CHANGES IN ACCUMULATED OTHER COMPREHENSIVE LOSS BY COMPONENT (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019		May 31, 2020	Aug. 31, 2019
Total AOCI, net of tax
Balance		$ 3,999	$ 5,307		$ 5,933		$ 8,785
OCI, before reclassifications, net of tax		37	(56)		(23)		40
Amounts reclassified to net income, net of tax		0	21	[1]	18	[1]	11	[1]
Total other comprehensive income (loss), net of tax		37	(35)		(5)		51
Balance		4,913	5,951		4,913		5,951
Trip.com Group convertible debt securities | Trip.com Group 1% Note Due 2020
Total AOCI, net of tax
Amortized cost of available-for-sale debt securities									$ 250
Trip.com Group convertible debt securities | Trip.com Group Note Due 2019
Total AOCI, net of tax
Amortized cost of available-for-sale debt securities										$ 500
Total AOCI
Total AOCI, net of tax
Balance		(233)	(230)		(191)		(316)
Balance		(196)	(265)		(196)		(265)
Foreign currency translation adjustments, net of tax
Total AOCI, net of tax
Balance		(184)	(157)		(139)		(129)
OCI, before reclassifications, net of tax		42	(35)		(3)		(63)
Amounts reclassified to net income, net of tax		0	0	[1]	0	[1]	0	[1]
Total other comprehensive income (loss), net of tax		42	(35)		(3)		(63)
Balance		(142)	(192)		(142)		(192)
Foreign currency translation adjustments, net of tax | Net Investment Hedging | Foreign Currency Forward
Before tax
Balance		(53)	(53)		(53)		(53)
Balance		(53)	(53)		(53)		(53)
Total AOCI, net of tax
Balance		(35)	(35)		(35)		(35)
Balance		(35)	(35)		(35)		(35)
Foreign currency translation
Before tax
Balance		(249)	(179)		(186)		(109)
OCI before reclassifications, before tax		99	(128)		36		(198)
Amounts reclassified to net income, before tax		0	0	[1]	0	[1]	0	[1]
OCI after reclassifications, before tax		99	(128)		36		(198)
Balance		(150)	(307)		(150)		(307)
Tax (expense) benefit
Balance	[2]	58	56		54		41
OCI before reclassifications, tax	[2]	(4)	8		0		23
Amounts reclassified to net income, tax	[2]	0	0	[1]	0	[1]	0	[1]
OCI after reclassifications, tax	[2]	(4)	8		0		23
Balance	[2]	54	64		54		64
Net Investment Hedges
Before tax
Balance	[3]	17	(38)		(2)		(73)
OCI before reclassifications, before tax	[3]	(69)	111		(50)		146
Amounts reclassified to net income, before tax	[3]	0	0	[1]	0	[1]	0	[1]
OCI after reclassifications, before tax	[3]	(69)	111		(50)		146
Balance	[3]	(52)	73		(52)		73
Tax (expense) benefit
Balance	[3]	(10)	4		(5)		12
OCI before reclassifications, tax	[3]	16	(26)		11		(34)
Amounts reclassified to net income, tax	[3]	0	0	[1]	0	[1]	0	[1]
OCI after reclassifications, tax	[3]	16	(26)		11		(34)
Balance	[3]	6	(22)		6		(22)
Net unrealized (losses) gains on available-for-sale securities, net of tax
Before tax
Balance		(24)	(7)		(7)		(157)
OCI before reclassifications, before tax		(5)	(28)		(26)		133
Amounts reclassified to net income, before tax		0	0	[1]	4	[1]	(11)	[1]
OCI after reclassifications, before tax		(5)	(28)		(22)		122
Balance		(29)	(35)		(29)		(35)
Tax (expense) benefit
Balance		(25)	(66)		(45)		(30)
OCI before reclassifications, tax		0	7		6		(30)
Amounts reclassified to net income, tax		0	21	[1]	14	[1]	22	[1]
OCI after reclassifications, tax		0	28		20		(8)
Balance		(25)	(38)		(25)		(38)
Total AOCI, net of tax
Balance		(49)	(73)		(52)		(187)
OCI, before reclassifications, net of tax		(5)	(21)		(20)		103
Amounts reclassified to net income, net of tax		0	21	[1]	18	[1]	11	[1]
Total other comprehensive income (loss), net of tax		(5)	0		(2)		114
Balance		$ (54)	(73)		(54)		(73)
Net unrealized (losses) gains on available-for-sale securities, net of tax | Trip.com Group convertible debt securities | Trip.com Group 1% Note Due 2020
Tax (expense) benefit
Amounts reclassified to net income, tax					$ 15
Net unrealized (losses) gains on available-for-sale securities, net of tax | Trip.com Group convertible debt securities | Trip.com Group Note Due 2019
Tax (expense) benefit
Amounts reclassified to net income, tax			$ 21				$ 21

[1] (3)         The reclassified net realized (losses) gains before tax from sales of investments in debt securities and impairment losses before tax related to debt securities are included in "Foreign currency transactions and other" and the related reclassified tax benefits (expenses) are included in "Income tax expense" in the Unaudited Consolidated Statements of Operations. The cost of marketable debt securities sold is determined using a first-in and first-out method. For the nine months ended September 30, 2020, the reclassified tax expenses include a tax expense of $15 million related to the maturity in May 2020 of the Company's investment of $250 million in Trip.com Group convertible notes (see Note 5). For the three and nine months ended September 30, 2019, the reclassified tax expenses include a tax expense of $21 million related to the maturity in August 2019 of the Company's investment of $500 million in Trip.com Group convertible notes.

[2] (2)         The tax benefits relate to foreign currency translation adjustments to the Company's one-time deemed repatriation tax liability recorded at December 31, 2017 and foreign earnings for periods after December 31, 2017 that are subject to U.S. federal and state income tax, resulting from the enactment of the U.S. Tax Cuts and Jobs Act (the "Tax Act").

[3] (1)         Net investment hedges balance, net of tax, at September 30, 2020 and earlier dates presented above, include accumulated net losses from fair value adjustments of $35 million after tax ($53 million before tax) associated with previously settled derivatives that were designated as net investment hedges. The remaining balances relate to foreign currency transaction gains (losses) and related tax benefits (expenses) associated with the Company's Euro-denominated debt that is designated as a hedge against the impact of currency fluctuations on the net assets of a Euro functional currency subsidiary (see Note 9).