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Fair value of financial instruments and marketable securities - Unrealized Loss Positions (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Unrealized losses    
Securities in an unrealized loss position less than 12 months $ (4) $ (8)
Securities in an unrealized loss position greater than 12 months 0 (3)
Total (4) (11)
Fair Value    
Securities in an unrealized loss position less than 12 months 3,017 16,223
Securities in an unrealized loss position greater than 12 months 0 10,087
Total 3,017 26,310
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   1,993
Securities in an unrealized loss position greater than 12 months   0
Total   1,993
Corporate debt securities    
Unrealized losses    
Securities in an unrealized loss position less than 12 months (4) (7)
Securities in an unrealized loss position greater than 12 months 0 (3)
Total (4) (10)
Fair Value    
Securities in an unrealized loss position less than 12 months 3,017 14,230
Securities in an unrealized loss position greater than 12 months 0 10,087
Total $ 3,017 $ 24,317