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Fair value of financial instruments and marketable securities - Unrealized Loss Positions (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2020
Dec. 31, 2019
Unrealized losses      
Securities in an unrealized loss position less than 12 months $ (53) $ (53) $ (28)
Securities in an unrealized loss position greater than 12 months 0 0 0
Total (53) (53) (28)
Fair Value      
Securities in an unrealized loss position less than 12 months 151,245 151,245 95,990
Securities in an unrealized loss position greater than 12 months 0 0 0
Total 151,245 151,245 95,990
Write downs of available for sale debt securities 0 0  
Commercial paper      
Unrealized losses      
Securities in an unrealized loss position less than 12 months (16) (16)  
Securities in an unrealized loss position greater than 12 months 0 0  
Total (16) (16)  
Fair Value      
Securities in an unrealized loss position less than 12 months 53,580 53,580  
Securities in an unrealized loss position greater than 12 months 0 0  
Total 53,580 53,580  
Corporate debt securities      
Unrealized losses      
Securities in an unrealized loss position less than 12 months (19) (19) (20)
Securities in an unrealized loss position greater than 12 months 0 0 0
Total (19) (19) (20)
Fair Value      
Securities in an unrealized loss position less than 12 months 62,811 62,811 71,779
Securities in an unrealized loss position greater than 12 months 0 0 0
Total 62,811 62,811 71,779
Asset-backed securities      
Unrealized losses      
Securities in an unrealized loss position less than 12 months (2) (2) (8)
Securities in an unrealized loss position greater than 12 months 0 0 0
Total (2) (2) (8)
Fair Value      
Securities in an unrealized loss position less than 12 months 2,899 2,899 24,211
Securities in an unrealized loss position greater than 12 months 0 0 0
Total 2,899 2,899 $ 24,211
Government obligations      
Unrealized losses      
Securities in an unrealized loss position less than 12 months (16) (16)  
Securities in an unrealized loss position greater than 12 months 0 0  
Total (16) (16)  
Fair Value      
Securities in an unrealized loss position less than 12 months 31,955 31,955  
Securities in an unrealized loss position greater than 12 months 0 0  
Total $ 31,955 $ 31,955