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Fair value of financial instruments and marketable securities - Marketable securities (Details) - USD ($)
$ in Thousands
Sep. 30, 2020
Dec. 31, 2019
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months $ 574,555 $ 342,418
Total Marketable securities, More Than 12 Months 132,494 56,117
Commercial paper    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 260,451 158,098
Total Marketable securities, More Than 12 Months 0 0
Corporate debt securities    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 260,199 139,596
Total Marketable securities, More Than 12 Months 58,062 49,738
Asset-backed securities    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 14,375 44,724
Total Marketable securities, More Than 12 Months 21,316 $ 6,379
Government obligations    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 39,530  
Total Marketable securities, More Than 12 Months $ 53,116