CASH, CASH EQUIVALENTS AND INVESTMENTS - Schedule of Cash, Cash Equivalents and Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Investments, Amortized Cost Basis	$ 282,961	$ 248,951
Investments, Gross Unrealized Gains	5	10
Investments, Gross Unrealized Losses	(401)	(382)
Investments	282,565	248,579
Certificates of Deposit [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	2,062	3,362
Cash Equivalents, Gross Unrealized Gain	2	3
Cash Equivalents, Gross Unrealized Losses	0
Cash and Cash Equivalents, Fair Value Disclosure	2,064	3,365
Commercial Paper [Member]
Cash and Cash Equivalents [Line Items]
Cash Equivalents, at Carrying Value	3,985	2,998
Cash Equivalents, Gross Unrealized Losses	(7)	(2)
Cash and Cash Equivalents, Fair Value Disclosure	3,978	2,996
Corporate Debt Securities [Member]
Cash and Cash Equivalents [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	274,914	209,595
Available-for-sale Securities, Gross Unrealized Gains	3	6
Available-for-sale Securities, Gross Unrealized Losses	(393)	(377)
Available-for-sale Securities, Fair Value Disclosure	274,524	209,224
US Treasury and Government [Member]
Cash and Cash Equivalents [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,000	32,996
Available-for-sale Securities, Gross Unrealized Gains		1
Available-for-sale Securities, Gross Unrealized Losses	(1)	(3)
Available-for-sale Securities, Fair Value Disclosure	$ 1,999	$ 32,994