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CASH, CASH EQUIVALENTS AND INVESTMENTS - Schedule of Cash, Cash Equivalents and Investments (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Cash and Cash Equivalents [Line Items]    
Investments, Amortized Cost Basis $ 275,115 $ 248,951
Investments, Gross Unrealized Gains 19 10
Investments, Gross Unrealized Losses (250) (382)
Investments 274,884 248,579
Certificates of Deposit [Member]    
Cash and Cash Equivalents [Line Items]    
Cash Equivalents, at Carrying Value 1,770 3,362
Cash Equivalents, Gross Unrealized Gain 1 3
Cash Equivalents, Gross Unrealized Losses (1)  
Cash and Cash Equivalents, Fair Value Disclosure 1,770 3,365
Commercial Paper [Member]    
Cash and Cash Equivalents [Line Items]    
Cash Equivalents, at Carrying Value 3,994 2,998
Cash Equivalents, Gross Unrealized Losses (1) (2)
Cash and Cash Equivalents, Fair Value Disclosure 3,993 2,996
Corporate Debt Securities [Member]    
Cash and Cash Equivalents [Line Items]    
Available-for-sale Debt Securities, Amortized Cost Basis 195,542 209,595
Available-for-sale Securities, Gross Unrealized Gains 14 6
Available-for-sale Securities, Gross Unrealized Losses (242) (377)
Available-for-sale Securities, Fair Value Disclosure 195,314 209,224
US Treasury and Government [Member]    
Cash and Cash Equivalents [Line Items]    
Available-for-sale Debt Securities, Amortized Cost Basis 73,809 32,996
Available-for-sale Securities, Gross Unrealized Gains 4 1
Available-for-sale Securities, Gross Unrealized Losses (6) (3)
Available-for-sale Securities, Fair Value Disclosure $ 73,807 $ 32,994