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Fair value of financial instruments and marketable securities - Unrealized Loss Positions (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2021
Dec. 31, 2020
Unrealized losses      
Securities in an unrealized loss position less than 12 months $ (50) $ (50) $ (75)
Total (50) (50) (75)
Fair Value      
Securities in an unrealized loss position less than 12 months 117,895 117,895 260,970
Total 117,895 117,895 260,970
Write downs of available for sale debt securities 0 0  
Commercial paper      
Unrealized losses      
Securities in an unrealized loss position less than 12 months (1) (1) (37)
Total (1) (1) (37)
Fair Value      
Securities in an unrealized loss position less than 12 months 7,991 7,991 129,630
Total 7,991 7,991 129,630
Corporate debt securities      
Unrealized losses      
Securities in an unrealized loss position less than 12 months (48) (48) (29)
Total (48) (48) (29)
Fair Value      
Securities in an unrealized loss position less than 12 months 80,058 80,058 102,426
Total 80,058 80,058 102,426
Asset-backed securities      
Unrealized losses      
Securities in an unrealized loss position less than 12 months     (3)
Total     (3)
Fair Value      
Securities in an unrealized loss position less than 12 months     1,830
Total     1,830
Government obligations      
Unrealized losses      
Securities in an unrealized loss position less than 12 months (1) (1) (6)
Total (1) (1) (6)
Fair Value      
Securities in an unrealized loss position less than 12 months 29,846 29,846 27,084
Total $ 29,846 $ 29,846 $ 27,084