	980,455
Convertible debt	0	710,910
Total current liabilities	3,930,224	3,497,868
Deferred income taxes	491,160	481,139
Long-term U.S. transition tax liability	1,144,682	1,250,846
Other long-term liabilities	148,699	148,061
Long-term debt	8,703,901	8,809,788
Total liabilities	14,418,666	14,187,702
Commitments and Contingencies
Convertible debt	0	2,963
Stockholders' equity:
Common stock, $0.008 par value; authorized 1,000,000,000 shares, 62,938,503 and 62,689,097 shares issued, respectively	489	487
Treasury stock, 16,311,836 and 14,216,819 shares, respectively	(12,879,305)	(8,698,829)
Additional paid-in capital	5,343,776	5,783,089
Retained earnings	17,720,656	13,938,869
Accumulated other comprehensive income (loss)	(233,806)	236,982
Total stockholders' equity	9,951,810	11,260,598
Total liabilities and stockholders' equity	$ 24,370,476	$ 25,451,263

UNAUDITED CONSOLIDATED BALANC_2

UNAUDITED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 55,683	$ 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)	62,938,503	62,689,097
Treasury stock, shares (in shares)	16,311,836	14,216,819

UNAUDITED CONSOLIDATED STATEMEN

UNAUDITED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Agency revenues	$ 3,540,874		$ 8,220,506
Agency revenues		$ 3,523,706		$ 7,641,390
Merchant revenues	1,049,661		2,285,992
Merchant revenues		684,289		1,624,467
Advertising and other revenues	258,555	226,034	807,887	612,132
Total revenues	4,849,090	4,434,029	11,314,385	9,877,989
Cost of revenues		54,181		202,007
Gross profit		4,379,848		9,675,982
Operating expenses:
Performance marketing	1,314,055	1,231,074	3,562,155	3,364,589
Brand marketing	160,126	125,877	385,262	337,016
Sales and other expenses	242,974	151,024	612,367	382,538
Personnel, including stock-based compensation of $70,090, $66,421, $216,035 and $192,248, respectively	536,735	483,438	1,557,872	1,220,176
General and administrative	183,228	142,823	504,120	422,252
Information technology	57,742	47,901	177,133	132,677
Depreciation and amortization	107,641	95,910	317,397	265,212
Total operating expenses	2,602,501	2,278,047	7,116,306	6,124,460
Operating income	2,246,589	2,101,801	4,198,079	3,551,522
Investment Income, Net	48,641	41,483	141,210	110,296
Other (expense) income:
Interest expense	(68,170)	(66,338)	(203,242)	(182,997)
Net unrealized gains on marketable equity securities	30,858	0	107,221	0
Foreign currency transactions and other	(17,072)	(10,101)	(40,174)	(21,249)
Total other (expense) income	(5,743)	(34,956)	5,015	(93,950)
Earnings before income taxes	2,240,846	2,066,845	4,203,094	3,457,572
Income tax expense	473,268	346,454	850,934	561,349
Net income	$ 1,767,578	$ 1,720,391	$ 3,352,160	$ 2,896,223
Net income applicable to common stockholders per basic common share	$ 37.39	$ 35.12	$ 70	$ 58.99
Weighted-average number of basic common shares outstanding	47,268	48,981	47,887	49,100
Net income applicable to common stockholders per diluted common share	$ 37.02	$ 34.43	$ 69.07	$ 57.85
Weighted-average number of diluted common shares outstanding	47,751	49,972	48,530	50,064

UNAUDITED CONSOLIDATED STATEM_2

UNAUDITED CONSOLIDATED STATEMENTS OF OPERATIONS (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Stock-based compensation expense	$ 70,090	$ 66,421	$ 216,035	$ 192,248

UNAUDITED CONSOLIDATED STATEM_3

UNAUDITED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income	$ 1,767,578	$ 1,720,391	$ 3,352,160		$ 2,896,223
Other comprehensive income, net of tax
Foreign currency translation adjustments, net of tax charge of $36,381, tax benefit of $100,608, tax charge of $20,358 and tax benefit of $120,341, respectively	(23,952)	111,628	(90,938)	[1]	287,316	[1]
Reclassification of net unrealized gains on marketable equity securities to retained earnings, net of tax charge of $57,597	0	0	(241,088)	[2]	0	[2]
Net unrealized (losses) gains on marketable securities, net of tax charge of $430 and $8,076, tax benefit of $998 and tax charge of $15,931, respectively	(133,571)	(31,877)	(138,762)	[3]	501,034	[3]
Comprehensive income	$ 1,610,055	$ 1,800,142	$ 2,881,372		$ 3,684,573
[1]	(1) Foreign currency translation adjustments result from currency fluctuations on the translation of the Company's non-U.S. Dollar denominated net assets. Foreign currency translation adjustments were unfavorable for the three and nine months ended September 30, 2018 compared to the three and nine months ended September 30, 2017 because the U.S. Dollar strengthened against certain currencies in which the Company's net assets are denominated. During the three and nine months ended September 30, 2018, the Company recorded deferred tax benefits of $5.7 million and $16.2 million, respectively, related to foreign currency translation adjustments 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 tax charges of $5.9 million and $36.7 million for the three and nine months ended September 30, 2018, respectively, and tax benefits of $59.6 million and $179.9 million for the three and nine months ended September 30, 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 from accumulated other comprehensive income to retained earnings.
[3]	(3) Net unrealized losses before tax on marketable securities of $155.3 million and $138.5 million for the three and nine months ended September 30, 2018, respectively, were not subject to income tax in the Netherlands. Net unrealized gains before tax of $22.2 million and net unrealized losses before tax of $0.5 million for the three and nine months ended September 30, 2018, respectively, were taxable at a 25% tax rate in the Netherlands, resulting in a tax charge of $5.5 million and a tax benefit of $0.2 million, respectively. The Company also recorded a U.S. tax benefit of $5.0 million and $0.3 million for the three and nine months ended September 30, 2018, respectively, related to these investments, which represented the reversal of a previously recorded U.S. deferred tax liability. The remaining net unrealized losses on marketable securities and related tax benefits for three and nine months ended September 30, 2018 were associated with marketable debt securities held by a U.S. subsidiary. Net unrealized losses before tax on marketable securities of $57.7 million for the three months ended September 30, 2017 and net unrealized gains before tax of $427.4 million for the nine months ended September 30, 2017 were not subject to income tax in the Netherlands. Net unrealized gains before tax of $34.5 million and $98.2 million for the three and nine months ended September 30, 2017, respectively, were taxable at a 25% tax rate in the Netherlands, resulting in tax charges of $8.7 million and $24.6 million, respectively.

UNAUDITED CONSOLIDATED STATEM_4

UNAUDITED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Tax (benefit) associated with gain (loss) on components of foreign currency translation	$ 161	$ (59,607)	$ 20,519	$ (179,948)
Reclassification of net unrealized gains on marketable equity securities to retained earnings, tax/(benefit)	0	0	57,597	0
Tax (benefit) associated with gain (loss) on marketable securities	349	8,618	(649)	24,549
Tax exempt net unrealized (losses) gains on marketable securities, before tax, in the Netherlands	(155,300)	(57,700)	(138,500)	427,400
Taxable net unrealized (losses) gains on marketable securities, before tax	22,200	34,500	(500)	98,200
Tax and Customs Administration, Netherlands
Tax (benefit) associated with gain (loss) on marketable securities	5,500	8,700	(200)	24,600
Domestic Tax Authority [Member]
Tax (benefit) associated with gain (loss) on marketable securities	(5,000)		(300)	0
Currency translation adjustment on deemed repatriation tax liability [Member]
Tax (benefit) associated with gain (loss) on components of foreign currency translation	(5,700)		(16,200)	0
Net Investment Hedging
Tax (benefit) associated with gain (loss) on components of foreign currency translation	$ 5,900	$ (59,600)	$ 36,700	$ (179,900)

UNAUDITED CONSOLIDATED STATEM_5

UNAUDITED CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total		Common Stock	Treasury Stock	Additional Paid-In Capital	Retained Earnings	AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity
Cumulative effect of adoption of accounting standard updates	$ 299,305				$ 8,988	$ 290,317
Balance (in shares) at Dec. 31, 2016			62,379,000	(13,191,000)
Balance at Dec. 31, 2016	9,820,142		$ 485	$ (6,855,164)	5,482,653	11,326,852	$ (134,684)
Increase (Decrease) in Stockholders' Equity
Net income	2,896,223					2,896,223
Foreign currency translation adjustments, net of taxes	287,316	[1]					287,316
Net unrealized gains (losses) on marketable securities, net of taxes	501,034	[2]					501,034
Reclassification adjustment for convertible debt in mezzanine	19,137				19,137
Exercise of stock options and vesting of restricted stock units and/or performance share units (in shares)			150,000
Exercise of stock options and vesting of restricted stock units and/or performance share units	$ 4,303		$ 1		4,302
Repurchase of common stock (in shares)	(632,006)			(632,000)
Repurchase of common stock	$ (1,142,717)			$ (1,142,717)
Stock-based compensation and other stock-based payments	192,548				192,548
Conversion of debt (in shares)			46,000
Conversion of debt	(297)				(297)
Balance (in shares) at Sep. 30, 2017			62,575,000	(13,823,000)
Balance at Sep. 30, 2017	12,876,994		$ 486	$ (7,997,881)	5,707,331	14,513,392	653,666
Balance (in shares) at Jun. 30, 2017			62,560,000	(13,503,000)
Balance at Jun. 30, 2017	11,589,669		$ 486	$ (7,411,392)	5,633,659	12,793,001	573,915
Increase (Decrease) in Stockholders' Equity
Net income	1,720,391					1,720,391
Foreign currency translation adjustments, net of taxes	111,628						111,628
Net unrealized gains (losses) on marketable securities, net of taxes	(31,877)						(31,877)
Reclassification adjustment for convertible debt in mezzanine	5,676				5,676
Exercise of stock options and vesting of restricted stock units and/or performance share units (in shares)			12,000
Exercise of stock options and vesting of restricted stock units and/or performance share units	$ 1,513		$ 0		1,513
Repurchase of common stock (in shares)	(319,488)			(320,000)
Repurchase of common stock	$ (586,489)			$ (586,489)
Stock-based compensation and other stock-based payments	66,501				66,501
Conversion of debt (in shares)			3,000
Conversion of debt	(18)				(18)
Balance (in shares) at Sep. 30, 2017			62,575,000	(13,823,000)
Balance at Sep. 30, 2017	12,876,994		$ 486	$ (7,997,881)	5,707,331	14,513,392	653,666
Increase (Decrease) in Stockholders' Equity
Cumulative effect of adoption of accounting standard updates	188,539					429,627	(241,088)
Balance (in shares) at Dec. 31, 2017			62,689,000	(14,217,000)
Balance at Dec. 31, 2017	11,260,598		$ 487	$ (8,698,829)	5,783,089	13,938,869	236,982
Increase (Decrease) in Stockholders' Equity
Net income	3,352,160					3,352,160
Foreign currency translation adjustments, net of taxes	(90,938)	[1]					(90,938)
Net unrealized gains (losses) on marketable securities, net of taxes	(138,762)	[2]					(138,762)
Reclassification adjustment for convertible debt in mezzanine	2,963				2,963
Exercise of stock options and vesting of restricted stock units and/or performance share units (in shares)			198,000
Exercise of stock options and vesting of restricted stock units and/or performance share units	$ 1,629		$ 2		1,627
Repurchase of common stock (in shares)	(2,095,017)			(2,095,000)
Repurchase of common stock	$ (4,180,476)			$ (4,180,476)
Stock-based compensation and other stock-based payments	218,834				218,834
Conversion of debt	(773,178)				(773,178)
Common stock issued in an acquisition (in shares)			52,000
Common stock issued in an acquisition	110,441				110,441
Balance (in shares) at Sep. 30, 2018			62,939,000	(16,312,000)
Balance at Sep. 30, 2018	9,951,810		$ 489	$ (12,879,305)	5,343,776	17,720,656	(233,806)
Balance (in shares) at Jun. 30, 2018			62,927,000	(15,163,000)
Balance at Jun. 30, 2018	10,472,632		$ 489	$ (10,630,361)	5,225,709	15,953,078	(76,283)
Increase (Decrease) in Stockholders' Equity
Net income	1,767,578					1,767,578
Foreign currency translation adjustments, net of taxes	(23,952)						(23,952)
Net unrealized gains (losses) on marketable securities, net of taxes	(133,571)						(133,571)
Reclassification adjustment for convertible debt in mezzanine	45,643				45,643
Exercise of stock options and vesting of restricted stock units and/or performance share units (in shares)			12,000
Exercise of stock options and vesting of restricted stock units and/or performance share units	$ 300		$ 0		300
Repurchase of common stock (in shares)	(1,149,324)			(1,149,000)
Repurchase of common stock	$ (2,248,944)			$ (2,248,944)
Stock-based compensation and other stock-based payments	72,152				72,152
Conversion of debt	(28)				(28)
Balance (in shares) at Sep. 30, 2018			62,939,000	(16,312,000)
Balance at Sep. 30, 2018	$ 9,951,810		$ 489	$ (12,879,305)	$ 5,343,776	$ 17,720,656	$ (233,806)
[1]	(1) Foreign currency translation adjustments result from currency fluctuations on the translation of the Company's non-U.S. Dollar denominated net assets. Foreign currency translation adjustments were unfavorable for the three and nine months ended September 30, 2018 compared to the three and nine months ended September 30, 2017 because the U.S. Dollar strengthened against certain currencies in which the Company's net assets are denominated. During the three and nine months ended September 30, 2018, the Company recorded deferred tax benefits of $5.7 million and $16.2 million, respectively, related to foreign currency translation adjustments 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 tax charges of $5.9 million and $36.7 million for the three and nine months ended September 30, 2018, respectively, and tax benefits of $59.6 million and $179.9 million for the three and nine months ended September 30, 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 before tax on marketable securities of $155.3 million and $138.5 million for the three and nine months ended September 30, 2018, respectively, were not subject to income tax in the Netherlands. Net unrealized gains before tax of $22.2 million and net unrealized losses before tax of $0.5 million for the three and nine months ended September 30, 2018, respectively, were taxable at a 25% tax rate in the Netherlands, resulting in a tax charge of $5.5 million and a tax benefit of $0.2 million, respectively. The Company also recorded a U.S. tax benefit of $5.0 million and $0.3 million for the three and nine months ended September 30, 2018, respectively, related to these investments, which represented the reversal of a previously recorded U.S. deferred tax liability. The remaining net unrealized losses on marketable securities and related tax benefits for three and nine months ended September 30, 2018 were associated with marketable debt securities held by a U.S. subsidiary. Net unrealized losses before tax on marketable securities of $57.7 million for the three months ended September 30, 2017 and net unrealized gains before tax of $427.4 million for the nine months ended September 30, 2017 were not subject to income tax in the Netherlands. Net unrealized gains before tax of $34.5 million and $98.2 million for the three and nine months ended September 30, 2017, respectively, were taxable at a 25% tax rate in the Netherlands, resulting in tax charges of $8.7 million and $24.6 million, respectively.

