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Marketable Securities - Unrealized losses (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Fair value    
Fair value, less than 12 months $ 236,605 $ 207,603
Fair value, 12 months or greater 276,228 139,197
Unrealized losses    
Unrealized losses, Less than 12 months (1,131) (598)
Unrealized losses, 12 months or greater (1,199) (1,201)
Corporate Securities    
Fair value    
Fair value, less than 12 months 69,494 79,290
Fair value, 12 months or greater 28,534 43,980
Unrealized losses    
Unrealized losses, Less than 12 months (411) (137)
Unrealized losses, 12 months or greater (60) (453)
Government Securities    
Fair value    
Fair value, less than 12 months 167,111 128,313
Fair value, 12 months or greater 247,694 95,217
Unrealized losses    
Unrealized losses, Less than 12 months (720) (461)
Unrealized losses, 12 months or greater $ (1,139) $ (748)