FAIR VALUE MEASUREMENTS - Financial Assets and Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020	Dec. 31, 2020
LIABILITIES:
Impairment of investment	[1]	$ 0		$ 100
Goodwill		1,882			$ 1,895
Impairment of goodwill		0		489
OpenTable And KAYAK
LIABILITIES:
Goodwill			$ 1,000	1,500
Impairment of goodwill			$ 573	489
Didi Chuxing | Investment in private company equity securities | Long-term Investments
LIABILITIES:
Impairment of investment				100
Recurring Basis
ASSETS:
Assets at fair value		15,657			14,054
Recurring Basis | Money market fund investments | Cash Equivalents and Restricted Cash Equivalents
ASSETS:
Assets at fair value		11,773			10,208
Recurring Basis | Time deposits and certificates of deposit | Cash Equivalents and Restricted Cash Equivalents
ASSETS:
Assets at fair value		41			32
Recurring Basis | Investments in private company debt securities | Long-term Investments
ASSETS:
Assets at fair value		200			200
Recurring Basis | Equity securities | Long-term Investments
ASSETS:
Assets at fair value		3,112			3,080
Recurring Basis | Trip.com Group | Convertible Debt Securities | Short-term Investments
ASSETS:
Assets at fair value		500			501
Recurring Basis | Trip.com Group | Convertible Debt Securities | Long-term Investments
ASSETS:
Assets at fair value		24			24
Recurring Basis | Not Designated as Hedging Instrument | Foreign currency exchange derivatives
ASSETS:
Assets at fair value		7			9
LIABILITIES:
Liabilities at fair value		11			7
Nonrecurring Basis
ASSETS:
Assets at fair value					1,404
Nonrecurring Basis | Investment in private company equity securities | Long-term Investments
ASSETS:
Assets at fair value	[2]				404
Nonrecurring Basis | Goodwill | OpenTable And KAYAK
ASSETS:
Assets at fair value	[3]				1,000
Nonrecurring Basis | Didi Chuxing | Investment in private company equity securities | Long-term Investments
ASSETS:
Assets at fair value				$ 400
Level 1 | Recurring Basis
ASSETS:
Assets at fair value		14,926			13,320
Level 1 | Recurring Basis | Money market fund investments | Cash Equivalents and Restricted Cash Equivalents
ASSETS:
Assets at fair value		11,773			10,208
Level 1 | Recurring Basis | Time deposits and certificates of deposit | Cash Equivalents and Restricted Cash Equivalents
ASSETS:
Assets at fair value		41			32
Level 1 | Recurring Basis | Investments in private company debt securities | Long-term Investments
ASSETS:
Assets at fair value		0			0
Level 1 | Recurring Basis | Equity securities | Long-term Investments
ASSETS:
Assets at fair value		3,112			3,080
Level 1 | Recurring Basis | Trip.com Group | Convertible Debt Securities | Short-term Investments
ASSETS:
Assets at fair value		0			0
Level 1 | Recurring Basis | Trip.com Group | Convertible Debt Securities | Long-term Investments
ASSETS:
Assets at fair value		0			0
Level 1 | Recurring Basis | Not Designated as Hedging Instrument | Foreign currency exchange derivatives
ASSETS:
Assets at fair value		0			0
LIABILITIES:
Liabilities at fair value		0			0
Level 1 | Nonrecurring Basis
ASSETS:
Assets at fair value					0
Level 1 | Nonrecurring Basis | Investment in private company equity securities | Long-term Investments
ASSETS:
Assets at fair value	[2]				0
Level 1 | Nonrecurring Basis | Goodwill | OpenTable And KAYAK
ASSETS:
Assets at fair value	[3]				0
Level 2 | Recurring Basis
ASSETS:
Assets at fair value		531			534
Level 2 | Recurring Basis | Money market fund investments | Cash Equivalents and Restricted Cash Equivalents
ASSETS:
Assets at fair value		0			0
Level 2 | Recurring Basis | Time deposits and certificates of deposit | Cash Equivalents and Restricted Cash Equivalents
ASSETS:
Assets at fair value		0			0
Level 2 | Recurring Basis | Investments in private company debt securities | Long-term Investments
ASSETS:
Assets at fair value		0			0
Level 2 | Recurring Basis | Equity securities | Long-term Investments
ASSETS:
Assets at fair value		0			0
Level 2 | Recurring Basis | Trip.com Group | Convertible Debt Securities | Short-term Investments
ASSETS:
Assets at fair value		500			501
Level 2 | Recurring Basis | Trip.com Group | Convertible Debt Securities | Long-term Investments
ASSETS:
Assets at fair value		24			24
Level 2 | Recurring Basis | Not Designated as Hedging Instrument | Foreign currency exchange derivatives
ASSETS:
Assets at fair value		7			9
LIABILITIES:
Liabilities at fair value		11			7
Level 2 | Nonrecurring Basis
ASSETS:
Assets at fair value					0
Level 2 | Nonrecurring Basis | Investment in private company equity securities | Long-term Investments
ASSETS:
Assets at fair value	[2]				0
Level 2 | Nonrecurring Basis | Goodwill | OpenTable And KAYAK
ASSETS:
Assets at fair value	[3]				0
Level 3 | Recurring Basis
ASSETS:
Assets at fair value		200			200
Level 3 | Recurring Basis | Money market fund investments | Cash Equivalents and Restricted Cash Equivalents
ASSETS:
Assets at fair value		0			0
Level 3 | Recurring Basis | Time deposits and certificates of deposit | Cash Equivalents and Restricted Cash Equivalents
ASSETS:
Assets at fair value		0			0
Level 3 | Recurring Basis | Investments in private company debt securities | Long-term Investments
ASSETS:
Assets at fair value		200			200
Level 3 | Recurring Basis | Equity securities | Long-term Investments
ASSETS:
Assets at fair value		0			0
Level 3 | Recurring Basis | Trip.com Group | Convertible Debt Securities | Short-term Investments
ASSETS:
Assets at fair value		0			0
Level 3 | Recurring Basis | Trip.com Group | Convertible Debt Securities | Long-term Investments
ASSETS:
Assets at fair value		0			0
Level 3 | Recurring Basis | Not Designated as Hedging Instrument | Foreign currency exchange derivatives
ASSETS:
Assets at fair value		0			0
LIABILITIES:
Liabilities at fair value		$ 0			0
Level 3 | Nonrecurring Basis
ASSETS:
Assets at fair value					1,404
Level 3 | Nonrecurring Basis | Investment in private company equity securities | Long-term Investments
ASSETS:
Assets at fair value	[2]				404
Level 3 | Nonrecurring Basis | Goodwill | OpenTable And KAYAK
ASSETS:
Assets at fair value	[3]				$ 1,000

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

[2] At March 31, 2020, the investment in Didi Chuxing was written down to its estimated fair value of $400 million, resulting in an impairment charge of $100 million (see Note 5).

[3] At March 31, 2020, the goodwill of the OpenTable and KAYAK reporting unit was written down to its estimated fair value of $1.5 billion, resulting in an impairment charge of $489 million. At September 30, 2020, the goodwill was further written down to its estimated fair value of $1.0 billion, resulting in an additional impairment charge of $573 million (see Note 8).