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Fair value of financial instruments and marketable securities - Unrealized Loss Positions (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Unrealized losses    
Securities in an unrealized loss position less than 12 months $ (144) $ (75)
Total (144) (75)
Fair Value    
Securities in an unrealized loss position less than 12 months 266,050 260,970
Total 266,050 260,970
Write downs of available for sale debt securities 0  
Commercial paper    
Unrealized losses    
Securities in an unrealized loss position less than 12 months (1) (37)
Total (1) (37)
Fair Value    
Securities in an unrealized loss position less than 12 months 69,096 129,630
Total 69,096 129,630
Corporate debt securities    
Unrealized losses    
Securities in an unrealized loss position less than 12 months (139) (29)
Total (139) (29)
Fair Value    
Securities in an unrealized loss position less than 12 months 177,817 102,426
Total 177,817 102,426
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 (4) (6)
Total (4) (6)
Fair Value    
Securities in an unrealized loss position less than 12 months 19,137 27,084
Total $ 19,137 $ 27,084