UNAUDITED CONSOLIDATED STATEM_6

UNAUDITED CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS' EQUITY (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Stockholders' Equity [Abstract]
Tax (benefit) associated with gain (loss) on components of foreign currency translation	$ 161	$ (59,607)	$ 20,519	$ (179,948)
Tax (benefit) associated with gain (loss) on marketable securities	$ 349	$ 8,618	$ (649)	$ 24,549

UNAUDITED CONSOLIDATED STATEM_7

UNAUDITED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
OPERATING ACTIVITIES:
Net income	$ 3,352,160	$ 2,896,223
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	182,052	135,736
Amortization	135,345	129,476
Provision for uncollectible accounts	108,081	42,575
Deferred income tax benefit	(25,955)	(25,655)
Net unrealized gains on marketable equity securities	(107,221)	0
Stock-based compensation expense and other stock-based payments	225,883	192,548
Amortization of debt issuance costs	5,477	6,827
Amortization of debt discount	39,699	52,909
Loss on early extinguishment of debt	0	1,093
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	7,933	0
Changes in assets and liabilities, net of effects of an acquisition:
Accounts receivable	(450,071)	(479,184)
Prepaid expenses and other current assets	(200,665)	(136,304)
Accounts payable, accrued expenses and other current liabilities	1,004,748	640,960
Other long-term assets and liabilities	(23,278)	31,221
Net cash provided by operating activities	4,254,188	3,488,425
INVESTING ACTIVITIES:
Purchase of investments	(2,194,796)	(5,338,444)
Proceeds from sale of investments	4,495,994	2,471,883
Additions to property and equipment	(356,011)	(223,692)
Acquisitions and other investments, net of cash acquired	(139,386)	(552,805)
Net cash provided by (used in) investing activities	1,805,801	(3,643,058)
FINANCING ACTIVITIES:
Proceeds from short-term borrowing	3,912	0
Proceeds from the issuance of long-term debt	0	2,044,952
Payments for conversion of senior notes	(1,487,187)	(89,575)
Payment of debt	(348)	(15,118)
Payments for repurchase of common stock	(4,110,304)	(1,123,102)
Proceeds from exercise of stock options	1,629	4,303
Net cash (used in) provided by financing activities	(5,592,298)	821,460
Effect of exchange rate changes on cash, cash equivalents and restricted cash and cash equivalents	(36,560)	99,037
Net increase in cash, cash equivalents and restricted cash and cash equivalents	431,131	765,864
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	2,994,472	2,847,871
SUPPLEMENTAL CASH FLOW INFORMATION:
Cash paid during the period for income taxes	1,044,312	601,248
Cash paid during the period for interest	155,056	110,745
Non-cash financing activity for loan forgiveness	0	1,000
Non-cash operating and financing activity for an acquisition (see Note 7)	50,751	0
Non-cash investing and financing activity for an acquisition (see Note 7)	$ 59,690	$ 0

BASIS OF PRESENTATION

BASIS OF PRESENTATION	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
BASIS OF PRESENTATION	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, 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 (loss) " 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" or "General and administrative" expenses in its Unaudited Consolidated Statement of Operations for the three and nine months ended September 30, 2017 to conform to the current period presentation. The change in presentation and the reclassification for the three and nine months ended September 30, 2017 had no impact on operating income or net income and are summarized below (in thousands): Previously Reported Three Months Ended Nine Months Ended Cost of revenues $ 59,476 $ 217,387 Performance advertising 1,224,345 3,352,707 Brand advertising 112,796 306,995 Sales and marketing 165,539 411,309 General and administrative 142,823 420,004 Current Presentation Three Months Ended Nine Months Ended Cost of revenues $ 54,181 $ 202,007 Performance marketing 1,231,074 3,364,589 Brand marketing 125,877 337,016 Sales and other expenses 151,024 382,538 General and administrative 142,823 422,252 Balance Sheet Reclassification In the second quarter of 2018, the Company changed the presentation of "Long-term investments" to include investments in private companies, which were previously included in "Other assets" in the Unaudited Consolidated Balance Sheets. Therefore, the Company reclassified $450.9 million for investments in private companies to conform its Consolidated Balance Sheet at December 31, 2017 to this current period presentation. See the section "Long-term Equity Investments without Readily Determinable Fair Value" within Note 5 for more detail. Restricted Cash and Cash Equivalents: Restricted cash and cash equivalents at September 30, 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): September 30, December 31, As included in the Unaudited Consolidated Balance Sheets: Cash and cash equivalents $ 2,973,096 $ 2,541,604 Restricted cash and cash equivalents included in prepaid expenses and other current assets 21,376 21,737 Total cash, cash equivalents and restricted cash and cash equivalents as shown in the Unaudited Consolidated Statements of Cash Flows $ 2,994,472 $ 2,563,341 Recent Accounting Pronouncements Adopted Premium Amortization on Purchased Callable Debt Securities In March 2017, the Financial Accounting Standards Board (“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, 2018, including interim periods within those fiscal years, with early adoption allowed. The Company early adopted this new standard in the third quarter of 2018. The adoption of this update did not have an impact to the Unaudited Consolidated Financial Statements. Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued a new accounting update to make targeted improvement to the disclosure requirement for fair value measurements as part of its disclosure framework project. This update eliminates, adds and modifies certain disclosure requirements primarily related to Level 3 fair value measurements. For public business entities, this update is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years, with early adoption allowed. The Company early adopted this new standard in the third quarter of 2018. The adoption of this update did not have an impact to the Unaudited Consolidated Financial Statements. Improvements to Non-employee Share-Based Payment Accounting In June 2018, the FASB issued a new accounting update which amends the guidance on share-based payments granted to non-employees for goods and services to align it with the guidance for share-based payments to employees. Under this new guidance, share-based awards to non-employees will be generally measured at fair value on the grant date of the awards and entities will need to assess the probability of satisfying performance conditions, if any are present, to determine expense to be recognized. This update is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years, with early adoption allowed. The Company early adopted this new standard in the second quarter of 2018 and applied this update as of January 1, 2018. The adoption of this update did not have a material impact to the Unaudited Consolidated Financial Statements. Recognition and Measurement of Financial Instruments In January 2016, the 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 investments in marketable 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 Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In August 2018, the FASB issued a new accounting update to address a customer's accounting for implementation costs incurred in a cloud computing arrangement that is a service contract and also adds certain disclosure requirements related to implementation costs incurred for internal-use software and cloud computing arrangements. The amendment aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). For public business entities, this update is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years, with early adoption allowed. The Company plans to adopt this update on January 1, 2019 and apply it on a prospective basis. The Company does not expect a material impact to the Unaudited Consolidated Financial Statements resulting from the adoption. 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 rights and obligations created by entering into a lease transaction. The new standard retains the dual-model concept by requiring entities to determine if a lease is an operating or financing lease. 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 using a modified retrospective approach applied to the earliest comparative period in the financial statements. In July 2018, the FASB approved a new transition method that would permit issuers to apply the standard as of January 1, 2019. Early adoption is permitted. The Company will adopt this standard in the first quarter of 2019 and apply it as of January 1, 2019. The Company is in the process of implementing a software platform to facilitate compliance with the new accounting and disclosure requirements. The Company will elect to use its previous evaluations regarding if an arrangement contains a lease, if a lease is an operating or financing lease and what costs are capitalized as initial direct costs prior to adoption, as permitted under this standard. The most significant change will be related to the recognition of right-of-use assets and lease liabilities in the Company's Consolidated Balance Sheet for real estate operating leases. The Company will elect to combine lease and non-lease components and to include leases which have an initial term of less than one year in the calculation of the lease liability and right-of-use assets for its real estate leases. The Company performed an assessment of its real estate leases, which comprises a substantial portion of its total operating leases, and estimates that the lease liability and corresponding right-of-use asset would be approximately $450 million to $550 million at January 1, 2019, based on outstanding leases, incremental borrowing rates and foreign exchange rates in effect as of September 30, 2018. This estimate could change significantly by the adoption date due to several factors, including changes in the Company’s real estate lease portfolio, changes in foreign exchange rates and/or changes in the interest rate environment. The Company is in the process of reviewing its remaining operating leases, including its data center leases. The Company does not expect a material impact to its Consolidated Statement of Operations and Statement of Cash Flows resulting from the adoption.

REVENUE RECOGNITION

REVENUE RECOGNITION	9 Months Ended
	Sep. 30, 2018
Revenue from Contract with Customer [Abstract]
REVENUE RECOGNITION	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 all periods prior to January 1, 2018 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 that 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 ® 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, Name Your Own Price ® 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 at September 30, 2018 : As reported (current revenue standard) Current period adjustments As adjusted (previous revenue standard) ASSETS Current assets: Accounts receivable, net $ 1,719,059 $ (93,439 ) $ 1,625,620 Prepaid expenses and other current assets 617,616 18,563 636,179 LIABILITIES AND STOCKHOLDERS' EQUITY Current liabilities: Accounts payable $ 1,174,599 $ (70,609 ) $ 1,103,990 Accrued expenses and other current liabilities 1,654,370 (1,108 ) 1,653,262 Deferred merchant bookings 1,101,255 88,594 1,189,849 Deferred income taxes 491,160 (95 ) 491,065 Stockholders' equity: Retained earnings 17,720,656 (94,781 ) 17,625,875 Accumulated other comprehensive income (loss) (233,806 ) 3,123 (230,683 ) Unaudited Consolidated Statements of Operations for the Three Months Ended September 30, 2018 : As reported (current revenue standard) Current period adjustments As adjusted (previous revenue standard) Agency revenues $ 3,540,874 $ 55,148 $ 3,596,022 Merchant revenues 1,049,661 48,712 1,098,373 Advertising and other revenues 258,555 (6 ) 258,549 Cost of revenues 44,936 44,936 Operating expenses: Performance marketing 1,314,055 3,250 1,317,305 Sales and other expenses 242,974 206 243,180 General and administrative 183,228 86 183,314 Foreign currency transactions and other (17,072 ) 951 (16,121 ) Income tax expense 473,268 11,742 485,010 Net income 1,767,578 44,585 1,812,163 Net income applicable to common stockholders per basic common share 37.39 0.95 38.34 Net income applicable to common stockholders per diluted common share 37.02 0.93 37.95 Unaudited Consolidated Statements of Operations for the Nine Months Ended September 30, 2018 : As reported (current revenue standard) Current period adjustments As adjusted (previous revenue standard) Agency revenues $ 8,220,506 $ 107,826 $ 8,328,332 Merchant revenues 2,285,992 150,389 2,436,381 Advertising and other revenues 807,887 98 807,985 Cost of revenues 134,886 134,886 Operating expenses: Performance marketing 3,562,155 4,254 3,566,409 Sales and other expenses 612,367 206 612,573 General and administrative 504,120 1,120 505,240 Foreign currency transactions and other (40,174 ) 1,740 (38,434 ) Income tax expense 850,934 25,829 876,763 Net income 3,352,160 93,758 (1) 3,445,918 Net income applicable to common stockholders per basic common share 70.00 1.96 71.96 Net income applicable to common stockholders per diluted common share 69.07 1.94 71.01 (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 $94.8 million that would have been recognized in the fourth 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 (e.g., 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, in certain merchant transactions, the travelers. 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 receive 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 system ("GDS") reservation booking fees and certain travel insurance fees. • Merchant revenues are derived from services where the Company receives 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 travel insurance-related revenues, credit card processing rebates 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. Pursuant to the terms of the Company's merchant services, 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 provider within the specified period of time, the Company increases its revenue by the unbilled amount. Tax Recovery Charge, Occupancy Taxes and State and Local Taxes For merchant transactions, the Company charges the traveler an amount intended to cover the taxes that the Company anticipates the travel service provider will remit to the local taxing authorities ("tax recovery charge"). Tax rate information for calculating the tax recovery charge is provided to the Company by the travel service providers. In certain taxing jurisdictions, the Company is required by statute, regulation or court order to collect and remit certain local occupancy tax, general excise and/or sales tax (“transaction-related taxes”) imposed upon its margin and/or service fees. In other taxing jurisdictions, the Company is required to collect from the traveler and remit directly to the taxing jurisdiction transaction-related taxes imposed on the full amount of the transaction, which includes taxes on the margin, service fees and the underlying rate provided by the travel service provider. The rate information for calculating these taxes is provided to the Company directly from the taxing jurisdictions. The taxes collected from travelers are reported on a net basis in Revenues in the Unaudited Consolidated Statements of Operations. 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 their websites and by Booking.com for its 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 September 30, 2018 and December 31, 2017 , liabilities of $80.0 million and $104.7 million , respectively, for loyalty incentives were included in "Accrued expenses and other current liabilities" in the Unaudited Consolidated Balance Sheets. Disaggregation of revenue Geographic Information The Company's international information consists of the results of Booking.com, Rentalcars.com and agoda 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 September 30, United States The Netherlands Other Total 2018 $ 441,470 (1) $ 3,888,255 $ 519,365 $ 4,849,090 (1) 2017 429,587 (2) 3,533,906 470,536 4,434,029 (2) (1) Total revenues have been reduced for cost of revenues for Name Your Own Price ® transactions of $44.9 million in 2018. (2) Total revenues have not been reduced for cost of revenues for Name Your Own Price ® transactions of $54.2 million in 2017. International Total revenues for the Nine Months Ended September 30, United States The Netherlands Other Total 2018 $ 1,243,428 (1) $ 8,717,681 $ 1,353,276 $ 11,314,385 (1) 2017 1,251,079 (2) 7,499,464 1,127,446 9,877,989 (2) (1) Total revenues have been reduced for cost of revenues for Name Your Own Price ® transactions of $134.9 million in 2018. (2) Total revenues have not been reduced for cost of revenues for Name Your Own Price ® transactions of $202.0 million in 2017. Revenue by Type of Service Approximately 88% and 87% of the Company's revenue for the three and nine months ended September 30, 2018 , respectively, 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. At September 30, 2018 and December 31, 2017 , deferred merchant bookings includes deferred revenue of $176.8 million and $151.2 million , respectively. The Company expects to complete its service obligation within one year of booking. In the nine months ended September 30, 2018 , the Company recognized revenue of $107.3 million and cancellations of $10.3 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 impact of the adoption of the current revenue standard on January 1, 2018. The offsetting increase in the deferred revenue balance for the nine months ended September 30, 2018 is principally driven by payments received from travelers, net of amounts payable to travel service providers, of $185.5 million for those online travel reservations in the current period.

