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Fair Value of Financial Instruments - Unrestricted Marketable Securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Fair Value Disclosures [Abstract]    
Amortized cost $ 757,206 $ 586,726
Gross unrealized gains 7,966 55
Gross unrealized losses (196) (4,213)
Contractual maturities of the unrestricted available for sale marketable securities held    
Less than 1 year 171,222  
1 year to 2 years 192,488  
2 years to 3 years 200,597  
3 years to 4 years 200,669  
Greater than 4 years 0  
Total $ 764,976 $ 582,568