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Fair Value of Financial Instruments (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Fair Value of Financial Instruments      
Money Market Funds $ 34,996 $ 16,528  
Securities 512,833    
Total Fair Value 528,465 351,975  
Transfers from level 1 to level 2 0   $ 0
Transfers from level 2 to level 1 0   $ 0
Money Market Funds      
Fair Value of Financial Instruments      
Money Market Funds 15,632 5,175  
Corporate Securities      
Fair Value of Financial Instruments      
Securities 98,028 123,270  
Government Securities      
Fair Value of Financial Instruments      
Securities 414,805 223,530  
Level 1      
Fair Value of Financial Instruments      
Total Fair Value 15,632 5,175  
Level 1 | Money Market Funds      
Fair Value of Financial Instruments      
Money Market Funds 15,632 5,175  
Level 2      
Fair Value of Financial Instruments      
Total Fair Value 512,833 346,800  
Level 2 | Corporate Securities      
Fair Value of Financial Instruments      
Securities 98,028 123,270  
Level 2 | Government Securities      
Fair Value of Financial Instruments      
Securities $ 414,805 $ 223,530