INVESTMENTS - Narrative (Details) - USD ($) $ in Millions		3 Months Ended			12 Months Ended
		Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2017	Dec. 31, 2020
Schedule of Investments [Line Items]
Proceeds from sale and maturity of investments		$ 0		$ 1,885
Impairment of investment	[1]	0		100
Trip.com Group
Schedule of Investments [Line Items]
Cost basis of securities sold				124
Proceeds from sale and maturity of investments				94
Loss on sale of securities				40
Unrealized loss on securities held				178
Trip.com Group | Convertible Debt Securities
Schedule of Investments [Line Items]
Amortized cost of available-for-sale debt securities		525
Meituan
Schedule of Investments [Line Items]
Unrealized loss on securities held				81
Payments to acquire other investments					$ 450
Unrealized gain on securities held		29
Long-term Investments | Investment in private company equity securities
Schedule of Investments [Line Items]
Cost of investment		552				$ 552
Investment in equity securities without readily determinable FV		452				455
Long-term Investments | Trip.com Group | Convertible Debt Securities
Schedule of Investments [Line Items]
Amortized cost of available-for-sale debt securities		$ 25				25
Term of available-for-sale debt securities		6 years
Estimated fair value		$ 24				24
Long-term Investments | Meituan
Schedule of Investments [Line Items]
Fair value of equity securities		3,100				3,100
Long-term Investments | Didi Chuxing | Investment in private company equity securities
Schedule of Investments [Line Items]
Cost of investment		500				500
Impairment of investment				100
Investment in equity securities without readily determinable FV		400		400		400
Long-term Investments | Grab | Redeemable Convertible Preferred Stock
Schedule of Investments [Line Items]
Amortized cost of available-for-sale debt securities		200				200
Estimated fair value		200				200
Unrealized gain (loss) on debt securities			$ 20	$ (20)
Short-term Investments | Trip.com Group | Convertible Debt Securities
Schedule of Investments [Line Items]
Amortized cost of available-for-sale debt securities		$ 500				500
Term of available-for-sale debt securities		10 years
Estimated fair value		$ 500				$ 501

[1] See Note 5 for additional information related to the net gains (losses) on marketable equity securities and impairment of investment.