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Fair value of financial instruments and marketable securities - Unrealized Loss Positions (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Unrealized losses    
Securities in an unrealized loss position less than 12 months $ (30)  
Securities in an unrealized loss position greater than 12 months 0  
Total (30)  
Fair Value    
Securities in an unrealized loss position less than 12 months 36,502  
Securities in an unrealized loss position greater than 12 months 0  
Total 36,502  
Commercial paper    
Unrealized losses    
Securities in an unrealized loss position less than 12 months (6)  
Securities in an unrealized loss position greater than 12 months 0  
Total (6)  
Fair Value    
Securities in an unrealized loss position less than 12 months 18,435  
Securities in an unrealized loss position greater than 12 months 0  
Total 18,435  
Corporate debt securities    
Unrealized losses    
Securities in an unrealized loss position less than 12 months (24) $ (28)
Securities in an unrealized loss position greater than 12 months 0 (2)
Total (24) (30)
Fair Value    
Securities in an unrealized loss position less than 12 months 18,067 59,108
Securities in an unrealized loss position greater than 12 months 0 6,519
Total $ 18,067 $ 65,627