FAIR VALUE MEASUREMENTS Fair value measurements (Details) - Recurring Basis - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
ASSETS:
Total assets at fair value	$ 15,865,996	$ 17,321,668
Foreign Currency Contracts
ASSETS:
Total assets at fair value	318	1,767
LIABILITIES:
Total liabilities at fair value	2,036	127
Money market funds | Cash Equivalents
ASSETS:
Total assets at fair value	2,673,212	1,895,272
International government securities | Short-term Investments
ASSETS:
Total assets at fair value	408,882	725,376
International government securities | Long-term Investments
ASSETS:
Total assets at fair value	809,291	607,910
U.S. government securities | Cash Equivalents
ASSETS:
Total assets at fair value	9,973	22,265
U.S. government securities | Short-term Investments
ASSETS:
Total assets at fair value	869,209	994,118
U.S. government securities | Long-term Investments
ASSETS:
Total assets at fair value	456,340	834,276
Commercial paper | Cash Equivalents
ASSETS:
Total assets at fair value	28,490	96,321
Commercial paper | Short-term Investments
ASSETS:
Total assets at fair value	37,808	72,650
Certificate of deposit | Short-term Investments
ASSETS:
Total assets at fair value	1,001
U.S. government agency securities | Cash Equivalents
ASSETS:
Total assets at fair value	1,000
Time deposits | Cash Equivalents
ASSETS:
Total assets at fair value	7,983	17,896
Corporate debt securities | Cash Equivalents
ASSETS:
Total assets at fair value	5,268	6,674
Corporate debt securities | Short-term Investments
ASSETS:
Total assets at fair value	2,823,947	3,063,315
Corporate debt securities | Long-term Investments
ASSETS:
Total assets at fair value	5,302,171	6,656,424
U.S. government agency securities | Short-term Investments
ASSETS:
Total assets at fair value	4,906	4,414
U.S. government agency securities | Long-term Investments
ASSETS:
Total assets at fair value		494
Level 1
ASSETS:
Total assets at fair value	3,712,555	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	2,673,212	1,895,272
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 | Certificate of deposit | Short-term Investments
ASSETS:
Total assets at fair value	1,001
Level 1 | U.S. government agency securities | Cash Equivalents
ASSETS:
Total assets at fair value	0
Level 1 | Time deposits | Cash Equivalents
ASSETS:
Total assets at fair value	7,983	17,896
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 | 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
Level 2
ASSETS:
Total assets at fair value	12,153,441	14,454,504
Level 2 | Foreign Currency Contracts
ASSETS:
Total assets at fair value	318	1,767
LIABILITIES:
Total liabilities at fair value	2,036	127
Level 2 | Money market funds | Cash Equivalents
ASSETS:
Total assets at fair value	0	0
Level 2 | International government securities | Short-term Investments
ASSETS:
Total assets at fair value	408,882	725,376
Level 2 | International government securities | Long-term Investments
ASSETS:
Total assets at fair value	809,291	607,910
Level 2 | U.S. government securities | Cash Equivalents
ASSETS:
Total assets at fair value	9,973	22,265
Level 2 | U.S. government securities | Short-term Investments
ASSETS:
Total assets at fair value	869,209	994,118
Level 2 | U.S. government securities | Long-term Investments
ASSETS:
Total assets at fair value	456,340	834,276
Level 2 | Commercial paper | Cash Equivalents
ASSETS:
Total assets at fair value	28,490	96,321
Level 2 | Commercial paper | Short-term Investments
ASSETS:
Total assets at fair value	37,808	72,650
Level 2 | Certificate of deposit | Short-term Investments
ASSETS:
Total assets at fair value	0
Level 2 | U.S. government agency securities | Cash Equivalents
ASSETS:
Total assets at fair value	1,000
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	5,268	6,674
Level 2 | Corporate debt securities | Short-term Investments
ASSETS:
Total assets at fair value	2,823,947	3,063,315
Level 2 | Corporate debt securities | Long-term Investments
ASSETS:
Total assets at fair value	5,302,171	6,656,424
Level 2 | U.S. government agency securities | Short-term Investments
ASSETS:
Total assets at fair value	4,906	4,414
Level 2 | U.S. government agency securities | Long-term Investments
ASSETS:
Total assets at fair value		494
Ctrip.com International, Ltd. | Convertible debt securities | Long-term Investments
ASSETS:
Total assets at fair value	1,395,838	1,368,500
Ctrip.com International, Ltd. | Equity securities | Long-term Investments
ASSETS:
Total assets at fair value	1,030,359	953,996
Ctrip.com International, Ltd. | Level 1 | Convertible debt securities | Long-term Investments
ASSETS:
Total assets at fair value	0	0
Ctrip.com International, Ltd. | Level 1 | Equity securities | Long-term Investments
ASSETS:
Total assets at fair value	1,030,359	953,996
Ctrip.com International, Ltd. | Level 2 | Convertible debt securities | Long-term Investments
ASSETS:
Total assets at fair value	1,395,838	1,368,500
Ctrip.com International, Ltd. | Level 2 | Equity securities | Long-term Investments
ASSETS:
Total assets at fair value	$ 0	$ 0