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Marketable Securities - Unrealized losses (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Fair value    
Fair value, less than 12 months $ 268,115 $ 207,603
Fair value, 12 months or greater 236,108 139,197
Unrealized losses    
Unrealized losses, Less than 12 months (977) (598)
Unrealized gain (losses), 12 months or greater 16 (1,201)
Corporate Securities    
Fair value    
Fair value, less than 12 months 84,770 79,290
Fair value, 12 months or greater 20,198 43,980
Unrealized losses    
Unrealized losses, Less than 12 months (310) (137)
Unrealized gain (losses), 12 months or greater (34) (453)
Government Securities    
Fair value    
Fair value, less than 12 months 183,345 128,313
Fair value, 12 months or greater 215,910 95,217
Unrealized losses    
Unrealized losses, Less than 12 months (667) (461)
Unrealized gain (losses), 12 months or greater $ 50 $ (748)