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Fair value of financial instruments and marketable securities - Marketable securities (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months $ 127,888 $ 58,088
Total Marketable securities, More Than 12 Months 57,184 0
Commercial paper    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 47,978 31,699
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 58,533 26,389
Total Marketable securities, More Than 12 Months 57,184 $ 0
Asset-backed securities    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 21,377  
Total Marketable securities, More Than 12 Months $ 0