STOCK-BASED EMPLOYEE COMPENSATI

STOCK-BASED EMPLOYEE COMPENSATION	9 Months Ended
	Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
STOCK-BASED EMPLOYEE COMPENSATION	STOCK-BASED EMPLOYEE COMPENSATION Stock-based compensation expense included in personnel expenses in the Unaudited Consolidated Statements of Operations was approximately $70.1 million and $66.4 million for the three months ended September 30, 2018 and 2017 , respectively, and $216.0 million and $192.2 million for the nine months ended September 30, 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 nine months ended September 30, 2018 : Share-Based Awards Shares Weighted-Average Grant Date Fair Value Unvested at December 31, 2017 524,696 $ 1,431.88 Granted 156,896 $ 2,032.81 Vested (194,461 ) $ 1,288.07 Performance Shares Adjustment 15,338 $ 1,657.61 Forfeited/Canceled (33,396 ) $ 1,690.38 Unvested at September 30, 2018 469,073 $ 1,681.48 At September 30, 2018 , there was $418.8 million of total future compensation cost related to unvested share-based awards to be recognized over a weighted-average period of 1.9 years. During the nine months ended September 30, 2018 , the Company made broad-based grants of 107,175 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 $217.8 million based on a weighted-average grant date fair value per share of $2,032.36 . In addition, during the nine months ended September 30, 2018 , the Company granted 49,721 performance share units to executives and certain other employees. The performance share units had a total grant date fair value of $101.1 million based on a weighted-average 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. At September 30, 2018 , the estimated number of probable shares to be issued is a total of 47,575 shares, net of performance share units that were forfeited or vested since the grant date. If the maximum performance thresholds are met at the end of the performance period, a maximum number of 95,150 total shares could be issued. If the minimum performance thresholds are not met, 36,265 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 September 30, 2018 , there were 63,265 unvested 2017 performance share units outstanding, net of performance share units that were forfeited or vested since the grant date. At September 30, 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 86,630 shares. If the maximum performance thresholds are met at the end of the performance period, a maximum of 125,622 shares could be issued pursuant to these performance share units. If the minimum performance thresholds are not met, 49,554 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 September 30, 2018 , there were 65,218 unvested 2016 performance share units outstanding, net of performance share units that were forfeited or vested since the grant date. At September 30, 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 105,037 shares. If the maximum thresholds are met at the end of the performance period, a maximum of 143,240 shares could be issued pursuant to these performance share units. If the minimum performance thresholds are not met, 39,418 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 nine months ended September 30, 2018 : Employee Stock Options Number of Shares Weighted-Average Aggregate Intrinsic Value (in thousands) Weighted-Average Remaining Contractual Term Balance, December 31, 2017 30,675 $ 401.61 $ 40,986 3.9 Exercised (3,300 ) $ 493.87 Forfeited (13 ) $ 241.83 Balance, September 30, 2018 27,362 $ 390.64 $ 43,598 3.0 Vested and exercisable at September 30, 2018 27,289 $ 399.10 $ 43,541 3.0 Vested and exercisable at September 30, 2018 and expected to vest thereafter 27,362 $ 390.64 $ 43,598 3.0 The aggregate intrinsic value of employee stock options exercised during the nine months ended September 30, 2018 and 2017 was $5.1 million and $21.4 million , respectively. During the nine months ended September 30, 2018 and 2017 , stock options vested for 98 and 1,246 shares, respectively.

NET INCOME PER SHARE

NET INCOME PER SHARE	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
NET INCOME PER SHARE	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 Nine Months Ended 2018 2017 2018 2017 Weighted-average number of basic common shares outstanding 47,268 48,981 47,887 49,100 Weighted-average dilutive stock options, restricted stock units and performance share units 230 273 270 278 Assumed conversion of Convertible Senior Notes 253 718 373 686 Weighted-average number of diluted common and common equivalent shares outstanding 47,751 49,972 48,530 50,064 Anti-dilutive potential common shares 1,428 1,948 1,366 2,006 Anti-dilutive potential common shares for the three and nine months ended September 30, 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

INVESTMENTS	9 Months Ended
	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
INVESTMENTS	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 (loss) " 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 September 30, 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 these 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 (loss)" to "Retained earnings" in the Unaudited Consolidated Balance Sheet. For the three and nine months ended September 30, 2018 , the Company recognized $30.9 million and $107.2 million , respectively, in "Net unrealized gains on marketable equity securities" in the Unaudited Consolidated Statements of Operations. The following table summarizes, by major security type, the Company's investments in marketable securities at September 30, 2018 (in thousands): Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Short-term investments in marketable securities: Debt securities: International government securities $ 389,789 $ 6 $ (523 ) $ 389,272 U.S. government securities 809,241 — (3,856 ) 805,385 Corporate debt securities 2,952,065 426 (11,561 ) 2,940,930 U.S. government agency securities 4,916 — (7 ) 4,909 Commercial paper 17,080 — — 17,080 Certificate of deposit 500 — — 500 Total $ 4,173,591 $ 432 $ (15,947 ) $ 4,158,076 Long-term investments in marketable securities: Debt securities: International government securities $ 790,653 $ 1,205 $ (986 ) $ 790,872 U.S. government securities 370,987 — (10,053 ) 360,934 Corporate debt securities 4,747,084 4,817 (59,563 ) 4,692,338 Investments in Ctrip: Convertible debt securities 1,275,000 21,500 (38,668 ) 1,257,832 Equity securities 655,311 153,553 (4,781 ) 804,083 Meituan Dianping equity securities 450,270 257,134 — 707,404 Total $ 8,289,305 $ 438,209 $ (114,051 ) $ 8,613,463 The Company's investment policy seeks to preserve capital and maintain sufficient liquidity to meet operational and other needs of the business. At September 30, 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 1.2 years with an average credit quality of A+/A1/A+. The Company invests in international government securities with high credit quality. At September 30, 2018 , investments in international government securities principally included debt securities issued by the governments of the Netherlands, France, Belgium, Austria, Finland, and Germany. The following table summarizes, by major security type, the Company's investments in marketable securities at December 31, 2017 (in thousands): Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Short-term investments in marketable securities: 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 $ 4,866,475 $ 700 $ (7,302 ) $ 4,859,873 Long-term investments in marketable securities: 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 Investments in Ctrip: Convertible debt securities 1,275,000 103,100 (9,600 ) 1,368,500 Equity securities 655,311 299,697 (1,012 ) 953,996 Total $ 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. At September 30, 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. At September 30, 2018 , the Company did not have significant influence over Ctrip. Investment in Meituan Dianping In October 2017, the Company invested $450 million in preferred shares of Meituan Dianping, the leading e-commerce platform for local services in China. As a result of Meituan Dianping's initial public offering in September 2018, the Company classified its investment as a marketable equity security (see the table above that summarizes the Company's investments in marketable securities at September 30, 2018 ) and recognized an unrealized gain of $257.1 million for the three and nine months ended September 30, 2018 , which was included in "Net unrealized gains on marketable equity securities" in the Unaudited Consolidated Statements of Operations. Long-term 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 $500.8 million and $450.9 million at September 30, 2018 and December 31, 2017 , respectively. The investments of $450.9 million at December 31, 2017 principally related to the Company's investment in Meituan Dianping prior to its initial public offering in September 2018. In July 2018, the Company invested $500 million in preferred shares of Didi Chuxing, the leading mobile transportation and ride-hailing platform in China. 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 "Long-term investments" in the Company's Unaudited Consolidated Balance Sheets. The Company determined that there was no change in the carrying value of these investments at September 30, 2018 . Subsequent Event In October 2018, the Company invested $200 million in preferred shares of Grab, the leading on-demand transportation and mobile service platform in Southeast Asia.

FAIR VALUE MEASUREMENTS

FAIR VALUE MEASUREMENTS	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	FAIR VALUE MEASUREMENTS Financial assets and liabilities carried at fair value at September 30, 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 $ 2,385,451 $ — $ 2,385,451 International government securities — 6,883 6,883 U.S. government securities — 2,009 2,009 Commercial paper — 4,372 4,372 Time deposits 4,519 — 4,519 Short-term investments in marketable securities: International government securities — 389,272 389,272 U.S. government securities — 805,385 805,385 Corporate debt securities — 2,940,930 2,940,930 U.S. government agency securities — 4,909 4,909 Commercial paper — 17,080 17,080 Certificate of deposit 500 — 500 Long-term investments in marketable securities: International government securities — 790,872 790,872 U.S. government securities — 360,934 360,934 Corporate debt securities — 4,692,338 4,692,338 Ctrip convertible debt securities — 1,257,832 1,257,832 Ctrip equity securities 804,083 — 804,083 Meituan Dianping equity securities 707,404 — 707,404 Derivatives: Currency exchange derivatives — 914 914 Total assets at fair value $ 3,901,957 $ 11,273,730 $ 15,175,687 Level 1 Level 2 Total LIABILITIES: Currency exchange derivatives $ — $ 1,617 $ 1,617 Financial assets and liabilities carried at fair value at 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 in marketable securities: 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 in marketable securities: 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. See Note 5 for information on the carrying value of the Company's investments in marketable securities. 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. At September 30, 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. See Note 8 for the estimated fair value of the Company's outstanding Senior Notes and Note 12 for the Company's contingent liability associated with a business acquisition. In the normal course of business, the Company is exposed to the impact of foreign currency fluctuations. The Company mitigates 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 (loss) ." 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. At September 30, 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 losses of $8.6 million and $38.2 million for the three and nine months ended September 30, 2018 , respectively, compared to foreign currency gains of $10.7 million and $38.3 million for the three and nine months ended September 30, 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 $15.1 million and $32.3 million for the three and nine months ended September 30, 2018 , respectively, and net losses of $7.6 million and $18.7 million for the three and nine months ended September 30, 2017 , respectively. The 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 outflows of $35.6 million and net cash inflows of $38.2 million for the nine months ended September 30, 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 September 30, 2018 and December 31, 2017 , the embedded derivative had an estimated fair value of $0.4 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 Statements of Operations.

INTANGIBLE ASSETS AND GOODWILL

INTANGIBLE ASSETS AND GOODWILL	9 Months Ended
	Sep. 30, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
INTANGIBLE ASSETS AND GOODWILL	INTANGIBLE ASSETS AND GOODWILL The Company's intangible assets at September 30, 2018 and December 31, 2017 consisted of the following (in thousands): September 30, 2018 December 31, 2017 Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Amortization Weighted- Supply and distribution agreements $ 1,081,543 $ (396,584 ) $ 684,959 $ 1,056,660 $ (355,000 ) $ 701,660 3 - 20 years 16 years Technology 171,419 (118,017 ) 53,402 137,288 (104,478 ) 32,810 1 - 7 years 5 years Patents 1,623 (1,623 ) — 1,623 (1,623 ) — 15 years 15 years Internet domain names 40,903 (29,767 ) 11,136 42,265 (28,802 ) 13,463 5 - 20 years 8 years Trade names 1,785,284 (417,600 ) 1,367,684 1,779,076 (350,447 ) 1,428,629 4-20 years 19 years Non-compete agreements 900 (806 ) 94 21,900 (21,639 ) 261 4 years 4 years Total intangible assets $ 3,081,672 $ (964,397 ) $ 2,117,275 $ 3,038,812 $ (861,989 ) $ 2,176,823 Intangible assets are amortized on a straight-line basis. Amortization expense was approximately $43.0 million and $135.3 million for the three and nine months ended September 30, 2018 , respectively, and $45.3 million and $129.5 million for the three and nine months ended September 30, 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 $ 42,591 2019 168,473 2020 160,708 2021 154,745 2022 152,006 2023 150,007 Thereafter 1,288,745 $ 2,117,275 A roll-forward of goodwill for the nine months ended September 30, 2018 consisted of the following (in thousands): Balance at December 31, 2017 $ 2,737,671 Acquisition 131,583 Currency translation adjustments (24,125 ) Balance at September 30, 2018 $ 2,845,129 Annual Goodwill Impairment Test A substantial portion of the Company's intangibles and goodwill relates to the acquisitions of OpenTable in July 2014 and KAYAK in May 2013. As of September 30, 2018 , the Company performed its annual goodwill impairment testing and concluded that there was no impairment of goodwill. Other than OpenTable, the fair values of the Company's reporting units substantially exceeded their respective carrying values as of September 30, 2018 . In addition, at September 30, 2018 , the Company did not identify any impairment indicator for the Company's other long-lived assets. The Company estimated OpenTable’s fair value using a combination of standard valuation techniques, including an income approach (discounted cash flows) and market approaches (EBITDA multiples of comparable publicly-traded companies and precedent transactions). At September 30, 2018 , OpenTable's estimated fair value was approximately 13% higher than its carrying value, therefore there was no impairment to OpenTable's goodwill. Acquisition In April 2018, the Company paid $139.4 million , net of cash acquired, and issued shares of the Company's common stock in the amount of $110.4 million in connection with an acquisition of a local activities and experiences booking software provider. In respect to the shares issued, as shown in the supplemental disclosure in the Unaudited Consolidated Statement of Cash Flows, $59.7 million relates to purchase price consideration and $50.7 million relates to shares restricted for trading purposes until the required post-acquisition services are completed by certain employees. At September 30, 2018, the Company's Unaudited Consolidated Balance Sheet includes approximately $17 million in "Prepaid expenses and other current assets" and approximately $27 million in "Other assets" related to this deferred compensation charge associated with these restricted shares. The purchase price allocation was completed as of September 30, 2018 .

DEBT

DEBT	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
DEBT	DEBT Short-term Borrowing On September 30, 2018 , the Company had a bank overdraft of $3.9 million , which was reported in "Accrued expenses and other current liabilities" in the Unaudited Consolidated Balance Sheet at September 30, 2018 and was repaid in October 2018. 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 $4.7 million and $3.8 million of letters of credit issued under the facility, respectively, at September 30, 2018 and December 31, 2017 . Outstanding Debt Outstanding debt at September 30, 2018 consisted of the following (in thousands): September 30, 2018 Outstanding Principal Amount Unamortized Debt Discount and Debt Issuance Cost Carrying Value Long-term debt: 0.35% Convertible Senior Notes due June 2020 $ 999,950 $ (45,225 ) $ 954,725 0.9% Convertible Senior Notes due September 2021 1,000,000 (66,841 ) 933,159 0.8% (€1 Billion) Senior Notes due March 2022 1,161,500 (5,073 ) 1,156,427 2.15% (€750 Million) Senior Notes due November 2022 871,125 (3,927 ) 867,198 2.75% Senior Notes due March 2023 500,000 (2,747 ) 497,253 2.375% (€1 Billion) Senior Notes due September 2024 1,161,500 (10,709 ) 1,150,791 3.65% Senior Notes due March 2025 500,000 (2,959 ) 497,041 3.6% Senior Notes due June 2026 1,000,000 (6,250 ) 993,750 1.8% (€1 Billion) Senior Notes due March 2027 1,161,500 (4,709 ) 1,156,791 3.55% Senior Notes due March 2028 500,000 (3,234 ) 496,766 Total long-term debt $ 8,855,575 $ (151,674 ) $ 8,703,901 Outstanding debt at 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 on the closing price of the Company's common stock for the prescribed measurement period for three months ended September 30, 2018 and December 31, 2017 , the contingent conversion thresholds on the 2020 Notes (as defined below) and 2021 Notes (as defined below) were not exceeded, therefore, these notes were not convertible at the option of the holder and were reported as non-current liabilities in the Unaudited Consolidated Balance Sheets. The 2020 Notes were convertible at the option of the holders at March 31 and June 30, 2018 as the contingent conversion threshold was exceeded. Therefore, the Company reported the carrying value of the 2020 Notes as a current liability and reclassified the unamortized debt discount from additional paid-in-capital to convertible debt in mezzanine in the Company's Unaudited Consolidated Balance Sheets at March 31 and June 30, 2018. As the 2020 Notes were not convertible at September 30, 2018 , the Company reclassified the unamortized debt discount in the amount of $45.6 million from convertible debt in mezzanine to additional paid-in-capital in the Company's Unaudited Consolidated Balance Sheet at September 30, 2018 . 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, at 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. Fair Value of Debt At September 30, 2018 and December 31, 2017 , the estimated fair value of the outstanding Senior Notes was approximately $9.6 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 $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 $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 remaining outstanding 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 September 30, 2018 and 2017 , the Company recognized interest expense of $15.2 million and $23.4 million , respectively, related to convertible notes, which was comprised of $3.1 million and $5.4 million , respectively, related to the contractual coupon interest, $11.8 million and $16.9 million , respectively, related to the amortization of debt discount and $0.3 million and $1.1 million , respectively, related to the amortization of debt issuance costs. For the three months ended September 30, 2018 and 2017 , included in the amortization of debt discount mentioned above was $0.7 million and $0.8 million , respectively, 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 September 30, 2018 and 2017 are 3.2% and 3.4% , respectively. For the nine months ended September 30, 2018 and 2017 , the Company recognized interest expense of $50.2 million and $71.4 million , respectively, related to convertible notes, which was comprised of $10.8 million and $16.4 million , respectively, related to the contractual coupon interest, $37.9 million and $51.4 million , respectively, related to the amortization of debt discount and $1.5 million and $3.6 million respectively, related to the amortization of debt issuance costs. For the nine months ended September 30, 2018 and 2017 , included in the amortization of debt discount mentioned above was $2.2 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 nine months ended September 30, 2018 and 2017 are 3.2% 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 (loss) " 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 the effects of foreign currency changes also reported in " Accumulated other comprehensive income (loss) " 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 September 30, 2018 and 2017 , the Company recognized interest expense of $42.3 million and $38.5 million , respectively, related to other long-term debt, which was almost entirely comprised of $40.6 million and $36.9 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. For the nine months ended September 30, 2018 and 2017 , the Company recognized interest expense of $128.4 million and $102.2 million , respectively, related to other long-term debt, which was principally comprised of $123.3 million and $98.1 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 additional employment and salary conditions are met. At September 30, 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

