CASH, CASH EQUIVALENTS AND INVESTMENTS - Schedule of Cash, Cash Equivalents and Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Investments, Amortized Cost Basis	$ 378,739	$ 320,563
Investments, Gross Unrealized Gains	2	1
Investments, Gross Unrealized Losses	(174)	(392)
Investments	378,567	320,172
Certificates of Deposit [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	1,250	1,296
Cash Equivalents, Gross Unrealized Gain		1
Cash Equivalents, Gross Unrealized Losses	(1)	(1)
Cash and Cash Equivalents, Fair Value Disclosure	1,249	1,296
Corporate Debt Securities [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value		17,900
Available-for-sale Debt Securities, Amortized Cost Basis	204,396	104,626
Available-for-sale Securities, Gross Unrealized Gains	2
Available-for-sale Securities, Gross Unrealized Losses	(63)	(252)
Available-for-sale Securities, Fair Value Disclosure	204,335	104,374
U.S. Government Securities [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	24,900
Available-for-sale Debt Securities, Amortized Cost Basis	173,093	214,641
Available-for-sale Securities, Gross Unrealized Losses	(110)	(139)
Available-for-sale Securities, Fair Value Disclosure	$ 172,983	$ 214,502