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Fair value of financial instruments and marketable securities - Marketable securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months $ 255,587 $ 342,418
Total Marketable securities, More Than 12 Months 84,843 56,117
Commercial paper    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 53,816 158,098
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 168,250 139,596
Total Marketable securities, More Than 12 Months 55,700 49,738
Asset-backed securities    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 26,320 44,724
Total Marketable securities, More Than 12 Months 18,503 $ 6,379
Government obligations    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 7,201  
Total Marketable securities, More Than 12 Months $ 10,640