TREASURY STOCK	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
TREASURY STOCK	TREASURY STOCK At December 31, 2017, the Company had a total remaining authorization of $2.4 billion to repurchase its common stock related to programs authorized by the Company's Board of Directors in 2016 and 2017 for $3.0 billion and $2.0 billion , respectively. 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. At September 30, 2018 , the Company had a remaining authorization of $6.4 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 withheld to satisfy employee withholding tax obligations related to stock-based compensation. In the three months ended September 30, 2018 , the Company repurchased a total of 1,149,324 shares of its common stock in the open market for an aggregate cost of $2.2 billion , which included 1,145,120 shares for $2.2 billion acquired through its general repurchase programs and 4,204 shares for $8.5 million withheld to satisfy employee withholding tax obligations related to stock-based compensation. In the nine months ended September 30, 2018 , the Company repurchased a total of 2,095,017 shares of its common stock in the open market for an aggregate cost of $4.2 billion , which included 2,019,106 shares for $4.0 billion acquired through its general repurchase programs and 75,911 shares for $154.8 million withheld to satisfy employee withholding tax obligations related to stock-based compensation. In the three months ended September 30, 2017 , the Company repurchased a total of 319,488 shares of its common stock in the open market for an aggregate cost of $586.5 million , which included 316,016 shares for $580.0 million acquired through its general repurchase programs and 3,472 shares for $6.5 million withheld to satisfy employee withholding tax obligations related to stock-based compensation. In the nine months ended September 30, 2017 , the Company repurchased a total of 632,006 shares of its common stock in the open market for an aggregate cost of $1.1 billion , which included 577,568 shares for $1.0 billion acquired through its general repurchase programs and 54,438 shares for $95.1 million withheld to satisfy employee withholding tax obligations related to stock-based compensation. In the three months ended September 30, 2018 , stock repurchases in September 2018 of 51,240 shares for an aggregate cost of $101.3 million were settled in October 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. The Company remitted employee withholding taxes of $8.9 million and $153.9 million for the three and nine months ended September 30, 2018 , respectively, and $9.0 million and $95.5 million for the three and nine months ended September 30, 2017 , 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. At September 30, 2018 , there were 16,311,836 shares of the Company's common stock held in treasury.

INCOME TAXES

INCOME TAXES	9 Months Ended
	Sep. 30, 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 and nine months ended September 30, 2018 was 21.1% and 20.2% , respectively, compared to 16.8% and 16.2% for the three and nine months ended September 30, 2017 , respectively. The Company's 2018 effective tax rates differ from the 2018 U.S. federal statutory tax rate of 21% , primarily due to the benefit of the Netherlands Innovation Box Tax (discussed below) and current year excess tax benefits 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, as well as certain nondeductible expenses. The Company's 2017 effective tax rates differ from the 2017 U.S. federal statutory tax rate of 35% , primarily as a result of lower international tax rates and current year excess tax benefits recognized from the vesting of equity awards, partially offset by certain nondeductible expenses. The Company's effective tax rates were higher for the three and nine months ended September 30, 2018 , compared to the three and nine months ended September 30, 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 discussed below. During the three and nine months ended September 30, 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") for periods beginning on or after January 1, 2018 rather than the Dutch statutory rate of 25% . Previously, the Innovation Box Tax rate had been 5% . A portion of Booking.com's earnings during the three and nine months ended September 30, 2018 and 2017 qualified 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% . At September 30, 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, the majority of which will be generated by entities with a tax year that ends subsequent to September 30, 2018, that are subject to the U.S. federal deemed repatriation tax. This analysis includes a review of income tax returns, some of which are not yet due. The Company expects to complete its accounting within the measurement period. 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 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 the 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 the GILTI inclusion upon reversal.

ACCUMULATED OTHER COMPREHENSIVE

ACCUMULATED OTHER COMPREHENSIVE INCOME	9 Months Ended
	Sep. 30, 2018
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
ACCUMULATED OTHER COMPREHENSIVE INCOME	ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) The table below provides the balances for each classification of accumulated other comprehensive income (loss) at September 30, 2018 and December 31, 2017 (in thousands): September 30, December 31, Foreign currency translation adjustments, net of tax (1) $ (106,638 ) $ (15,700 ) Net unrealized gains on marketable securities, net of tax: Net unrealized gains on marketable equity securities, net of tax (2) — 241,088 Net unrealized (losses) gains on marketable debt securities, net of tax (3) (127,168 ) 11,594 Accumulated other comprehensive income (loss) $ (233,806 ) $ 236,982 (1) Foreign currency translation adjustments, net of tax, at September 30, 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 $79.4 million after tax ( $89.5 million before tax) and $190.4 million after tax ( $237.2 million before tax) at September 30, 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 non-U.S. Dollar denominated net assets. During the three and nine months ended September 30, 2018 , the Company recorded deferred tax benefits of $5.7 million and $16.2 million , respectively, 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 not subject to income 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 losses before tax on marketable debt securities of $223.8 million and $85.3 million at September 30, 2018 and December 31, 2017 , respectively, were not subject to income tax in the Netherlands. Unrealized gains before tax of $129.3 million and $129.8 million at September 30, 2018 and December 31, 2017 , respectively, were taxable at a 25% tax rate in the Netherlands, resulting in tax charges of $32.2 million and $32.4 million , respectively. The Company also recorded U.S. tax charges of $0.2 million and $0.5 million at September 30, 2018 and December 31, 2017 , respectively, related to these investments. The remaining net unrealized losses on marketable debt securities and related tax benefit at September 30, 2018 were associated with marketable debt securities held by a U.S. subsidiary.

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES

BASIS OF PRESENTATION (Policies

BASIS OF PRESENTATION (Policies)	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Accounting, Policy	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, 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 (loss) " 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.
Reclassification, Policy	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" or "General and administrative" expenses in its Unaudited Consolidated Statement of Operations for the three and nine months ended September 30, 2017 to conform to the current period presentation. The change in presentation and the reclassification for the three and nine months ended September 30, 2017 had no impact on operating income or net income and are summarized below (in thousands): Previously Reported Three Months Ended Nine Months Ended Cost of revenues $ 59,476 $ 217,387 Performance advertising 1,224,345 3,352,707 Brand advertising 112,796 306,995 Sales and marketing 165,539 411,309 General and administrative 142,823 420,004 Current Presentation Three Months Ended Nine Months Ended Cost of revenues $ 54,181 $ 202,007 Performance marketing 1,231,074 3,364,589 Brand marketing 125,877 337,016 Sales and other expenses 151,024 382,538 General and administrative 142,823 422,252 Balance Sheet Reclassification In the second quarter of 2018, the Company changed the presentation of "Long-term investments" to include investments in private companies, which were previously included in "Other assets" in the Unaudited Consolidated Balance Sheets. Therefore, the Company reclassified $450.9 million for investments in private companies to conform its Consolidated Balance Sheet at December 31, 2017 to this current period presentation. See the section "Long-term Equity Investments without Readily Determinable Fair Value" within Note 5 for more detail.
Cash and Cash Equivalents, Restricted Cash and Cash Equivalents, Policy	Restricted Cash and Cash Equivalents: Restricted cash and cash equivalents at September 30, 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): September 30, December 31, As included in the Unaudited Consolidated Balance Sheets: Cash and cash equivalents $ 2,973,096 $ 2,541,604 Restricted cash and cash equivalents included in prepaid expenses and other current assets 21,376 21,737 Total cash, cash equivalents and restricted cash and cash equivalents as shown in the Unaudited Consolidated Statements of Cash Flows $ 2,994,472 $ 2,563,341
Recent Accounting Pronouncements Adopted	Recent Accounting Pronouncements Adopted Premium Amortization on Purchased Callable Debt Securities In March 2017, the Financial Accounting Standards Board (“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, 2018, including interim periods within those fiscal years, with early adoption allowed. The Company early adopted this new standard in the third quarter of 2018. The adoption of this update did not have an impact to the Unaudited Consolidated Financial Statements. Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued a new accounting update to make targeted improvement to the disclosure requirement for fair value measurements as part of its disclosure framework project. This update eliminates, adds and modifies certain disclosure requirements primarily related to Level 3 fair value measurements. For public business entities, this update is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years, with early adoption allowed. The Company early adopted this new standard in the third quarter of 2018. The adoption of this update did not have an impact to the Unaudited Consolidated Financial Statements. Improvements to Non-employee Share-Based Payment Accounting In June 2018, the FASB issued a new accounting update which amends the guidance on share-based payments granted to non-employees for goods and services to align it with the guidance for share-based payments to employees. Under this new guidance, share-based awards to non-employees will be generally measured at fair value on the grant date of the awards and entities will need to assess the probability of satisfying performance conditions, if any are present, to determine expense to be recognized. This update is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years, with early adoption allowed. The Company early adopted this new standard in the second quarter of 2018 and applied this update as of January 1, 2018. The adoption of this update did not have a material impact to the Unaudited Consolidated Financial Statements. Recognition and Measurement of Financial Instruments In January 2016, the 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 investments in marketable 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	Other Recent Accounting Pronouncements Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In August 2018, the FASB issued a new accounting update to address a customer's accounting for implementation costs incurred in a cloud computing arrangement that is a service contract and also adds certain disclosure requirements related to implementation costs incurred for internal-use software and cloud computing arrangements. The amendment aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). For public business entities, this update is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years, with early adoption allowed. The Company plans to adopt this update on January 1, 2019 and apply it on a prospective basis. The Company does not expect a material impact to the Unaudited Consolidated Financial Statements resulting from the adoption. 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 rights and obligations created by entering into a lease transaction. The new standard retains the dual-model concept by requiring entities to determine if a lease is an operating or financing lease. 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 using a modified retrospective approach applied to the earliest comparative period in the financial statements. In July 2018, the FASB approved a new transition method that would permit issuers to apply the standard as of January 1, 2019. Early adoption is permitted. The Company will adopt this standard in the first quarter of 2019 and apply it as of January 1, 2019. The Company is in the process of implementing a software platform to facilitate compliance with the new accounting and disclosure requirements. The Company will elect to use its previous evaluations regarding if an arrangement contains a lease, if a lease is an operating or financing lease and what costs are capitalized as initial direct costs prior to adoption, as permitted under this standard. The most significant change will be related to the recognition of right-of-use assets and lease liabilities in the Company's Consolidated Balance Sheet for real estate operating leases. The Company will elect to combine lease and non-lease components and to include leases which have an initial term of less than one year in the calculation of the lease liability and right-of-use assets for its real estate leases. The Company performed an assessment of its real estate leases, which comprises a substantial portion of its total operating leases, and estimates that the lease liability and corresponding right-of-use asset would be approximately $450 million to $550 million at January 1, 2019, based on outstanding leases, incremental borrowing rates and foreign exchange rates in effect as of September 30, 2018. This estimate could change significantly by the adoption date due to several factors, including changes in the Company’s real estate lease portfolio, changes in foreign exchange rates and/or changes in the interest rate environment. The Company is in the process of reviewing its remaining operating leases, including its data center leases. The Company does not expect a material impact to its Consolidated Statement of Operations and Statement of Cash Flows resulting from the adoption.

REVENUE RECOGNITION (Policies)

REVENUE RECOGNITION (Policies)	9 Months Ended
	Sep. 30, 2018
Revenue from Contract with Customer [Abstract]
Revenue Recognition, Policy	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 (e.g., 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, in certain merchant transactions, the travelers. 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 receive 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 system ("GDS") reservation booking fees and certain travel insurance fees. • Merchant revenues are derived from services where the Company receives 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 travel insurance-related revenues, credit card processing rebates 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. Pursuant to the terms of the Company's merchant services, 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 provider within the specified period of time, the Company increases its revenue by the unbilled amount. Tax Recovery Charge, Occupancy Taxes and State and Local Taxes For merchant transactions, the Company charges the traveler an amount intended to cover the taxes that the Company anticipates the travel service provider will remit to the local taxing authorities ("tax recovery charge"). Tax rate information for calculating the tax recovery charge is provided to the Company by the travel service providers. In certain taxing jurisdictions, the Company is required by statute, regulation or court order to collect and remit certain local occupancy tax, general excise and/or sales tax (“transaction-related taxes”) imposed upon its margin and/or service fees. In other taxing jurisdictions, the Company is required to collect from the traveler and remit directly to the taxing jurisdiction transaction-related taxes imposed on the full amount of the transaction, which includes taxes on the margin, service fees and the underlying rate provided by the travel service provider. The rate information for calculating these taxes is provided to the Company directly from the taxing jurisdictions. The taxes collected from travelers are reported on a net basis in Revenues in the Unaudited Consolidated Statements of Operations. 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 their websites and by Booking.com for its 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 September 30, 2018 and December 31, 2017 , liabilities of $80.0 million and $104.7 million , respectively, for loyalty incentives were included in "Accrued expenses and other current liabilities" in the Unaudited Consolidated Balance Sheets.

