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Fair Value of Financial Instruments - Unrestricted Marketable Securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Fair Value of Financial Instruments    
Corporate debt securities, Amortized Cost $ 135,864 $ 99,487
Corporate debt securities, Gross Unrealized Gains 229 86
Corporate debt securities, Gross Unrealized Losses (232) (440)
Corporate debt securities, Fair Value 135,861 99,133
Contractual maturities of the unrestricted available for sale marketable securities held    
Less Than 1 Year 31,229  
1 year to 2 years 33,124  
3 years to 4 years 71,508  
Corporate debt securities, Fair Value $ 135,861 $ 99,133