FAIR VALUE MEASUREMENTS - Financial Assets and Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Recurring Basis
ASSETS:
Assets at fair value	$ 8,463	$ 10,751
Recurring Basis | Long-term Investments
ASSETS:
Assets at fair value	230	250
Recurring Basis | Money market funds | Cash Equivalents
ASSETS:
Assets at fair value	5,944	5,734
Recurring Basis | Time deposits and certificates of deposit | Cash Equivalents
ASSETS:
Assets at fair value	26	29
Recurring Basis | International government securities | Short-term Investments
ASSETS:
Assets at fair value		109
Recurring Basis | International government securities | Long-term Investments
ASSETS:
Assets at fair value		68
Recurring Basis | U.S. government securities | Short-term Investments
ASSETS:
Assets at fair value		138
Recurring Basis | U.S. government securities | Long-term Investments
ASSETS:
Assets at fair value		135
Recurring Basis | Corporate debt securities | Short-term Investments
ASSETS:
Assets at fair value	164	751
Recurring Basis | Corporate debt securities | Long-term Investments
ASSETS:
Assets at fair value		963
Recurring Basis | Corporate debt securities | Cash Equivalents
ASSETS:
Assets at fair value		2
Recurring Basis | Equity securities | Long-term Investments
ASSETS:
Assets at fair value	977	1,793
Recurring Basis | Trip.com Group | Trip.com Group convertible debt securities | Short-term Investments
ASSETS:
Assets at fair value	248
Recurring Basis | Trip.com Group | Trip.com Group convertible debt securities | Long-term Investments
ASSETS:
Assets at fair value	448	767
Recurring Basis | Trip.com Group | Equity securities | Short-term Investments
ASSETS:
Assets at fair value	414
Recurring Basis | Not Designated as Hedging Instrument | Foreign Currency Contracts
ASSETS:
Assets at fair value	12	12
LIABILITIES:
Liabilities at fair value	21	5
Nonrecurring Basis
ASSETS:
Assets at fair value	1,945
Nonrecurring Basis | Equity securities | Long-term Investments
ASSETS:
Assets at fair value	400
Nonrecurring Basis | Goodwill
ASSETS:
Assets at fair value	1,545
Level 1 | Recurring Basis
ASSETS:
Assets at fair value	7,361	7,556
Level 1 | Recurring Basis | Long-term Investments
ASSETS:
Assets at fair value	0	0
Level 1 | Recurring Basis | Money market funds | Cash Equivalents
ASSETS:
Assets at fair value	5,944	5,734
Level 1 | Recurring Basis | Time deposits and certificates of deposit | Cash Equivalents
ASSETS:
Assets at fair value	26	29
Level 1 | Recurring Basis | International government securities | Short-term Investments
ASSETS:
Assets at fair value		0
Level 1 | Recurring Basis | International government securities | Long-term Investments
ASSETS:
Assets at fair value		0
Level 1 | Recurring Basis | U.S. government securities | Short-term Investments
ASSETS:
Assets at fair value		0
Level 1 | Recurring Basis | U.S. government securities | Long-term Investments
ASSETS:
Assets at fair value		0
Level 1 | Recurring Basis | Corporate debt securities | Short-term Investments
ASSETS:
Assets at fair value	0	0
Level 1 | Recurring Basis | Corporate debt securities | Long-term Investments
ASSETS:
Assets at fair value		0
Level 1 | Recurring Basis | Corporate debt securities | Cash Equivalents
ASSETS:
Assets at fair value		0
Level 1 | Recurring Basis | Equity securities | Long-term Investments
ASSETS:
Assets at fair value	977	1,793
Level 1 | Recurring Basis | Trip.com Group | Trip.com Group convertible debt securities | Short-term Investments
ASSETS:
Assets at fair value	0
Level 1 | Recurring Basis | Trip.com Group | Trip.com Group convertible debt securities | Long-term Investments
ASSETS:
Assets at fair value	0	0
Level 1 | Recurring Basis | Trip.com Group | Equity securities | Short-term Investments
ASSETS:
Assets at fair value	414
Level 1 | Recurring Basis | Not Designated as Hedging Instrument | Foreign Currency Contracts
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 | Equity securities | Long-term Investments
ASSETS:
Assets at fair value	0
Level 1 | Nonrecurring Basis | Goodwill
ASSETS:
Assets at fair value	0
Level 2 | Recurring Basis
ASSETS:
Assets at fair value	872	2,945