BASIS OF PRESENTATION (Tables)

BASIS OF PRESENTATION (Tables)	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Reclassifications	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" or "General and administrative" expenses in its Unaudited Consolidated Statement of Operations for the three and nine months ended September 30, 2017 to conform to the current period presentation. The change in presentation and the reclassification for the three and nine months ended September 30, 2017 had no impact on operating income or net income and are summarized below (in thousands): Previously Reported Three Months Ended Nine Months Ended Cost of revenues $ 59,476 $ 217,387 Performance advertising 1,224,345 3,352,707 Brand advertising 112,796 306,995 Sales and marketing 165,539 411,309 General and administrative 142,823 420,004 Current Presentation Three Months Ended Nine Months Ended Cost of revenues $ 54,181 $ 202,007 Performance marketing 1,231,074 3,364,589 Brand marketing 125,877 337,016 Sales and other expenses 151,024 382,538 General and administrative 142,823 422,252
Restrictions on Cash and Cash Equivalents	Restricted Cash and Cash Equivalents: Restricted cash and cash equivalents at September 30, 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): September 30, December 31, As included in the Unaudited Consolidated Balance Sheets: Cash and cash equivalents $ 2,973,096 $ 2,541,604 Restricted cash and cash equivalents included in prepaid expenses and other current assets 21,376 21,737 Total cash, cash equivalents and restricted cash and cash equivalents as shown in the Unaudited Consolidated Statements of Cash Flows $ 2,994,472 $ 2,563,341

REVENUE RECOGNITION (Tables)

REVENUE RECOGNITION (Tables)	9 Months Ended
	Sep. 30, 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 at September 30, 2018 : As reported (current revenue standard) Current period adjustments As adjusted (previous revenue standard) ASSETS Current assets: Accounts receivable, net $ 1,719,059 $ (93,439 ) $ 1,625,620 Prepaid expenses and other current assets 617,616 18,563 636,179 LIABILITIES AND STOCKHOLDERS' EQUITY Current liabilities: Accounts payable $ 1,174,599 $ (70,609 ) $ 1,103,990 Accrued expenses and other current liabilities 1,654,370 (1,108 ) 1,653,262 Deferred merchant bookings 1,101,255 88,594 1,189,849 Deferred income taxes 491,160 (95 ) 491,065 Stockholders' equity: Retained earnings 17,720,656 (94,781 ) 17,625,875 Accumulated other comprehensive income (loss) (233,806 ) 3,123 (230,683 ) Unaudited Consolidated Statements of Operations for the Three Months Ended September 30, 2018 : As reported (current revenue standard) Current period adjustments As adjusted (previous revenue standard) Agency revenues $ 3,540,874 $ 55,148 $ 3,596,022 Merchant revenues 1,049,661 48,712 1,098,373 Advertising and other revenues 258,555 (6 ) 258,549 Cost of revenues 44,936 44,936 Operating expenses: Performance marketing 1,314,055 3,250 1,317,305 Sales and other expenses 242,974 206 243,180 General and administrative 183,228 86 183,314 Foreign currency transactions and other (17,072 ) 951 (16,121 ) Income tax expense 473,268 11,742 485,010 Net income 1,767,578 44,585 1,812,163 Net income applicable to common stockholders per basic common share 37.39 0.95 38.34 Net income applicable to common stockholders per diluted common share 37.02 0.93 37.95 Unaudited Consolidated Statements of Operations for the Nine Months Ended September 30, 2018 : As reported (current revenue standard) Current period adjustments As adjusted (previous revenue standard) Agency revenues $ 8,220,506 $ 107,826 $ 8,328,332 Merchant revenues 2,285,992 150,389 2,436,381 Advertising and other revenues 807,887 98 807,985 Cost of revenues 134,886 134,886 Operating expenses: Performance marketing 3,562,155 4,254 3,566,409 Sales and other expenses 612,367 206 612,573 General and administrative 504,120 1,120 505,240 Foreign currency transactions and other (40,174 ) 1,740 (38,434 ) Income tax expense 850,934 25,829 876,763 Net income 3,352,160 93,758 (1) 3,445,918 Net income applicable to common stockholders per basic common share 70.00 1.96 71.96 Net income applicable to common stockholders per diluted common share 69.07 1.94 71.01 (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 $94.8 million that would have been recognized in the fourth quarter of 2018 under the previous revenue standard.
Geographic Information	The Company's geographic information is as follows (in thousands): International Total revenues for the Three Months Ended September 30, United States The Netherlands Other Total 2018 $ 441,470 (1) $ 3,888,255 $ 519,365 $ 4,849,090 (1) 2017 429,587 (2) 3,533,906 470,536 4,434,029 (2) (1) Total revenues have been reduced for cost of revenues for Name Your Own Price ® transactions of $44.9 million in 2018. (2) Total revenues have not been reduced for cost of revenues for Name Your Own Price ® transactions of $54.2 million in 2017. International Total revenues for the Nine Months Ended September 30, United States The Netherlands Other Total 2018 $ 1,243,428 (1) $ 8,717,681 $ 1,353,276 $ 11,314,385 (1) 2017 1,251,079 (2) 7,499,464 1,127,446 9,877,989 (2) (1) Total revenues have been reduced for cost of revenues for Name Your Own Price ® transactions of $134.9 million in 2018. (2) Total revenues have not been reduced for cost of revenues for Name Your Own Price ® transactions of $202.0 million

STOCK-BASED EMPLOYEE COMPENSA_2

STOCK-BASED EMPLOYEE COMPENSATION (Tables)	9 Months Ended
	Sep. 30, 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 nine months ended September 30, 2018 : Share-Based Awards Shares Weighted-Average Grant Date Fair Value Unvested at December 31, 2017 524,696 $ 1,431.88 Granted 156,896 $ 2,032.81 Vested (194,461 ) $ 1,288.07 Performance Shares Adjustment 15,338 $ 1,657.61 Forfeited/Canceled (33,396 ) $ 1,690.38 Unvested at September 30, 2018 469,073 $ 1,681.48
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 nine months ended September 30, 2018 : Employee Stock Options Number of Shares Weighted-Average Aggregate Intrinsic Value (in thousands) Weighted-Average Remaining Contractual Term Balance, December 31, 2017 30,675 $ 401.61 $ 40,986 3.9 Exercised (3,300 ) $ 493.87 Forfeited (13 ) $ 241.83 Balance, September 30, 2018 27,362 $ 390.64 $ 43,598 3.0 Vested and exercisable at September 30, 2018 27,289 $ 399.10 $ 43,541 3.0 Vested and exercisable at September 30, 2018 and expected to vest thereafter 27,362 $ 390.64 $ 43,598 3.0

NET INCOME PER SHARE (Tables)

NET INCOME PER SHARE (Tables)	9 Months Ended
	Sep. 30, 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 Nine Months Ended 2018 2017 2018 2017 Weighted-average number of basic common shares outstanding 47,268 48,981 47,887 49,100 Weighted-average dilutive stock options, restricted stock units and performance share units 230 273 270 278 Assumed conversion of Convertible Senior Notes 253 718 373 686 Weighted-average number of diluted common and common equivalent shares outstanding 47,751 49,972 48,530 50,064 Anti-dilutive potential common shares 1,428 1,948 1,366 2,006

INVESTMENTS (Tables)

INVESTMENTS (Tables)	9 Months Ended
	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Investments	The following table summarizes, by major security type, the Company's investments in marketable securities at December 31, 2017 (in thousands): Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Short-term investments in marketable securities: 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 $ 4,866,475 $ 700 $ (7,302 ) $ 4,859,873 Long-term investments in marketable securities: 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 Investments in Ctrip: Convertible debt securities 1,275,000 103,100 (9,600 ) 1,368,500 Equity securities 655,311 299,697 (1,012 ) 953,996 Total $ 10,072,468 $ 412,786 $ (63,654 ) $ 10,421,600 The following table summarizes, by major security type, the Company's investments in marketable securities at September 30, 2018 (in thousands): Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Short-term investments in marketable securities: Debt securities: International government securities $ 389,789 $ 6 $ (523 ) $ 389,272 U.S. government securities 809,241 — (3,856 ) 805,385 Corporate debt securities 2,952,065 426 (11,561 ) 2,940,930 U.S. government agency securities 4,916 — (7 ) 4,909 Commercial paper 17,080 — — 17,080 Certificate of deposit 500 — — 500 Total $ 4,173,591 $ 432 $ (15,947 ) $ 4,158,076 Long-term investments in marketable securities: Debt securities: International government securities $ 790,653 $ 1,205 $ (986 ) $ 790,872 U.S. government securities 370,987 — (10,053 ) 360,934 Corporate debt securities 4,747,084 4,817 (59,563 ) 4,692,338 Investments in Ctrip: Convertible debt securities 1,275,000 21,500 (38,668 ) 1,257,832 Equity securities 655,311 153,553 (4,781 ) 804,083 Meituan Dianping equity securities 450,270 257,134 — 707,404 Total $ 8,289,305 $ 438,209 $ (114,051 ) $ 8,613,463

FAIR VALUE MEASUREMENTS (Tables

FAIR VALUE MEASUREMENTS (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Financial instruments carried at fair value	Financial assets and liabilities carried at fair value at September 30, 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 $ 2,385,451 $ — $ 2,385,451 International government securities — 6,883 6,883 U.S. government securities — 2,009 2,009 Commercial paper — 4,372 4,372 Time deposits 4,519 — 4,519 Short-term investments in marketable securities: International government securities — 389,272 389,272 U.S. government securities — 805,385 805,385 Corporate debt securities — 2,940,930 2,940,930 U.S. government agency securities — 4,909 4,909 Commercial paper — 17,080 17,080 Certificate of deposit 500 — 500 Long-term investments in marketable securities: International government securities — 790,872 790,872 U.S. government securities — 360,934 360,934 Corporate debt securities — 4,692,338 4,692,338 Ctrip convertible debt securities — 1,257,832 1,257,832 Ctrip equity securities 804,083 — 804,083 Meituan Dianping equity securities 707,404 — 707,404 Derivatives: Currency exchange derivatives — 914 914 Total assets at fair value $ 3,901,957 $ 11,273,730 $ 15,175,687 Level 1 Level 2 Total LIABILITIES: Currency exchange derivatives $ — $ 1,617 $ 1,617 Financial assets and liabilities carried at fair value at 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 in marketable securities: 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 in marketable securities: 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

INTANGIBLE ASSETS AND GOODWILL (Tables)	9 Months Ended
	Sep. 30, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible assets	The Company's intangible assets at September 30, 2018 and December 31, 2017 consisted of the following (in thousands): September 30, 2018 December 31, 2017 Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Amortization Weighted- Supply and distribution agreements $ 1,081,543 $ (396,584 ) $ 684,959 $ 1,056,660 $ (355,000 ) $ 701,660 3 - 20 years 16 years Technology 171,419 (118,017 ) 53,402 137,288 (104,478 ) 32,810 1 - 7 years 5 years Patents 1,623 (1,623 ) — 1,623 (1,623 ) — 15 years 15 years Internet domain names 40,903 (29,767 ) 11,136 42,265 (28,802 ) 13,463 5 - 20 years 8 years Trade names 1,785,284 (417,600 ) 1,367,684 1,779,076 (350,447 ) 1,428,629 4-20 years 19 years Non-compete agreements 900 (806 ) 94 21,900 (21,639 ) 261 4 years 4 years Total intangible assets $ 3,081,672 $ (964,397 ) $ 2,117,275 $ 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 $ 42,591 2019 168,473 2020 160,708 2021 154,745 2022 152,006 2023 150,007 Thereafter 1,288,745 $ 2,117,275
Goodwill	A roll-forward of goodwill for the nine months ended September 30, 2018 consisted of the following (in thousands): Balance at December 31, 2017 $ 2,737,671 Acquisition 131,583 Currency translation adjustments (24,125 ) Balance at September 30, 2018 $ 2,845,129

DEBT (Tables)

DEBT (Tables)	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Schedule of Debt	Outstanding Debt Outstanding debt at September 30, 2018 consisted of the following (in thousands): September 30, 2018 Outstanding Principal Amount Unamortized Debt Discount and Debt Issuance Cost Carrying Value Long-term debt: 0.35% Convertible Senior Notes due June 2020 $ 999,950 $ (45,225 ) $ 954,725 0.9% Convertible Senior Notes due September 2021 1,000,000 (66,841 ) 933,159 0.8% (€1 Billion) Senior Notes due March 2022 1,161,500 (5,073 ) 1,156,427 2.15% (€750 Million) Senior Notes due November 2022 871,125 (3,927 ) 867,198 2.75% Senior Notes due March 2023 500,000 (2,747 ) 497,253 2.375% (€1 Billion) Senior Notes due September 2024 1,161,500 (10,709 ) 1,150,791 3.65% Senior Notes due March 2025 500,000 (2,959 ) 497,041 3.6% Senior Notes due June 2026 1,000,000 (6,250 ) 993,750 1.8% (€1 Billion) Senior Notes due March 2027 1,161,500 (4,709 ) 1,156,791 3.55% Senior Notes due March 2028 500,000 (3,234 ) 496,766 Total long-term debt $ 8,855,575 $ (151,674 ) $ 8,703,901 Outstanding debt at 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 COMPREHENSI_2

ACCUMULATED OTHER COMPREHENSIVE INCOME (Tables)	9 Months Ended
	Sep. 30, 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 (loss) at September 30, 2018 and December 31, 2017 (in thousands): September 30, December 31, Foreign currency translation adjustments, net of tax (1) $ (106,638 ) $ (15,700 ) Net unrealized gains on marketable securities, net of tax: Net unrealized gains on marketable equity securities, net of tax (2) — 241,088 Net unrealized (losses) gains on marketable debt securities, net of tax (3) (127,168 ) 11,594 Accumulated other comprehensive income (loss) $ (233,806 ) $ 236,982 (1) Foreign currency translation adjustments, net of tax, at September 30, 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 $79.4 million after tax ( $89.5 million before tax) and $190.4 million after tax ( $237.2 million before tax) at September 30, 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 non-U.S. Dollar denominated net assets. During the three and nine months ended September 30, 2018 , the Company recorded deferred tax benefits of $5.7 million and $16.2 million , respectively, 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 not subject to income 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 losses before tax on marketable debt securities of $223.8 million and $85.3 million at September 30, 2018 and December 31, 2017 , respectively, were not subject to income tax in the Netherlands. Unrealized gains before tax of $129.3 million and $129.8 million at September 30, 2018 and December 31, 2017 , respectively, were taxable at a 25% tax rate in the Netherlands, resulting in tax charges of $32.2 million and $32.4 million , respectively. The Company also recorded U.S. tax charges of $0.2 million and $0.5 million at September 30, 2018 and December 31, 2017 , respectively, related to these investments. The remaining net unrealized losses on marketable debt securities and related tax benefit at September 30, 2018 were associated with marketable debt securities held by a U.S. subsidiary.

