INVESTMENTS - Summary of Investments by Major Security Type (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Equity security investments in private companies:
Cost		$ 501
Short-term investments:
Carrying Value	$ 826	998
Long-term investments:
Carrying Value	2,056	4,477
Trip.com Group convertible debt securities \| Trip.com Group
Debt securities:
Cost	775
Short-term investments
Short-term investments:
Cost	945	998
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(119)	(1)
Carrying Value	826	998
Short-term investments \| International government securities
Debt securities:
Cost		109
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Carrying Value		109
Short-term investments \| U.S. government securities
Debt securities:
Cost		138
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Carrying Value		138
Short-term investments \| Corporate debt securities
Debt securities:
Cost	164	751
Gross Unrealized Gains	0	1
Gross Unrealized Losses	0	(1)
Carrying Value	164	751
Short-term investments \| Trip.com Group convertible debt securities \| Trip.com Group
Debt securities:
Cost	250
Gross Unrealized Gains	0
Gross Unrealized Losses	(2)
Carrying Value	248
Short-term investments \| Equity securities \| Trip.com Group
Equity securities:
Cost	531
Gross Unrealized Gains	0
Gross Unrealized Losses	(117)
Carrying Value	414
Long-term investments
Long-term investments:
Cost	1,737	3,810
Gross Unrealized Gains	522	686
Gross Unrealized Losses	(203)	(19)
Carrying Value	2,056	4,477
Long-term investments \| Trip.com Group
Equity securities:
Cost		655
Carrying Value		726
Long-term investments \| International government securities
Debt securities:
Cost		68
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Carrying Value		68
Long-term investments \| U.S. government securities
Debt securities:
Cost		136
Gross Unrealized Gains		0
Gross Unrealized Losses		(1)
Carrying Value		135
Long-term investments \| Corporate debt securities
Debt securities:
Cost		963
Gross Unrealized Gains		2
Gross Unrealized Losses		(2)
Carrying Value		963
Long-term investments \| Trip.com Group convertible debt securities \| Trip.com Group
Debt securities:
Cost	525	775
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(77)	(8)
Carrying Value	448	767
Long-term investments \| Debt securities
Debt securities:
Cost	250	250
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(20)	0
Carrying Value	230	250
Long-term investments \| Investment in private company equity securities
Equity security investments in private companies:
Cost	501	501
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(100)	0
Carrying Value	401	501
Long-term investments \| Equity securities
Equity securities:
Cost	461	1,117
Gross Unrealized Gains	522	684
Gross Unrealized Losses	(6)	(8)
Carrying Value	$ 977	$ 1,793

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