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Fair Value of Financial Instruments - Unrestricted Marketable Securities (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Fair Value of Financial Instruments    
Corporate and government debt securities, Amortized cost $ 690,853 $ 586,726
Corporate and government debt securities, Gross unrealized gains 2,905 55
Corporate and government debt securities, Gross unrealized losses (1,042) (4,213)
Corporate and government debt securities, Fair Value 692,716 582,568
Contractual maturities of the unrestricted available for sale marketable securities held    
Less than 1 year 170,165  
1 year to 2 years 170,246  
2 years to 3 years 181,872  
3 years to 4 years 170,433  
Corporate debt securities, Fair Value $ 692,716 $ 582,568