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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, 2021
Sep. 30, 2021
Dec. 31, 2020
Unrealized losses      
Securities in an unrealized loss position less than 12 months $ (42) $ (42) $ (75)
Total (42) (42) (75)
Fair Value      
Securities in an unrealized loss position less than 12 months 98,697 98,697 260,970
Total 98,697 98,697 260,970
Write downs of available for sale debt securities 0 0  
Commercial paper      
Unrealized losses      
Securities in an unrealized loss position less than 12 months     (37)
Total     (37)
Fair Value      
Securities in an unrealized loss position less than 12 months     129,630
Total     129,630
Corporate debt securities      
Unrealized losses      
Securities in an unrealized loss position less than 12 months (37) (37) (29)
Total (37) (37) (29)
Fair Value      
Securities in an unrealized loss position less than 12 months 78,272 78,272 102,426
Total 78,272 78,272 102,426
Asset-backed securities      
Unrealized losses      
Securities in an unrealized loss position less than 12 months (1) (1) (3)
Total (1) (1) (3)
Fair Value      
Securities in an unrealized loss position less than 12 months 499 499 1,830
Total 499 499 1,830
Government obligations      
Unrealized losses      
Securities in an unrealized loss position less than 12 months (4) (4) (6)
Total (4) (4) (6)
Fair Value      
Securities in an unrealized loss position less than 12 months 19,926 19,926 27,084
Total $ 19,926 $ 19,926 $ 27,084