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Fair Value of Financial Instruments - Unrestricted Marketable Securities (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Fair Value of Financial Instruments    
Corporate and government debt securities, Amortized Cost $ 586,726 $ 174,235
Corporate and government debt securities, Gross Unrealized Gains 55 48
Corporate and government debt securities, Gross Unrealized Losses (4,213) (909)
Corporate and government debt securities, Fair Value 582,568 173,374
Contractual maturities of the unrestricted available for sale marketable securities held    
Less Than 1 Year 163,770  
1 year to 2 years 166,482  
2 year to 3 years 163,687  
3 years to 4 years 88,629  
Corporate debt securities, Fair Value $ 582,568 $ 173,374