FAIR VALUE OF CERTAIN FINANCIAL ASSETS AND LIABILITIES - Assets - Recurring Basis (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Fair value amounts included in the carrying value of
Cash and cash equivalents	$ 465,559	$ 377,582	$ 341,526	$ 2,175,417
Short-term investments	630,348	220,554
Investments	7,003	2,394
Asset transfers between Level 1 and Level 2 measurements	0	0
Total fair value
Fair value amounts included in the carrying value of
Cash and cash equivalents	465,559	377,582
Short-term investments	630,348	220,554
Accounts receivable, net	174	236
Investments	7,003	2,394
Accrued liabilities	(513)	(764)
Assets measured at fair value	1,102,571	600,002
Cash	330,758	278,972
Foreign currency derivatives	(339)	(528)
Money market funds | Total fair value
Fair value amounts included in the carrying value of
Assets measured at fair value	69,273	76,112
Certificates of deposit | Total fair value
Fair value amounts included in the carrying value of
Assets measured at fair value	56,698
Commercial paper | Total fair value
Fair value amounts included in the carrying value of
Assets measured at fair value	81,708	47,855
Variable rate demand notes | Total fair value
Fair value amounts included in the carrying value of
Assets measured at fair value	46,054	13,923
U.S. Treasuries | Total fair value
Fair value amounts included in the carrying value of
Assets measured at fair value	11,981
Municipal securities | Total fair value
Fair value amounts included in the carrying value of
Assets measured at fair value	423,159	157,617
U.S. government agency securities | Total fair value
Fair value amounts included in the carrying value of
Assets measured at fair value	83,279	26,051
Level 1
Fair value amounts included in the carrying value of
Cash and cash equivalents	400,031	355,084
Assets measured at fair value	400,031	355,084
Cash	330,758	278,972
Level 1 | Money market funds
Fair value amounts included in the carrying value of
Assets measured at fair value	69,273	76,112
Level 2
Fair value amounts included in the carrying value of
Cash and cash equivalents	65,528	22,498
Short-term investments	630,348	220,554
Accounts receivable, net	174	236
Investments	7,003	2,394
Accrued liabilities	(513)	(764)
Assets measured at fair value	702,540	244,918
Foreign currency derivatives	(339)	(528)
Level 2 | Certificates of deposit
Fair value amounts included in the carrying value of
Assets measured at fair value	56,698
Level 2 | Commercial paper
Fair value amounts included in the carrying value of
Assets measured at fair value	81,708	47,855
Level 2 | Variable rate demand notes
Fair value amounts included in the carrying value of
Assets measured at fair value	46,054	13,923
Level 2 | U.S. Treasuries
Fair value amounts included in the carrying value of
Assets measured at fair value	11,981
Level 2 | Municipal securities
Fair value amounts included in the carrying value of
Assets measured at fair value	423,159	157,617
Level 2 | U.S. government agency securities
Fair value amounts included in the carrying value of
Assets measured at fair value	$ 83,279	$ 26,051