BASIS OF PRESENTATION Basis of

BASIS OF PRESENTATION Basis of Presentation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2019	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Cost of revenues		$ 54,181		$ 202,007
Performance marketing	$ 1,314,055	1,231,074	$ 3,562,155	3,364,589
Brand marketing	160,126	125,877	385,262	337,016
Sales and other expenses	242,974	151,024	612,367	382,538
General and administrative	183,228	142,823	504,120	422,252
Equity securities without readily determinable fair value reclassified from other assets to long-term investments							$ 450,900
Cash and cash equivalents	2,973,096		2,973,096				2,541,604
Restricted cash included in prepaid expenses and other current assets	21,376		21,376				21,737
Total cash, cash equivalents and restricted cash as shown in the Unaudited Consolidated Statements of Cash Flows	2,994,472	2,847,871	2,994,472	2,847,871			2,563,341	$ 2,082,007
Cumulative effect of adoption of accounting standard updates							188,539	299,305
Retained earnings	17,720,656		17,720,656			$ 14,127,408	13,938,869
Retained Earnings
Cumulative effect of adoption of accounting standard updates							429,627	$ 290,317
Accumulated Other Comprehensive Income (Loss)
Cumulative effect of adoption of accounting standard updates							$ (241,088)
Accounting Standards Update 2016-01 | Retained Earnings
Cumulative effect of adoption of accounting standard updates						241,100
Accounting Standards Update 2016-01 | Accumulated Other Comprehensive Income (Loss)
Cumulative effect of adoption of accounting standard updates						(241,088)
Accounting Standards Update 2016-02 | Minimum | Forecast
Right-of-use asset under operating lease					$ 450,000
Lease liability under operating lease					450,000
Accounting Standards Update 2016-02 | Maximum | Forecast
Right-of-use asset under operating lease					550,000
Lease liability under operating lease					$ 550,000
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Cost of revenues	44,936		134,886
Performance marketing	3,250		4,254
Sales and other expenses	206		206
General and administrative	86		1,120
Retained earnings	$ (94,781)		$ (94,781)			$ 188,539
Previously Reported
Cost of revenues		59,476		217,387
Performance advertising		1,224,345		3,352,707
Brand advertising		112,796		306,995
Sales and marketing		165,539		411,309
General and administrative		$ 142,823		$ 420,004

REVENUE RECOGNITION - Narrative

REVENUE RECOGNITION - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Retained earnings	$ 17,720,656		$ 17,720,656		$ 14,127,408	$ 13,938,869
Reduction in customer incentive liability			27,000
Customer incentive liability	80,000		80,000			104,700
Revenues	4,849,090	$ 4,434,029	11,314,385	$ 9,877,989
Deferred revenue for merchant revenue transactions	176,800		176,800			$ 151,200
Revenue recognized related to the deferred revenue balance			107,300
Deferred Revenue, Refund Payments			10,300
Cash payments received from travelers reservations			185,500
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	$ (94,781)		$ (94,781)		188,539
Deferred revenue for merchant revenue transactions					$ (32,400)
Sales Revenue, Net [Member] | Product Concentration Risk [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Concentration Risk, Percentage	88.00%		87.00%

REVENUE RECOGNITION - Cumulativ

REVENUE RECOGNITION - Cumulative Effect of Revenue Accounting Changes on the Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Current assets:
Accounts receivable, net	$ 1,719,059	$ 1,423,125	$ 1,217,801
Current liabilities:
Accounts payable	1,174,599	839,167	667,523
Accrued expenses and other current liabilities	1,654,370	1,183,354	1,138,980
Deferred merchant bookings	1,101,255	778,808	980,455
Deferred income taxes	491,160	483,553	481,139
Stockholders' equity:
Retained earnings	17,720,656	14,127,408	13,938,869
Adjustments | Accounting Standards Update 2014-09
Current assets:
Accounts receivable, net	(93,439)	205,324
Current liabilities:
Accounts payable	(70,609)	171,644
Accrued expenses and other current liabilities	(1,108)	44,374
Deferred merchant bookings	88,594	(201,647)
Deferred income taxes	(95)	2,414
Stockholders' equity:
Retained earnings	(94,781)	$ 188,539
As Adjusted (Previous Revenue Standard)
Current assets:
Accounts receivable, net	1,625,620		1,217,801
Current liabilities:
Accounts payable	1,103,990		667,523
Accrued expenses and other current liabilities	1,653,262		1,138,980
Deferred merchant bookings	1,189,849		980,455
Deferred income taxes	491,065		481,139
Stockholders' equity:
Retained earnings	$ 17,625,875		$ 13,938,869

REVENUE RECOGNITION - Impacts o

REVENUE RECOGNITION - Impacts of Adopting Current Revenue Standard (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Current assets:
Accounts receivable, net	$ 1,719,059		$ 1,719,059		$ 1,423,125	$ 1,217,801
Prepaid expenses and other current assets	617,616		617,616			415,527
Current liabilities:
Accounts payable	1,174,599		1,174,599		839,167	667,523
Accrued expenses and other current liabilities	1,654,370		1,654,370		1,183,354	1,138,980
Deferred merchant bookings	1,101,255		1,101,255		778,808	980,455
Deferred income taxes	491,160		491,160		483,553	481,139
Stockholders' equity:
Retained earnings	17,720,656		17,720,656		14,127,408	13,938,869
Accumulated other comprehensive income (loss)	(233,806)		(233,806)			236,982
Consolidated Statements of Operations
Agency revenues	3,540,874		8,220,506
Agency revenues		$ 3,523,706		$ 7,641,390
Merchant revenues	1,049,661		2,285,992
Merchant revenues		684,289		1,624,467
Advertising and other revenues	258,555	226,034	807,887	612,132
Cost of revenues		54,181		202,007
Operating expenses:
Performance marketing	1,314,055	1,231,074	3,562,155	3,364,589
Sales and other expenses	242,974	151,024	612,367	382,538
General and administrative	183,228	142,823	504,120	422,252
Foreign currency transactions and other	(17,072)	(10,101)	(40,174)	(21,249)
Income tax expense	473,268	346,454	850,934	561,349
Net income	$ 1,767,578	$ 1,720,391	$ 3,352,160	$ 2,896,223
Net income applicable to common stockholders per basic common share	$ 37.39	$ 35.12	$ 70	$ 58.99
Net income applicable to common stockholders per diluted common share	$ 37.02	$ 34.43	$ 69.07	$ 57.85
Adjustments | Accounting Standards Update 2014-09
Current assets:
Accounts receivable, net	$ (93,439)		$ (93,439)		205,324
Prepaid expenses and other current assets	18,563		18,563
Current liabilities:
Accounts payable	(70,609)		(70,609)		171,644
Accrued expenses and other current liabilities	(1,108)		(1,108)		44,374
Deferred merchant bookings	88,594		88,594		(201,647)
Deferred income taxes	(95)		(95)		2,414
Stockholders' equity:
Retained earnings	(94,781)		(94,781)		$ 188,539
Accumulated other comprehensive income (loss)	3,123		3,123
Consolidated Statements of Operations
Agency revenues	55,148		107,826
Merchant revenues	48,712		150,389
Advertising and other revenues	(6)		98
Cost of revenues	44,936		134,886
Operating expenses:
Performance marketing	3,250		4,254
Sales and other expenses	206		206
General and administrative	86		1,120
Foreign currency transactions and other	951		1,740
Income tax expense	11,742		25,829
Net income	$ 44,585		$ 93,758
Net income applicable to common stockholders per basic common share	$ 0.95		$ 1.96
Net income applicable to common stockholders per diluted common share	$ 0.93		$ 1.94
As Adjusted (Previous Revenue Standard)
Current assets:
Accounts receivable, net	$ 1,625,620		$ 1,625,620			1,217,801
Prepaid expenses and other current assets	636,179		636,179
Current liabilities:
Accounts payable	1,103,990		1,103,990			667,523
Accrued expenses and other current liabilities	1,653,262		1,653,262			1,138,980
Deferred merchant bookings	1,189,849		1,189,849			980,455
Deferred income taxes	491,065		491,065			481,139
Stockholders' equity:
Retained earnings	17,625,875		17,625,875			$ 13,938,869
Accumulated other comprehensive income (loss)	(230,683)		(230,683)
Consolidated Statements of Operations
Agency revenues	3,596,022		8,328,332
Merchant revenues	1,098,373		2,436,381
Advertising and other revenues	258,549		807,985
Cost of revenues	44,936		134,886
Operating expenses:
Performance marketing	1,317,305		3,566,409
Sales and other expenses	243,180		612,573
General and administrative	183,314		505,240
Foreign currency transactions and other	(16,121)		(38,434)
Income tax expense	485,010		876,763
Net income	$ 1,812,163		$ 3,445,918
Net income applicable to common stockholders per basic common share	$ 38.34		$ 71.96
Net income applicable to common stockholders per diluted common share	$ 37.95		$ 71.01

REVENUE RECOGNITION - Geographi

REVENUE RECOGNITION - Geographic Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 4,849,090	$ 4,434,029	$ 11,314,385	$ 9,877,989
Cost of revenues		54,181		202,007
United States
Disaggregation of Revenue [Line Items]
Revenues	441,470	429,587	1,243,428	1,251,079
The Netherlands
Disaggregation of Revenue [Line Items]
Revenues	3,888,255	3,533,906	8,717,681	7,499,464
Other
Disaggregation of Revenue [Line Items]
Revenues	$ 519,365	$ 470,536	$ 1,353,276	$ 1,127,446

STOCK-BASED EMPLOYEE COMPENSA_3

STOCK-BASED EMPLOYEE COMPENSATION (Stock-based Employee Compensation) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-based compensation expense	$ 70,090	$ 66,421	$ 216,035	$ 192,248

STOCK-BASED EMPLOYEE COMPENSA_4

STOCK-BASED EMPLOYEE COMPENSATION (Restricted Stock Units and Performance Share Units) (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 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)	156,896
Vested (in shares)	(194,461)
Performance Share Units Adjustment (in shares)	15,338
Forfeited (in shares)	(33,396)
Unvested at September 30, 2018	469,073	524,696
Share-Based Awards - Weighted Average Grant Date Fair Value
Unvested at December 31, 2017	$ 1,431.88
Granted (in dollars per share)	2,032.81
Vested (in dollars per share)	1,288.07
Performance Share Units Adjustment (in dollars per share)	1,657.61
Forfeited (in shares)	1,690.38
Unvested at September 30, 2018	$ 1,681.48	$ 1,431.88
Total unrecognized estimated compensation expense, unvested share-based awards	$ 418.8
Total future compensation cost related to unvested share-based awards, expected period of recognition	1 year 11 months
Restricted Stock Units (RSUs)
Share-Based Awards - Shares
Granted (in shares)	107,175
Share-Based Awards - Weighted Average Grant Date Fair Value
Granted (in dollars per share)	$ 2,032.36
Vesting period (in years)	3 years
Grant date fair value	$ 217.8
Performance Share Units 2018 Grants
Share-Based Awards - Shares
Granted (in shares)	49,721
Share-Based Awards - Weighted Average Grant Date Fair Value
Granted (in dollars per share)	$ 2,033.79
Grant date fair value	$ 101.1
Estimated number of probable shares to be issued (in shares)	47,575
Maximum shares that could be issued (in shares)	95,150
Minimum shares that could be issued (in shares)	36,265
Performance Share Units 2017 Grants
Share-Based Awards - Shares
Granted (in shares)		73,893
Unvested at September 30, 2018	63,265
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)	86,630
Maximum shares that could be issued (in shares)	125,622
Minimum shares that could be issued (in shares)	49,554
Performance Share Units 2016 Grants
Share-Based Awards - Shares
Granted (in shares)			85,735
Unvested at September 30, 2018	65,218
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)	105,037
Maximum shares that could be issued (in shares)	143,240
Minimum shares that could be issued (in shares)	39,418

STOCK-BASED EMPLOYEE COMPENSA_5

STOCK-BASED EMPLOYEE COMPENSATION (Stock Options) (Details) - Employee Stock Option [Member] - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Number of Shares
Beginning balance (in shares)	30,675
Exercised (in shares)	(3,300)
Forfeited (in shares)	(13)
Ending balance (in shares)	27,362		30,675
Weighted-Average Exercise Price
Beginning balance (in dollars per share)	$ 401.61
Exercised (in dollars per share)	493.87
Forfeited (in dollars per share)	241.83
Ending balance (in dollars per share)	$ 390.64		$ 401.61
Aggregate Intrinsic Value	$ 43,598		$ 40,986
Weighted Average Remaining Contractual Term	3 years		3 years 11 months
Vested and exercisable at September 30, 2018
Number of Shares	27,289
Weighted Average Exercise Price (in dollars per share)	$ 399.10
Aggregate Intrinsic Value	$ 43,541
Weighted Average Remaining Contractual Term, Exercisable	3 years
Vested and exercisable at September 30, 2018 and expected to vest thereafter
Number of Shares	27,362
Weighted Average Exercise Price (in dollars per share)	$ 390.64
Aggregate Intrinsic Value	$ 43,598
Weighted Average Remaining Contractual Term	3 years
Stock options exercised, total intrinsic value	$ 5,100	$ 21,400
Options, vested, number of shares (in shares)	98	1,246

NET INCOME PER SHARE (Details)

NET INCOME PER SHARE (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Weighted-average number of basic common shares outstanding	47,268	48,981	47,887	49,100
Weighted-average dilutive stock options, restricted stock units and performance share units (in shares)	230	273	270	278
Assumed conversion of Convertible Senior Notes (in shares)	253	718	373	686
Weighted-average number of diluted common and common equivalent shares outstanding	47,751	49,972	48,530	50,064
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	1,428	1,948	1,366	2,006
Convertible Debt [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive potential common shares	1,000		1,000

INVESTMENTS (Details)

