INVESTMENTS - Summary of Investments by Major Security Type (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Long-term investments:
Carrying Value	$ 3,283	$ 3,759
Trip.com Group
Equity security investments with readily determinable fair value:
Cost			$ 655
Convertible Debt Securities \| Trip.com Group
Debt Securities, Available-for-sale [Abstract]
Cost	525
Short-term Investments \| Convertible Debt Securities \| Trip.com Group
Debt Securities, Available-for-sale [Abstract]
Cost	525	500
Gross Unrealized Gains/Upward Adjustments	0	1
Gross Unrealized Losses/Downward Adjustments	(3)	0
Carrying Value	522	501
Long-term investments
Long-term investments:
Cost	1,229	1,240
Gross Unrealized Gains /Upward Adjustments	2,248	2,620
Gross Unrealized Losses /Downward Adjustments	(194)	(101)
Carrying Value	3,283	3,759
Long-term investments \| Convertible Debt Securities \| Trip.com Group
Debt Securities, Available-for-sale [Abstract]
Cost		25
Gross Unrealized Gains/Upward Adjustments		0
Gross Unrealized Losses/Downward Adjustments		(1)
Carrying Value		24
Long-term investments \| Investments in private company debt securities
Debt Securities, Available-for-sale [Abstract]
Cost	200	200
Gross Unrealized Gains/Upward Adjustments	130	0
Gross Unrealized Losses/Downward Adjustments	0	0
Carrying Value	330	200
Long-term investments \| Investment in private company equity securities
Equity security investments in private companies:
Cost	66	552
Gross Unrealized Gains /Upward Adjustments	3	3
Gross Unrealized Losses /Downward Adjustments	0	(100)
Carrying Value	69	455
Long-term investments \| Equity securities
Equity security investments with readily determinable fair value:
Cost	963	463
Gross Unrealized Gains /Upward Adjustments	2,115	2,617
Gross Unrealized Losses /Downward Adjustments	(194)	0
Carrying Value	$ 2,884	$ 3,080

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