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Fair Value of Financial Instruments - Unrestricted Marketable Securities (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value of Financial Instruments    
Corporate and government debt securities, Amortized Cost $ 174,235 $ 99,487
Corporate and government debt securities, Gross Unrealized Gains 48 86
Corporate and government debt securities, Gross Unrealized Losses (909) (440)
Corporate and government debt securities, Fair Value 173,374 99,133
Contractual maturities of the unrestricted available for sale marketable securities held    
Less Than 1 Year 39,736  
1 year to 2 years 42,921  
2 year to 3 years 43,021  
3 years to 4 years 47,696  
Corporate debt securities, Fair Value $ 173,374 $ 99,133