INVESTMENTS (Details) - USD ($) $ in Thousands	Sep. 12, 2016	Dec. 11, 2015	May 26, 2015	Aug. 07, 2014	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Oct. 29, 2018	Jul. 17, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Cumulative effect of adoption of accounting standard updates												$ 188,539	$ 299,305
Net unrealized gains on marketable equity securities					$ 30,858	$ 0	$ 107,221	$ 0
Marketable securities:
Short-term investments in marketable securities					4,158,076		$ 4,158,076					4,859,873
Investments [Abstract]
Weighted average life of all fixed income investments, excluding the Company's investment in Ctrip convertible debt securities							1 year 2 months
Investment in equity securities					500,800		$ 500,800					450,900
Ctrip.com International, Ltd.
Investments [Abstract]
Maximum ownership percentage in Ctrip			15.00%
Meituan-Dianping
Investments [Abstract]
Investment in equity securities					450,000		450,000
Didi Chuxing
Investments [Abstract]
Investment in equity securities										$ 500,000
Short-term Investments
Marketable securities:
Cost					4,173,591		4,173,591
Gross Unrealized Gains					432		432
Gross Unrealized Losses					(15,947)		(15,947)
Short-term investments in marketable securities					4,158,076		4,158,076
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					389,789		389,789
Gross Unrealized Gains					6		6
Gross Unrealized Losses					(523)		(523)
Fair Value					389,272		389,272
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					809,241		809,241
Gross Unrealized Gains					0		0
Gross Unrealized Losses					(3,856)		(3,856)
Fair Value					805,385		805,385
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					2,952,065		2,952,065
Gross Unrealized Gains					426		426
Gross Unrealized Losses					(11,561)		(11,561)
Fair Value					2,940,930		2,940,930
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					4,916		4,916
Gross Unrealized Gains					0		0
Gross Unrealized Losses					(7)		(7)
Fair Value					4,909		4,909
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					17,080		17,080
Gross Unrealized Gains					0		0
Gross Unrealized Losses					0		0
Fair Value					17,080		17,080
Available-for-sale securities:
Cost												72,650
Gross Unrealized Gains												0
Gross Unrealized Losses												0
Fair Value												72,650
Short-term Investments | Certificate of deposit
Debt securities:
Cost					500		500
Gross Unrealized Gains					0		0
Gross Unrealized Losses					0		0
Fair Value					500		500
Long-term Investments
Marketable securities:
Cost					8,289,305		8,289,305
Gross Unrealized Gains					438,209		438,209
Gross Unrealized Losses					(114,051)		(114,051)
Long-term investments					8,613,463		8,613,463
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					790,653		790,653
Gross Unrealized Gains					1,205		1,205
Gross Unrealized Losses					(986)		(986)
Fair Value					790,872		790,872
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					370,987		370,987
Gross Unrealized Gains					0		0
Gross Unrealized Losses					(10,053)		(10,053)
Fair Value					360,934		360,934
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					4,747,084		4,747,084
Gross Unrealized Gains					4,817		4,817
Gross Unrealized Losses					(59,563)		(59,563)
Fair Value					4,692,338		4,692,338
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
Available-for-sale securities:
Cost												500
Gross Unrealized Gains												0
Gross Unrealized Losses												(6)
Fair Value												494
Long-term Investments | Convertible debt securities | Ctrip.com International, Ltd.
Debt securities:
Cost	$ 25,000	$ 500,000	$ 250,000	$ 500,000	1,275,000		1,275,000
Gross Unrealized Gains					21,500		21,500
Gross Unrealized Losses					(38,668)		(38,668)
Fair Value					1,257,832		1,257,832
Available-for-sale securities:
Cost												1,275,000
Gross Unrealized Gains												103,100
Gross Unrealized Losses												(9,600)
Fair Value												1,368,500
Investments [Abstract]
Debt investment term	6 years	10 years	5 years	5 years
Long-term Investments | Equity securities | Ctrip.com International, Ltd.
Investments in Equity securities:
Cost					655,311		655,311
Gross Unrealized Gains					153,553		153,553
Gross Unrealized Losses					(4,781)		(4,781)
Fair Value					804,083		804,083
Available-for-sale securities:
Cost												655,311
Gross Unrealized Gains												299,697
Gross Unrealized Losses												(1,012)
Fair Value												953,996
Long-term Investments | Equity securities | Meituan-Dianping
Investments in Equity securities:
Cost					450,270		450,270
Gross Unrealized Gains					257,134		257,134
Gross Unrealized Losses					0		0
Accumulated Other Comprehensive Income (Loss)
Schedule of Investments [Line Items]
Cumulative effect of adoption of accounting standard updates												(241,088)
Retained Earnings
Schedule of Investments [Line Items]
Cumulative effect of adoption of accounting standard updates												429,627	$ 290,317
Accounting Standards Update 2016-01 | Accumulated Other Comprehensive Income (Loss)
Schedule of Investments [Line Items]
Cumulative effect of adoption of accounting standard updates											$ (241,088)
Accounting Standards Update 2016-01 | Retained Earnings
Schedule of Investments [Line Items]
Net unrealized gains reclassified from AOCI to retained earnings, before tax											298,700
Cumulative effect of adoption of accounting standard updates											$ 241,100
Subsequent Event [Member] | Grab [Member]
Investments [Abstract]
Investment in equity securities									$ 200,000
Fair Value, Measurements, Recurring [Member]
Investments in Equity securities:
Assets, Fair Value Disclosure					15,175,687		15,175,687					17,321,668
Fair Value, Measurements, Recurring [Member] | Short-term Investments | International government securities
Investments in Equity securities:
Assets, Fair Value Disclosure					389,272		389,272					725,376
Fair Value, Measurements, Recurring [Member] | Short-term Investments | U.S. government securities
Investments in Equity securities:
Assets, Fair Value Disclosure					805,385		805,385					994,118
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Corporate debt securities
Investments in Equity securities:
Assets, Fair Value Disclosure					2,940,930		2,940,930					3,063,315
Fair Value, Measurements, Recurring [Member] | Short-term Investments | U.S. government agency securities
Investments in Equity securities:
Assets, Fair Value Disclosure					4,909		4,909					4,414
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Commercial paper
Investments in Equity securities:
Assets, Fair Value Disclosure					17,080		17,080					72,650
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Certificate of deposit
Investments in Equity securities:
Assets, Fair Value Disclosure					500		500
Fair Value, Measurements, Recurring [Member] | Long-term Investments | International government securities
Investments in Equity securities:
Assets, Fair Value Disclosure					790,872		790,872					607,910
Fair Value, Measurements, Recurring [Member] | Long-term Investments | U.S. government securities
Investments in Equity securities:
Assets, Fair Value Disclosure					360,934		360,934					834,276
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Corporate debt securities
Investments in Equity securities:
Assets, Fair Value Disclosure					4,692,338		4,692,338					6,656,424
Fair Value, Measurements, Recurring [Member] | Long-term Investments | U.S. government agency securities
Investments in Equity securities:
Assets, Fair Value Disclosure												494
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Convertible debt securities | Ctrip.com International, Ltd.
Investments in Equity securities:
Assets, Fair Value Disclosure					1,257,832		1,257,832					1,368,500
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Equity securities | Ctrip.com International, Ltd.
Investments in Equity securities:
Assets, Fair Value Disclosure					804,083		804,083					953,996
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Equity securities | Meituan-Dianping
Investments in Equity securities:
Assets, Fair Value Disclosure					707,404		707,404
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Investments in Equity securities:
Assets, Fair Value Disclosure					3,901,957		3,901,957					2,867,164
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Short-term Investments | International government securities
Investments in Equity securities:
Assets, Fair Value Disclosure					0		0					0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Short-term Investments | U.S. government securities
Investments in Equity securities:
Assets, Fair Value Disclosure					0		0					0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Short-term Investments | Corporate debt securities
Investments in Equity securities:
Assets, Fair Value Disclosure					0		0					0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Short-term Investments | U.S. government agency securities
Investments in Equity securities:
Assets, Fair Value Disclosure					0		0					0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Short-term Investments | Commercial paper
Investments in Equity securities:
Assets, Fair Value Disclosure					0		0					0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Short-term Investments | Certificate of deposit
Investments in Equity securities:
Assets, Fair Value Disclosure					500		500
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Long-term Investments | International government securities
Investments in Equity securities:
Assets, Fair Value Disclosure					0		0					0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Long-term Investments | U.S. government securities
Investments in Equity securities:
Assets, Fair Value Disclosure					0		0					0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Long-term Investments | Corporate debt securities
Investments in Equity securities:
Assets, Fair Value Disclosure					0		0					0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Long-term Investments | U.S. government agency securities
Investments in Equity securities:
Assets, Fair Value Disclosure												0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Long-term Investments | Convertible debt securities | Ctrip.com International, Ltd.
Investments in Equity securities:
Assets, Fair Value Disclosure					0		0					0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Long-term Investments | Equity securities | Ctrip.com International, Ltd.
Investments in Equity securities:
Assets, Fair Value Disclosure					804,083		804,083					$ 953,996
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Long-term Investments | Equity securities | Meituan-Dianping
Investments in Equity securities:
Assets, Fair Value Disclosure					$ 707,404		$ 707,404

FAIR VALUE MEASUREMENTS Fair va

FAIR VALUE MEASUREMENTS Fair value measurements (Details) - Recurring Basis - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
ASSETS:
Total assets at fair value	$ 15,175,687	$ 17,321,668
Foreign Currency Contracts
ASSETS:
Total assets at fair value	914	1,767
LIABILITIES:
Total liabilities at fair value	1,617	127
Money market funds | Cash Equivalents
ASSETS:
Total assets at fair value	2,385,451	1,895,272
International government securities | Cash Equivalents
ASSETS:
Total assets at fair value	6,883
International government securities | Short-term Investments
ASSETS:
Total assets at fair value	389,272	725,376
International government securities | Long-term Investments
ASSETS:
Total assets at fair value	790,872	607,910
U.S. government securities | Cash Equivalents
ASSETS:
Total assets at fair value	2,009	22,265
U.S. government securities | Short-term Investments
ASSETS:
Total assets at fair value	805,385	994,118
U.S. government securities | Long-term Investments
ASSETS:
Total assets at fair value	360,934	834,276
Commercial paper | Cash Equivalents
ASSETS:
Total assets at fair value	4,372	96,321
Commercial paper | Short-term Investments
ASSETS:
Total assets at fair value	17,080	72,650
Certificate of deposit | Short-term Investments
ASSETS:
Total assets at fair value	500
Time deposits | Cash Equivalents
ASSETS:
Total assets at fair value	4,519	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	2,940,930	3,063,315
Corporate debt securities | Long-term Investments
ASSETS:
Total assets at fair value	4,692,338	6,656,424
U.S. government agency securities | Short-term Investments
ASSETS:
Total assets at fair value	4,909	4,414
U.S. government agency securities | Long-term Investments
ASSETS:
Total assets at fair value		494
Level 1
ASSETS:
Total assets at fair value	3,901,957	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	2,385,451	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	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	0
Level 1 | Commercial paper | Short-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 | Certificate of deposit | Short-term Investments
ASSETS:
Total assets at fair value	500
Level 1 | Time deposits | Cash Equivalents
ASSETS:
Total assets at fair value	4,519	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
Level 2
ASSETS:
Total assets at fair value	11,273,730	14,454,504
Level 2 | Foreign Currency Contracts
ASSETS:
Total assets at fair value	914	1,767
LIABILITIES:
Total liabilities at fair value	1,617	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	6,883
Level 2 | International government securities | Short-term Investments
ASSETS:
Total assets at fair value	389,272	725,376
Level 2 | International government securities | Long-term Investments
ASSETS:
Total assets at fair value	790,872	607,910
Level 2 | U.S. government securities | Cash Equivalents
ASSETS:
Total assets at fair value	2,009	22,265
Level 2 | U.S. government securities | Short-term Investments
ASSETS:
Total assets at fair value	805,385	994,118
Level 2 | U.S. government securities | Long-term Investments
ASSETS:
Total assets at fair value	360,934	834,276
Level 2 | Commercial paper | Cash Equivalents
ASSETS:
Total assets at fair value	4,372	96,321
Level 2 | Commercial paper | Short-term Investments
ASSETS:
Total assets at fair value	17,080	72,650
Level 2 | Certificate of deposit | Short-term Investments
ASSETS:
Total assets at fair value	0
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	2,940,930	3,063,315
Level 2 | Corporate debt securities | Long-term Investments
ASSETS:
Total assets at fair value	4,692,338	6,656,424
Level 2 | U.S. government agency securities | Short-term Investments
ASSETS:
Total assets at fair value	4,909	4,414
Level 2 | U.S. government agency securities | Long-term Investments
ASSETS:
Total assets at fair value		494
Ctrip.com International, Ltd. | Convertible debt securities | Long-term Investments
ASSETS:
Total assets at fair value	1,257,832	1,368,500
Ctrip.com International, Ltd. | Equity securities | Long-term Investments
ASSETS:
Total assets at fair value	804,083	953,996
Ctrip.com International, Ltd. | Level 1 | Convertible debt securities | Long-term Investments
ASSETS:
Total assets at fair value	0	0
Ctrip.com International, Ltd. | Level 1 | Equity securities | Long-term Investments
ASSETS:
Total assets at fair value	804,083	953,996
Ctrip.com International, Ltd. | Level 2 | Convertible debt securities | Long-term Investments
ASSETS:
Total assets at fair value	1,257,832	1,368,500
Ctrip.com International, Ltd. | Level 2 | Equity securities | Long-term Investments
ASSETS:
Total assets at fair value	0	$ 0
Meituan-Dianping [Member] | Equity securities | Long-term Investments
ASSETS:
Total assets at fair value	707,404
Meituan-Dianping [Member] | Level 1 | Equity securities | Long-term Investments
ASSETS:
Total assets at fair value	707,404
Meituan-Dianping [Member] | Level 2 | Equity securities | Long-term Investments
ASSETS:
Total assets at fair value	$ 0

FAIR VALUE MEASUREMENTS Equity

FAIR VALUE MEASUREMENTS Equity investments without readily determinable fair values and derivatives (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Sep. 12, 2016	Dec. 11, 2015	May 26, 2015	Aug. 07, 2014
Fair Value Disclosures [Abstract]
Investment in equity securities	$ 500,800		$ 500,800		$ 450,900
Derivatives Not Designated as Hedging Instruments
Foreign currency gains (losses), net of derivative activity	(15,100)	$ (7,600)	(32,300)	$ (18,700)
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		0
Foreign Currency Contracts, Transaction Risk
Derivatives Not Designated as Hedging Instruments
Foreign currency gains (losses) recorded in foreign currency transactions and other	(8,600)	$ 10,700	(38,200)	38,300
Foreign Currency Contracts
Derivatives Not Designated as Hedging Instruments
Payments for Derivative Instrument Operating Activities			35,600
Net cash inflow from settlement of derivative contracts included in operating activities				$ 38,200
Ctrip.com International, Ltd. | Convertible debt securities | Long-term Investments
Derivatives Not Designated as Hedging Instruments
Cost of Ctrip convertible notes	1,275,000		1,275,000			$ 25,000	$ 500,000	$ 250,000	$ 500,000
Embedded derivative, fair value of embedded derivative	$ 400		$ 400		$ 1,800

INTANGIBLE ASSETS AND GOODWIL_2

INTANGIBLE ASSETS AND GOODWILL (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Finite-lived intangible assets
Gross Carrying Amount	$ 3,081,672		$ 3,081,672		$ 3,038,812
Accumulated Amortization	(964,397)		(964,397)		(861,989)
Net Carrying Amount	2,117,275		2,117,275		2,176,823
Amortization expense	43,000	$ 45,300	135,345	$ 129,476
Annual estimated amortization expense for intangible assets
Remainder of 2018	42,591		42,591
2,019	168,473		168,473
2,020	160,708		160,708
2,021	154,745		154,745
2,022	152,006		152,006
2,023	150,007		150,007
Thereafter	1,288,745		1,288,745
Total	2,117,275		2,117,275
Goodwill:
Balance at December 31, 2017			2,737,671
Acquisition			131,583
Currency translation adjustments			(24,125)
Balance at September 30, 2018	2,845,129		2,845,129
Cash paid in business acquisition			139,386	552,805
Common stock issued in an acquisition			110,441
Non-cash investing and financing activity for an acquisition			59,690	$ 0
Restricted Shares | Prepaid Expenses and Other Current Assets
Goodwill:
Deferred compensation cost	17,000		17,000
Restricted Shares | Other Assets
Goodwill:
Deferred compensation cost	27,000		27,000
Supply and distribution agreements
Finite-lived intangible assets
Gross Carrying Amount	1,081,543		1,081,543		1,056,660
Accumulated Amortization	(396,584)		(396,584)		(355,000)
Net Carrying Amount	684,959		$ 684,959		701,660
Weighted- Average Useful Life			16 years
Technology
Finite-lived intangible assets
Gross Carrying Amount	171,419		$ 171,419		137,288
Accumulated Amortization	(118,017)		(118,017)		(104,478)
Net Carrying Amount	53,402		$ 53,402		32,810
Weighted- Average Useful Life			5 years
Patents
Finite-lived intangible assets
Gross Carrying Amount	1,623		$ 1,623		1,623
Accumulated Amortization	(1,623)		(1,623)		(1,623)
Net Carrying Amount	0		$ 0		0
Amortization Period			15 years
Weighted- Average Useful Life			15 years
Internet domain names
Finite-lived intangible assets
Gross Carrying Amount	40,903		$ 40,903		42,265
Accumulated Amortization	(29,767)		(29,767)		(28,802)
Net Carrying Amount	11,136		$ 11,136		13,463
Weighted- Average Useful Life			8 years
Trade Names
Finite-lived intangible assets
Gross Carrying Amount	1,785,284		$ 1,785,284		1,779,076
Accumulated Amortization	(417,600)		(417,600)		(350,447)
Net Carrying Amount	1,367,684		$ 1,367,684		1,428,629
Weighted- Average Useful Life			19 years
Noncompete Agreements
Finite-lived intangible assets
Gross Carrying Amount	900		$ 900		21,900
Accumulated Amortization	(806)		(806)		(21,639)
Net Carrying Amount	$ 94		$ 94		$ 261
Amortization Period			4 years
Weighted- Average Useful Life			4 years
Minimum | Supply and distribution agreements
Finite-lived intangible assets
Amortization Period			3 years
Minimum | Technology
Finite-lived intangible assets
Amortization Period			1 year
Minimum | Internet domain names
Finite-lived intangible assets
Amortization Period			5 years
Minimum | Trade Names
Finite-lived intangible assets
Amortization Period			4 years
Maximum | Supply and distribution agreements
Finite-lived intangible assets
Amortization Period			20 years
Maximum | Technology
Finite-lived intangible assets
Amortization Period			7 years
Maximum | Internet domain names
Finite-lived intangible assets
Amortization Period			20 years
Maximum | Trade Names
Finite-lived intangible assets
Amortization Period			20 years
Additional Paid-In Capital
Goodwill:
Common stock issued in an acquisition			$ 110,441
Business Acquisition [Member]
Goodwill:
Common stock issued in an acquisition			$ 50,700
OpenTable [Member]
Finite-lived intangible assets
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount	13.00%		13.00%

