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CASH EQUIVALENTS AND INVESTMENTS - Schedule of Cash Equivalents and Investments (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Cash and Cash Equivalents [Line Items]    
Investments, Amortized Cost Basis $ 248,951 $ 300,333
Investments, Gross Unrealized Gains 10 4
Investments, Gross Unrealized Losses (382) (169)
Investments 248,579 300,168
Certificates of Deposit [Member]    
Cash and Cash Equivalents [Line Items]    
Cash Equivalents, at Carrying Value 3,362 11,532
Cash Equivalents, Gross Unrealized Gain 3 3
Cash Equivalents, Gross Unrealized Losses   (14)
Cash and Cash Equivalents, Fair Value Disclosure 3,365 11,521
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  
Corporate Debt Securities [Member]    
Cash and Cash Equivalents [Line Items]    
Cash Equivalents, at Carrying Value 14,975  
Available-for-sale Debt Securities, Amortized Cost Basis 209,595 233,848
Available-for-sale Securities, Gross Unrealized Gains 6  
Available-for-sale Securities, Gross Unrealized Losses (377) (139)
Available-for-sale Securities, Fair Value Disclosure 209,224 233,709
US Treasury and Government [Member]    
Cash and Cash Equivalents [Line Items]    
Cash Equivalents, at Carrying Value 30,000  
Available-for-sale Debt Securities, Amortized Cost Basis 32,996 54,953
Available-for-sale Securities, Gross Unrealized Gains 1 1
Available-for-sale Securities, Gross Unrealized Losses (3) (16)
Available-for-sale Securities, Fair Value Disclosure $ 32,994 $ 54,938