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Fair Value of Financial Instruments - Unrestricted Marketable Securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Fair Value of Financial Instruments    
Corporate and government debt securities, Amortized Cost $ 647,253 $ 174,235
Corporate and government debt securities, Gross Unrealized Gains 12 48
Corporate and government debt securities, Gross Unrealized Losses (4,745) (909)
Corporate and government debt securities, Fair Value 642,520 173,374
Contractual maturities of the unrestricted available for sale marketable securities held    
Less Than 1 Year 139,208  
1 year to 2 years 165,451  
2 year to 3 years 169,167  
3 years to 4 years 168,694  
Corporate debt securities, Fair Value $ 642,520 $ 173,374