DEBT (Revolving Credit Facility

DEBT (Revolving Credit Facility) (Details) - USD ($)	Jun. 19, 2015	Sep. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Letters of credit issued		$ 4,700,000	$ 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
Revolving Credit Facility | Minimum
Line of Credit Facility [Line Items]
Commitment fee rate	0.085%
Revolving Credit Facility | Minimum | Rate 2C
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.00%
Revolving Credit Facility | Maximum
Line of Credit Facility [Line Items]
Commitment fee rate	0.20%
Revolving Credit Facility | Maximum | Rate 2C
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
Revolving Credit Facility | London Interbank Offered Rate (LIBOR) | Minimum | Rate 1
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.875%
Revolving Credit Facility | London Interbank Offered Rate (LIBOR) | Maximum | Rate 1
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.50%
Revolving Credit Facility | Federal Funds Purchased | Rate 2B
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
Revolving Credit Facility | One Month LIBOR | Rate 2C
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.00%
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

DEBT (Outstanding Debt) (Detail

DEBT (Outstanding Debt) (Details) $ / shares in Units, $ in Thousands, € in Millions	Mar. 31, 2016USD ($)	Aug. 20, 2014USD ($)Days$ / shares	May 31, 2013USD ($)Days$ / shares	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Sep. 30, 2018EUR (€)	Dec. 31, 2017EUR (€)	Aug. 15, 2017USD ($)	Mar. 10, 2017EUR (€)	May 23, 2016USD ($)	Nov. 25, 2015EUR (€)	Mar. 13, 2015USD ($)	Mar. 03, 2015EUR (€)	Sep. 23, 2014EUR (€)	Jun. 30, 2013USD ($)	Mar. 31, 2012USD ($)$ / shares
Debt Instrument
Bank overdraft				$ 3,900		$ 3,900
Outstanding Principal Amount, Long-term debt				8,855,575		8,855,575		$ 9,003,000
Unamortized Debt Discount and Debt Issuance Cost				(151,674)		(151,674)		(193,212)
Carrying Value, Long-term debt				8,703,901		8,703,901		8,809,788
Description of Senior Notes
Convertible debt				0		0		2,963
Payments Related to Conversion of Senior Notes						1,487,187	$ 89,575
Amortization of debt discount included in interest expense						39,699	52,909
0.35% Convertible Senior Notes due June 2020
Debt Instrument
Outstanding Principal Amount, Long-term debt				999,950		999,950		1,000,000
Unamortized Debt Discount and Debt Issuance Cost				(45,225)		(45,225)		(64,825)
Carrying Value, Long-term debt				954,725		954,725		$ 935,175
Description of Senior Notes
Convertible debt				$ (45,600)		$ (45,600)
Aggregate principal amount			$ 1,000,000
Interest rate on Long-term Debt			0.35%	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	$ 800	$ 2,200	2,200
0.9% Convertible Senior Notes due September 2021
Debt Instrument
Outstanding Principal Amount, Long-term debt				1,000,000		1,000,000		$ 1,000,000
Unamortized Debt Discount and Debt Issuance Cost				(66,841)		(66,841)		(83,272)
Carrying Value, Long-term debt				$ 933,159		$ 933,159		$ 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%	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				$ 1,161,500		$ 1,161,500		$ 1,200,800
Unamortized Debt Discount and Debt Issuance Cost				(5,073)		(5,073)		(6,238)
Carrying Value, Long-term debt				$ 1,156,427		$ 1,156,427		$ 1,194,562
Description of Senior Notes
Aggregate principal amount | €													€ 1,000	€ 1,000		€ 1,000
Interest rate on Long-term Debt				0.80%		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				$ 871,125		$ 871,125		900,600
Unamortized Debt Discount and Debt Issuance Cost				(3,927)		(3,927)		(4,683)
Carrying Value, Long-term debt				$ 867,198		$ 867,198		$ 895,917
Description of Senior Notes
Aggregate principal amount | €													€ 750	€ 750				€ 750
Interest rate on Long-term Debt				2.15%		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		$ 500,000
Unamortized Debt Discount and Debt Issuance Cost				(2,747)		(2,747)		(3,203)
Carrying Value, Long-term debt				$ 497,253		$ 497,253		$ 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%	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				$ 1,161,500		$ 1,161,500		1,200,800
Unamortized Debt Discount and Debt Issuance Cost				(10,709)		(10,709)		(12,240)
Carrying Value, Long-term debt				$ 1,150,791		$ 1,150,791		$ 1,188,560
Description of Senior Notes
Aggregate principal amount | €													€ 1,000	€ 1,000							€ 1,000
Interest rate on Long-term Debt				2.375%		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		$ 500,000
Unamortized Debt Discount and Debt Issuance Cost				(2,959)		(2,959)		(3,290)
Carrying Value, Long-term debt				$ 497,041		$ 497,041		$ 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%					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		$ 1,000,000
Unamortized Debt Discount and Debt Issuance Cost				(6,250)		(6,250)		(6,840)
Carrying Value, Long-term debt				$ 993,750		$ 993,750		$ 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%			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				$ 1,161,500		$ 1,161,500		$ 1,200,800
Unamortized Debt Discount and Debt Issuance Cost				(4,709)		(4,709)		(5,136)
Carrying Value, Long-term debt				$ 1,156,791		$ 1,156,791		$ 1,195,664
Description of Senior Notes
Aggregate principal amount | €													€ 1,000	€ 1,000						€ 1,000
Interest rate on Long-term Debt				1.80%		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		$ 500,000
Unamortized Debt Discount and Debt Issuance Cost				(3,234)		(3,234)		(3,485)
Carrying Value, Long-term debt				$ 496,766		$ 496,766		$ 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%	3.55%
Debt financing costs paid								$ 3,200
Unamortized debt discount															$ 400
Effective interest rate at debt origination or modification															3.56%
1.0% Convertible Senior Notes due March 2018
Debt Instrument
Short-term Debt, Gross								714,304
Unamortized Debt Discount and Debt Issuance Cost								(3,394)
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				$ 15,200	23,400	50,200	71,400
Coupon Interest expense				3,100	5,400	10,800	16,400
Amortization of debt discount included in interest expense				11,800	16,900	37,900	51,400
Amortization of debt issuance costs				$ 300	$ 1,100	$ 1,500	$ 3,600
Debt, weighted average interest rate				3.20%	3.40%	3.20%	3.40%
Other Long-term Debt
Description of Senior Notes
Interest expense related to debt				$ 42,300	$ 38,500	$ 128,400	$ 102,200
Coupon Interest expense				40,600	$ 36,900	123,300	98,100
1% CT Loan Due March 2016
Description of Senior Notes
Aggregate principal amount	$ 2,500			$ 1,500		$ 1,500		$ 1,500
Interest rate on Long-term Debt	1.00%			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				$ 9,600,000		$ 9,600,000		$ 11,100,000

TREASURY STOCK (Details)

TREASURY STOCK (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Equity, Class of Treasury Stock [Line Items]
Remaining authorization to repurchase common stock	$ 6,400,000	$ 2,400,000		$ 6,400,000
Treasury stock shares acquired	1,149,324		319,488	2,095,017	632,006
Treasury stock value acquired	$ 2,248,944		$ 586,489	$ 4,180,476	$ 1,142,717
Repurchase of common shares to satisfy employee withholding tax obligations related to stock-based compensation (in shares)	4,204		3,472	75,911	54,438
Repurchase of common shares to satisfy employee withholding tax obligations related to stock-based compensation	$ 8,500		$ 6,500	$ 154,800	$ 95,100
Payments Related to Tax Withholding for Share-based Compensation	$ 8,900		$ 9,000	$ 153,900	$ 95,500
Treasury stock, shares (in shares)	16,311,836	14,216,819		16,311,836
Repurchase Program (Q12016)
Equity, Class of Treasury Stock [Line Items]
Amount of common stock repurchases authorized								$ 3,000,000
Treasury stock shares acquired					577,568
Treasury stock value acquired					$ 1,000,000
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 shares acquired			316,016
Repurchase Program (Q12016) and (Q12017) [Member]
Equity, Class of Treasury Stock [Line Items]
Treasury stock value acquired			$ 580,000
Repurchase Program (Q12018)
Equity, Class of Treasury Stock [Line Items]
Amount of common stock repurchases authorized						$ 8,000,000
Treasury Stock Repurchased but unsettled by period end	51,240
Treasury Stock Repurchased but unsettled by period end, amount	$ 101,300
Repurchase Program (Q12017) and (Q12018) [Member]
Equity, Class of Treasury Stock [Line Items]
Treasury stock shares acquired	1,145,120
Treasury stock value acquired	$ 2,200,000
Repurchase Program (Q12016), (Q12017) and (Q12018) [Member]
Equity, Class of Treasury Stock [Line Items]
Treasury stock shares acquired				2,019,106
Treasury stock value acquired				$ 4,000,000

INCOME TAXES (Details)

INCOME TAXES (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Contingency [Line Items]
Effective income tax rate reconciliation, percent	21.10%	16.80%	20.20%	16.20%
Provisional income tax expense					$ 1,600
Accumulated international earnings repatriated					16,500
Provisional net income tax benefit					$ 217
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 COMPREHENSI_3

ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		$ 9,951,810	$ 12,876,994	$ 9,951,810	$ 12,876,994	$ 11,260,598	$ 10,472,632	$ 11,589,669	$ 9,820,142
Tax (benefit) associated with gain (loss) on components of foreign currency translation		161	(59,607)	$ 20,519	(179,948)
Tax and Customs Administration, Netherlands
Accumulated Other Comprehensive Income (Loss) [Line Items]
Statutory federal rate				25.00%		25.00%
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		(5,700)		$ (16,200)	0
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity	[1]	(106,638)		(106,638)		$ (15,700)
Foreign currency translation adjustments | Net Investment Hedging | Euro Senior Notes
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		(79,400)		(79,400)		(190,400)
Stockholders' equity, before tax		(89,500)		(89,500)		(237,200)
Foreign currency translation adjustments | Net Investment Hedging | Foreign Currency Forward
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		(35,000)		(35,000)		(35,000)
Stockholders' equity, before tax		(52,600)		(52,600)		(52,600)
Net unrealized gains/(losses) on marketable securities | Ctrip.com International, Ltd.
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity	[2]	0		0		241,088
Stockholders' equity, before tax						298,700
Net unrealized gains/(losses) on marketable securities | Ctrip.com International, Ltd. | Internal Revenue Service (IRS)
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI tax related to investments						62,900
Net unrealized gains/(losses) on marketable securities | Ctrip.com International, Ltd. | Tax and Customs Administration, Netherlands
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity, before tax, tax-exempt						319,900
Stockholders' equity, before tax, taxable						(21,200)
AOCI tax related to investments						(5,300)
Net unrealized gains/(losses) on marketable securities | Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity	[3]	(127,168)		(127,168)		11,594
Net unrealized gains/(losses) on marketable securities | Debt Securities | Internal Revenue Service (IRS)
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI tax related to investments		200		200		500
Net unrealized gains/(losses) on marketable securities | Debt Securities | Tax and Customs Administration, Netherlands
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity, before tax, tax-exempt		(223,800)		(223,800)		(85,300)
Stockholders' equity, before tax, taxable		129,300		129,300		129,800
AOCI tax related to investments		32,200		32,200		32,400
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		$ (233,806)	$ 653,666	$ (233,806)	$ 653,666	$ 236,982	$ (76,283)	$ 573,915	$ (134,684)
[1]	(1) Foreign currency translation adjustments, net of tax, at September 30, 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 $79.4 million after tax ($89.5 million before tax) and $190.4 million after tax ($237.2 million before tax) at September 30, 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 non-U.S. Dollar denominated net assets. During the three and nine months ended September 30, 2018, the Company recorded deferred tax benefits of $5.7 million and $16.2 million, respectively, 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 not subject to income 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 losses before tax on marketable debt securities of $223.8 million and $85.3 million at September 30, 2018 and December 31, 2017, respectively, were not subject to income tax in the Netherlands. Unrealized gains before tax of $129.3 million and $129.8 million at September 30, 2018 and December 31, 2017, respectively, were taxable at a 25% tax rate in the Netherlands, resulting in tax charges of $32.2 million and $32.4 million, respectively. The Company also recorded U.S. tax charges of $0.2 million and $0.5 million at September 30, 2018 and December 31, 2017, respectively, related to these investments.

COMMITMENTS AND CONTINGENCIES (

COMMITMENTS AND CONTINGENCIES (Details) € in Millions	Jul. 09, 2018USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2016EUR (€)	Sep. 30, 2018USD ($)country	Sep. 30, 2018EUR (€)country	Sep. 30, 2017USD ($)	Mar. 31, 2021EUR (€)	Mar. 31, 2021EUR (€)	Sep. 30, 2018EUR (€)	Dec. 31, 2017USD ($)
Commitments and Contingencies
Number of pricing parity working group members | country				10	10
Increase in the estimated fair value of contingent consideration liabilities				$ 7,933,000		$ 0
Payments for building construction				356,011,000		$ 223,692,000
Business Combination, Consideration Transferred	$ 140,000,000
Booking.com | Headquarters
Commitments and Contingencies
Contractual obligation for building construction | €			€ 270
Payments to developer | €			48
Acquisition of land use rights | €			43
Payments for building construction | €			€ 5		€ 61
Booking.com | Headquarters | Ground Lease
Commitments and Contingencies
Contractual obligation for building construction | €									€ 78
Series of individually immaterial business acquisitions
Commitments and Contingencies
Contingent consideration arrangements, range of outcomes, minimum value		$ 0		0
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High		90,000,000		90,000,000
Series of individually immaterial business acquisitions | Fair Value, Inputs, Level 3
Commitments and Contingencies
Contingent consideration liability		17,100,000		17,100,000						$ 9,200,000
Increase in the estimated fair value of contingent consideration liabilities				7,900,000
French Tax Audit
Commitments and Contingencies
Assessed taxes including interest and penalties | €					€ 356
Taxes Owed For Prior Periods
Commitments and Contingencies
Charge for estimated probable tax owed for prior periods, including applicable interest and penalties		29,000,000
Estimated reasonably possible loss in excess of amount accrued		$ 20,000,000		$ 20,000,000
Scenario, Forecast [Member] | Booking.com | Headquarters
Commitments and Contingencies
Payments for building construction | €							€ 161	€ 222