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Fair value of financial instruments and marketable securities - Unrealized Loss Positions (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Unrealized losses    
Securities in an unrealized loss position less than 12 months $ (47) $ (8)
Securities in an unrealized loss position greater than 12 months 0 (3)
Total (47) (11)
Fair Value    
Securities in an unrealized loss position less than 12 months 67,793 16,223
Securities in an unrealized loss position greater than 12 months 0 10,087
Total 67,793 26,310
Commercial paper    
Unrealized losses    
Securities in an unrealized loss position less than 12 months 0 (1)
Securities in an unrealized loss position greater than 12 months 0 0
Total 0 (1)
Fair Value    
Securities in an unrealized loss position less than 12 months 0 1,993
Securities in an unrealized loss position greater than 12 months 0 0
Total 0 1,993
Corporate debt securities    
Unrealized losses    
Securities in an unrealized loss position less than 12 months (46) (7)
Securities in an unrealized loss position greater than 12 months 0 (3)
Total (46) (10)
Fair Value    
Securities in an unrealized loss position less than 12 months 61,716 14,230
Securities in an unrealized loss position greater than 12 months 0 10,087
Total 61,716 $ 24,317
Asset-backed securities    
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 6,077  
Securities in an unrealized loss position greater than 12 months 0  
Total $ 6,077