FAIR VALUE MEASUREMENTS Fair value measurements (Details) - Recurring Basis - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
ASSETS:
Total assets at fair value	$ 15,712,056	$ 17,321,668
Foreign Currency Contracts
ASSETS:
Total assets at fair value	440	1,767
LIABILITIES:
Total liabilities at fair value	1,853	127
Money market funds | Cash Equivalents
ASSETS:
Total assets at fair value	1,930,983	1,895,272
International government securities | Cash Equivalents
ASSETS:
Total assets at fair value	70,025
International government securities | Short-term Investments
ASSETS:
Total assets at fair value	460,890	725,376
International government securities | Long-term Investments
ASSETS:
Total assets at fair value	788,134	607,910
U.S. government securities | Cash Equivalents
ASSETS:
Total assets at fair value		22,265
U.S. government securities | Short-term Investments
ASSETS:
Total assets at fair value	790,425	994,118
U.S. government securities | Long-term Investments
ASSETS:
Total assets at fair value	543,888	834,276
Commercial paper | Cash Equivalents
ASSETS:
Total assets at fair value		96,321
Commercial paper | Short-term Investments
ASSETS:
Total assets at fair value	46,581	72,650
Time deposits | Cash Equivalents
ASSETS:
Total assets at fair value	37,141	17,896
Corporate debt securities | Cash Equivalents
ASSETS:
Total assets at fair value		6,674
Corporate debt securities | Short-term Investments
ASSETS:
Total assets at fair value	2,938,400	3,063,315
Corporate debt securities | Long-term Investments
ASSETS:
Total assets at fair value	5,680,724	6,656,424
U.S government agency securities | Short-term Investments
ASSETS:
Total assets at fair value	4,408	4,414
U.S government agency securities | Long-term Investments
ASSETS:
Total assets at fair value	494	494
Level 1
ASSETS:
Total assets at fair value	2,976,634	2,867,164
Level 1 | Foreign Currency Contracts
ASSETS:
Total assets at fair value	0	0
LIABILITIES:
Total liabilities at fair value	0	0
Level 1 | Money market funds | Cash Equivalents
ASSETS:
Total assets at fair value	1,930,983	1,895,272
Level 1 | International government securities | Cash Equivalents
ASSETS:
Total assets at fair value	0
Level 1 | International government securities | Short-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 | International government securities | Long-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 | U.S. government securities | Cash Equivalents
ASSETS:
Total assets at fair value		0
Level 1 | U.S. government securities | Short-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 | U.S. government securities | Long-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 | Commercial paper | Cash Equivalents
ASSETS:
Total assets at fair value		0
Level 1 | Commercial paper | Short-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 | Time deposits | Cash Equivalents
ASSETS:
Total assets at fair value	37,141	17,896
Level 1 | Corporate debt securities | Cash Equivalents
ASSETS:
Total assets at fair value		0
Level 1 | Corporate debt securities | Short-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 | Corporate debt securities | Long-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 | U.S government agency securities | Short-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 | U.S government agency securities | Long-term Investments
ASSETS:
Total assets at fair value	0	0
Level 2
ASSETS:
Total assets at fair value	12,735,422	14,454,504
Level 2 | Foreign Currency Contracts
ASSETS:
Total assets at fair value	440	1,767
LIABILITIES:
Total liabilities at fair value	1,853	127
Level 2 | Money market funds | Cash Equivalents
ASSETS:
Total assets at fair value	0	0
Level 2 | International government securities | Cash Equivalents
ASSETS:
Total assets at fair value	70,025
Level 2 | International government securities | Short-term Investments
ASSETS:
Total assets at fair value	460,890	725,376
Level 2 | International government securities | Long-term Investments
ASSETS:
Total assets at fair value	788,134	607,910
Level 2 | U.S. government securities | Cash Equivalents
ASSETS:
Total assets at fair value		22,265
Level 2 | U.S. government securities | Short-term Investments
ASSETS:
Total assets at fair value	790,425	994,118
Level 2 | U.S. government securities | Long-term Investments
ASSETS:
Total assets at fair value	543,888	834,276
Level 2 | Commercial paper | Cash Equivalents
ASSETS:
Total assets at fair value		96,321
Level 2 | Commercial paper | Short-term Investments
ASSETS:
Total assets at fair value	46,581	72,650
Level 2 | Time deposits | Cash Equivalents
ASSETS:
Total assets at fair value	0	0
Level 2 | Corporate debt securities | Cash Equivalents
ASSETS:
Total assets at fair value		6,674
Level 2 | Corporate debt securities | Short-term Investments
ASSETS:
Total assets at fair value	2,938,400	3,063,315
Level 2 | Corporate debt securities | Long-term Investments
ASSETS:
Total assets at fair value	5,680,724	6,656,424
Level 2 | U.S government agency securities | Short-term Investments
ASSETS:
Total assets at fair value	4,408	4,414
Level 2 | U.S government agency securities | Long-term Investments
ASSETS:
Total assets at fair value	494	494
Ctrip.com International, Ltd. | Ctrip convertible debt securities | Long-term Investments
ASSETS:
Total assets at fair value	1,411,013	1,368,500
Ctrip.com International, Ltd. | Ctrip equity securities | Long-term Investments
ASSETS:
Total assets at fair value	1,008,510	953,996
Ctrip.com International, Ltd. | Level 1 | Ctrip convertible debt securities | Long-term Investments
ASSETS:
Total assets at fair value	0	0
Ctrip.com International, Ltd. | Level 1 | Ctrip equity securities | Long-term Investments
ASSETS:
Total assets at fair value	1,008,510	953,996
Ctrip.com International, Ltd. | Level 2 | Ctrip convertible debt securities | Long-term Investments
ASSETS:
Total assets at fair value	1,411,013	1,368,500
Ctrip.com International, Ltd. | Level 2 | Ctrip equity securities | Long-term Investments
ASSETS:
Total assets at fair value	$ 0	$ 0