FAIR VALUE MEASUREMENTS Fair value measurements (Details) - Recurring Basis - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
ASSETS:
Total assets at fair value	$ 17,685,737	$ 13,055,008
Foreign Currency Contracts
ASSETS:
Total assets at fair value	80	756
LIABILITIES:
Total liabilities at fair value	1,693	1,015
Money market funds | Cash Equivalents
ASSETS:
Total assets at fair value	2,013,334	977,468
International government securities | Cash Equivalents
ASSETS:
Total assets at fair value	21,308	30,266
International government securities | Short-term Investments
ASSETS:
Total assets at fair value	691,142	249,684
International government securities | Long-term Investments
ASSETS:
Total assets at fair value	593,508	659,344
U.S. government securities | Cash Equivalents
ASSETS:
Total assets at fair value	27,625	176,140
U.S. government securities | Short-term Investments
ASSETS:
Total assets at fair value	843,660	456,888
U.S. government securities | Long-term Investments
ASSETS:
Total assets at fair value	1,003,786	766,592
Commercial paper | Cash Equivalents
ASSETS:
Total assets at fair value	77,319	1,998
Commercial paper | Short-term Investments
ASSETS:
Total assets at fair value	53,998	1,998
Time deposits | Cash Equivalents
ASSETS:
Total assets at fair value	4,532	49,160
Corporate debt securities | Cash Equivalents
ASSETS:
Total assets at fair value	15,397	9,273
Corporate debt securities | Short-term Investments
ASSETS:
Total assets at fair value	2,818,228	1,510,310
Corporate debt securities | Long-term Investments
ASSETS:
Total assets at fair value	6,858,554	6,001,789
U.S. government agency securities | Cash Equivalents
ASSETS:
Total assets at fair value	4,800
U.S. government agency securities | Long-term Investments
ASSETS:
Total assets at fair value	4,927	4,952
Ctrip convertible debt securities | Long-term Investments
ASSETS:
Total assets at fair value	1,512,638	1,293,088
Ctrip equity securities | Long-term Investments
ASSETS:
Total assets at fair value	1,140,901	865,302
Level 1
ASSETS:
Total assets at fair value	3,158,767	1,891,930
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	2,013,334	977,468
Level 1 | International government securities | Cash Equivalents
ASSETS:
Total assets at fair value	0	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	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	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	4,532	49,160
Level 1 | Corporate debt securities | Cash Equivalents
ASSETS:
Total assets at fair value	0	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 | Cash Equivalents
ASSETS:
Total assets at fair value	0
Level 1 | U.S. government agency securities | Long-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 | Ctrip convertible debt securities | Long-term Investments
ASSETS:
Total assets at fair value	0	0
Level 1 | Ctrip equity securities | Long-term Investments
ASSETS:
Total assets at fair value	1,140,901	865,302
Level 2
ASSETS:
Total assets at fair value	14,526,970	11,163,078
Level 2 | Foreign Currency Contracts
ASSETS:
Total assets at fair value	80	756
LIABILITIES:
Total liabilities at fair value	1,693	1,015
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	21,308	30,266
Level 2 | International government securities | Short-term Investments
ASSETS:
Total assets at fair value	691,142	249,684
Level 2 | International government securities | Long-term Investments
ASSETS:
Total assets at fair value	593,508	659,344
Level 2 | U.S. government securities | Cash Equivalents
ASSETS:
Total assets at fair value	27,625	176,140
Level 2 | U.S. government securities | Short-term Investments
ASSETS:
Total assets at fair value	843,660	456,888
Level 2 | U.S. government securities | Long-term Investments
ASSETS:
Total assets at fair value	1,003,786	766,592
Level 2 | Commercial paper | Cash Equivalents
ASSETS:
Total assets at fair value	77,319	1,998
Level 2 | Commercial paper | Short-term Investments
ASSETS:
Total assets at fair value	53,998	1,998
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	15,397	9,273
Level 2 | Corporate debt securities | Short-term Investments
ASSETS:
Total assets at fair value	2,818,228	1,510,310
Level 2 | Corporate debt securities | Long-term Investments
ASSETS:
Total assets at fair value	6,858,554	6,001,789
Level 2 | U.S. government agency securities | Cash Equivalents
ASSETS:
Total assets at fair value	4,800
Level 2 | U.S. government agency securities | Long-term Investments
ASSETS:
Total assets at fair value	4,927	4,952
Level 2 | Ctrip convertible debt securities | Long-term Investments
ASSETS:
Total assets at fair value	1,512,638	1,293,088
Level 2 | Ctrip equity securities | Long-term Investments
ASSETS:
Total assets at fair value	$ 0	$ 0