FAIR VALUE MEASUREMENTS - Financial Assets and Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
LIABILITIES:
Impairment of investment	[1]	$ 0		$ 0		$ 0	$ 100
Goodwill		1,886				1,886		$ 1,895
Impairment of goodwill		0		$ 0		0	$ 489
OpenTable And KAYAK
LIABILITIES:
Goodwill			$ 1,000		$ 1,500
Impairment of goodwill			$ 573		489
DiDi Global Inc. | Investment in private company equity securities | Long-term Investments
LIABILITIES:
Impairment of investment					100
Recurring Basis
ASSETS:
Assets at fair value		15,579				15,579		14,054
Recurring Basis | Money market fund investments | Cash Equivalents and Restricted Cash Equivalents
ASSETS:
Assets at fair value		10,766				10,766		10,208
Recurring Basis | Time deposits and certificates of deposit | Cash Equivalents and Restricted Cash Equivalents
ASSETS:
Assets at fair value		52				52		32
Recurring Basis | Investments in private company debt securities | Long-term Investments
ASSETS:
Assets at fair value		326				326		200
Recurring Basis | Equity securities | Long-term Investments
ASSETS:
Assets at fair value		3,903				3,903		3,080
Recurring Basis | Trip.com Group | Convertible Debt Securities | Short-term Investments
ASSETS:
Assets at fair value		501				501		501
Recurring Basis | Trip.com Group | Convertible Debt Securities | Long-term Investments
ASSETS:
Assets at fair value		24				24		24
Recurring Basis | Not Designated as Hedging Instrument | Foreign currency exchange derivatives
ASSETS:
Assets at fair value		7				7		9
LIABILITIES:
Liabilities at fair value		12				12		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 Global Inc. | Investment in private company equity securities | Long-term Investments
ASSETS:
Assets at fair value					$ 400
Level 1 | Recurring Basis
ASSETS:
Assets at fair value		14,721				14,721		13,320
Level 1 | Recurring Basis | Money market fund investments | Cash Equivalents and Restricted Cash Equivalents
ASSETS:
Assets at fair value		10,766				10,766		10,208
Level 1 | Recurring Basis | Time deposits and certificates of deposit | Cash Equivalents and Restricted Cash Equivalents
ASSETS:
Assets at fair value		52				52		32
Level 1 | Recurring Basis | Investments in private company debt securities | Long-term Investments
ASSETS:
Assets at fair value		0				0		0
Level 1 | Recurring Basis | Equity securities | Long-term Investments
ASSETS:
Assets at fair value		3,903				3,903		3,080
Level 1 | Recurring Basis | Trip.com Group | Convertible Debt Securities | Short-term Investments
ASSETS:
Assets at fair value		0				0		0
Level 1 | Recurring Basis | Trip.com Group | Convertible Debt Securities | Long-term Investments
ASSETS:
Assets at fair value		0				0		0
Level 1 | Recurring Basis | Not Designated as Hedging Instrument | Foreign currency exchange derivatives
ASSETS:
Assets at fair value		0				0		0
LIABILITIES:
Liabilities at fair value		0				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		532				532		534
Level 2 | Recurring Basis | Money market fund investments | Cash Equivalents and Restricted Cash Equivalents
ASSETS:
Assets at fair value		0				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		0
Level 2 | Recurring Basis | Investments in private company debt securities | Long-term Investments
ASSETS:
Assets at fair value		0				0		0
Level 2 | Recurring Basis | Equity securities | Long-term Investments
ASSETS:
Assets at fair value		0				0		0
Level 2 | Recurring Basis | Trip.com Group | Convertible Debt Securities | Short-term Investments
ASSETS:
Assets at fair value		501				501		501
Level 2 | Recurring Basis | Trip.com Group | Convertible Debt Securities | Long-term Investments
ASSETS:
Assets at fair value		24				24		24
Level 2 | Recurring Basis | Not Designated as Hedging Instrument | Foreign currency exchange derivatives
ASSETS:
Assets at fair value		7				7		9
LIABILITIES:
Liabilities at fair value		12				12		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		326				326		200
Level 3 | Recurring Basis | Money market fund investments | Cash Equivalents and Restricted Cash Equivalents
ASSETS:
Assets at fair value		0				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		0
Level 3 | Recurring Basis | Investments in private company debt securities | Long-term Investments
ASSETS:
Assets at fair value		326				326		200
Level 3 | Recurring Basis | Equity securities | Long-term Investments
ASSETS:
Assets at fair value		0				0		0
Level 3 | Recurring Basis | Trip.com Group | Convertible Debt Securities | Short-term Investments
ASSETS:
Assets at fair value		0				0		0
Level 3 | Recurring Basis | Trip.com Group | Convertible Debt Securities | Long-term Investments
ASSETS:
Assets at fair value		0				0		0
Level 3 | Recurring Basis | Not Designated as Hedging Instrument | Foreign currency exchange derivatives
ASSETS:
Assets at fair value		0				0		0
LIABILITIES:
Liabilities at fair value		$ 0				$ 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 on equity securities and impairment of investment.

[2] At March 31, 2020, the investment in DiDi 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).