CASH, CASH EQUIVALENTS AND INVESTMENTS - Schedule of Cash, Cash Equivalents and Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Investments, Amortized Cost Basis	$ 320,563	$ 248,951
Investments, Gross Unrealized Gains	1	10
Investments, Gross Unrealized Losses	(392)	(382)
Investments	320,172	248,579
Certificates of Deposit [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	1,296	3,362
Cash Equivalents, Gross Unrealized Gain	1	3
Cash Equivalents, Gross Unrealized Losses	(1)
Cash and Cash Equivalents, Fair Value Disclosure	1,296	3,365
Corporate Debt Securities [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	17,900	15,000
Available-for-sale Debt Securities, Amortized Cost Basis	104,626	209,595
Available-for-sale Securities, Gross Unrealized Gains		6
Available-for-sale Securities, Gross Unrealized Losses	(252)	(377)
Available-for-sale Securities, Fair Value Disclosure	104,374	209,224
U.S. Government Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	0	30,000
Available-for-sale Debt Securities, Amortized Cost Basis	214,641	32,996
Available-for-sale Securities, Gross Unrealized Gains		1
Available-for-sale Securities, Gross Unrealized Losses	(139)	(3)
Available-for-sale Securities, Fair Value Disclosure	$ 214,502	32,994
Money Market Instruments [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value		2,998
Cash Equivalents, Gross Unrealized Losses		(2)
Cash and Cash Equivalents, Fair Value Disclosure		$ 2,996