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Fair value of financial instruments and marketable securities - Unrealized Loss Positions (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Unrealized losses    
Securities in an unrealized loss position less than 12 months $ (68)  
Securities in an unrealized loss position greater than 12 months 0  
Total (68)  
Fair Value    
Securities in an unrealized loss position less than 12 months 56,447  
Securities in an unrealized loss position greater than 12 months 0  
Total 56,447  
Commercial paper    
Unrealized losses    
Securities in an unrealized loss position less than 12 months (1)  
Securities in an unrealized loss position greater than 12 months 0  
Total (1)  
Fair Value    
Securities in an unrealized loss position less than 12 months 10,457  
Securities in an unrealized loss position greater than 12 months 0  
Total 10,457  
Corporate debt securities    
Unrealized losses    
Securities in an unrealized loss position less than 12 months (67) $ (28)
Securities in an unrealized loss position greater than 12 months 0 (2)
Total (67) (30)
Fair Value    
Securities in an unrealized loss position less than 12 months 45,990 59,108
Securities in an unrealized loss position greater than 12 months 0 6,519
Total $ 45,990 $ 65,627