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Fair value of financial instruments and marketable securities - Unrealized Loss Positions (Details) - Corporate debt securities - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Unrealized losses    
Securities in an unrealized loss position less than 12 months $ (131) $ (28)
Securities in an unrealized loss position greater than 12 months 0 (2)
Total (131) (30)
Fair Value    
Securities in an unrealized loss position less than 12 months 54,374 59,108
Securities in an unrealized loss position greater than 12 months 0 6,519
Total $ 54,374 $ 65,627