INVESTMENTS - Summary of Investments by Major Security Type (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Short-term investments:
Carrying Value	$ 0	$ 998
Long-term investments:
Carrying Value	3,672	4,477
Trip.com Group convertible debt securities \| Trip.com Group
Debt securities:
Cost	525
Short-term Investments
Short-term investments:
Cost		998
Gross Unrealized Gains /Upward Adjustments		1
Gross Unrealized Losses /Downward Adjustments		(1)
Carrying Value		998
Short-term Investments \| International government securities
Debt securities:
Cost		109
Gross Unrealized Gains/Upward Adjustments		0
Gross Unrealized Losses/Downward Adjustments		0
Carrying Value		109
Short-term Investments \| U.S. government securities
Debt securities:
Cost		138
Gross Unrealized Gains/Upward Adjustments		0
Gross Unrealized Losses/Downward Adjustments		0
Carrying Value		138
Short-term Investments \| Corporate debt securities
Debt securities:
Cost		751
Gross Unrealized Gains/Upward Adjustments		1
Gross Unrealized Losses/Downward Adjustments		(1)
Carrying Value		751
Long-term investments
Long-term investments:
Cost	1,737	3,810
Gross Unrealized Gains /Upward Adjustments	2,067	686
Gross Unrealized Losses /Downward Adjustments	(132)	(19)
Carrying Value	3,672	4,477
Long-term investments \| Trip.com Group
Equity security investments with readily determinable fair value:
Cost		655
Carrying Value		726
Long-term investments \| International government securities
Debt securities:
Cost		68
Gross Unrealized Gains/Upward Adjustments		0
Gross Unrealized Losses/Downward Adjustments		0
Carrying Value		68
Long-term investments \| U.S. government securities
Debt securities:
Cost		136
Gross Unrealized Gains/Upward Adjustments		0
Gross Unrealized Losses/Downward Adjustments		(1)
Carrying Value		135
Long-term investments \| Corporate debt securities
Debt securities:
Cost		963
Gross Unrealized Gains/Upward Adjustments		2
Gross Unrealized Losses/Downward Adjustments		(2)
Carrying Value		963
Long-term investments \| Investments in private company debt securities
Debt securities:
Cost	250	250
Gross Unrealized Gains/Upward Adjustments	0	0
Gross Unrealized Losses/Downward Adjustments	0	0
Carrying Value	250	250
Long-term investments \| Investment in private company equity securities
Equity security investments in private companies:
Cost	501	501
Gross Unrealized Gains /Upward Adjustments	3	0
Gross Unrealized Losses /Downward Adjustments	(100)	0
Carrying Value	404	501
Long-term investments \| Trip.com Group convertible debt securities \| Trip.com Group
Debt securities:
Cost	525	775
Gross Unrealized Gains/Upward Adjustments	0	0
Gross Unrealized Losses/Downward Adjustments	(32)	(8)
Carrying Value	493	767
Long-term investments \| Equity securities
Equity security investments with readily determinable fair value:
Cost	461	1,117
Gross Unrealized Gains /Upward Adjustments	2,064	684
Gross Unrealized Losses /Downward Adjustments	0	(8)
Carrying Value	2,525	1,793
Long-term investments \| Redeemable Convertible Preferred Stock
Debt securities:
Cost	250	250
Carrying Value	$ 250	$ 250

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