Level 2 | Recurring Basis | Long-term Investments
ASSETS:
Assets at fair value	0	0
Level 2 | Recurring Basis | Money market funds | Cash Equivalents
ASSETS:
Assets at fair value	0	0
Level 2 | Recurring Basis | Time deposits and certificates of deposit | Cash Equivalents
ASSETS:
Assets at fair value	0	0
Level 2 | Recurring Basis | International government securities | Short-term Investments
ASSETS:
Assets at fair value		109
Level 2 | Recurring Basis | International government securities | Long-term Investments
ASSETS:
Assets at fair value		68
Level 2 | Recurring Basis | U.S. government securities | Short-term Investments
ASSETS:
Assets at fair value		138
Level 2 | Recurring Basis | U.S. government securities | Long-term Investments
ASSETS:
Assets at fair value		135
Level 2 | Recurring Basis | Corporate debt securities | Short-term Investments
ASSETS:
Assets at fair value	164	751
Level 2 | Recurring Basis | Corporate debt securities | Long-term Investments
ASSETS:
Assets at fair value		963
Level 2 | Recurring Basis | Corporate debt securities | Cash Equivalents
ASSETS:
Assets at fair value		2
Level 2 | Recurring Basis | Equity securities | Long-term Investments
ASSETS:
Assets at fair value	0	0
Level 2 | Recurring Basis | Trip.com Group | Trip.com Group convertible debt securities | Short-term Investments
ASSETS:
Assets at fair value	248
Level 2 | Recurring Basis | Trip.com Group | Trip.com Group convertible debt securities | Long-term Investments
ASSETS:
Assets at fair value	448	767
Level 2 | Recurring Basis | Trip.com Group | Equity securities | Short-term Investments
ASSETS:
Assets at fair value	0
Level 2 | Recurring Basis | Not Designated as Hedging Instrument | Foreign Currency Contracts
ASSETS:
Assets at fair value	12	12
LIABILITIES:
Liabilities at fair value	21	5
Level 2 | Nonrecurring Basis
ASSETS:
Assets at fair value	0
Level 2 | Nonrecurring Basis | Equity securities | Long-term Investments
ASSETS:
Assets at fair value	0
Level 2 | Nonrecurring Basis | Goodwill
ASSETS:
Assets at fair value	0
Level 3 | Recurring Basis
ASSETS:
Assets at fair value	230	250
Level 3 | Recurring Basis | Long-term Investments
ASSETS:
Assets at fair value	230	250
Level 3 | Recurring Basis | Money market funds | Cash Equivalents
ASSETS:
Assets at fair value	0	0
Level 3 | Recurring Basis | Time deposits and certificates of deposit | Cash Equivalents
ASSETS:
Assets at fair value	0	0
Level 3 | Recurring Basis | International government securities | Short-term Investments
ASSETS:
Assets at fair value		0
Level 3 | Recurring Basis | International government securities | Long-term Investments
ASSETS:
Assets at fair value		0
Level 3 | Recurring Basis | U.S. government securities | Short-term Investments
ASSETS:
Assets at fair value		0
Level 3 | Recurring Basis | U.S. government securities | Long-term Investments
ASSETS:
Assets at fair value		0
Level 3 | Recurring Basis | Corporate debt securities | Short-term Investments
ASSETS:
Assets at fair value	0	0
Level 3 | Recurring Basis | Corporate debt securities | Long-term Investments
ASSETS:
Assets at fair value		0
Level 3 | Recurring Basis | Corporate debt securities | Cash Equivalents
ASSETS:
Assets at fair value		0
Level 3 | Recurring Basis | Equity securities | Long-term Investments
ASSETS:
Assets at fair value	0	0
Level 3 | Recurring Basis | Trip.com Group | Trip.com Group convertible debt securities | Short-term Investments
ASSETS:
Assets at fair value	0
Level 3 | Recurring Basis | Trip.com Group | Trip.com Group convertible debt securities | Long-term Investments
ASSETS:
Assets at fair value	0	0
Level 3 | Recurring Basis | Trip.com Group | Equity securities | Short-term Investments
ASSETS:
Assets at fair value	0
Level 3 | Recurring Basis | Not Designated as Hedging Instrument | Foreign Currency Contracts
ASSETS:
Assets at fair value	0	0
LIABILITIES:
Liabilities at fair value	0	$ 0
Level 3 | Nonrecurring Basis
ASSETS:
Assets at fair value	1,945
Level 3 | Nonrecurring Basis | Equity securities | Long-term Investments
ASSETS:
Assets at fair value	400
Level 3 | Nonrecurring Basis | Goodwill
ASSETS:
Assets at fair value	$ 1,545