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Fair value of financial instruments and marketable securities - Marketable securities (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months $ 42,491 $ 69,377
Total Marketable securities, More Than 12 Months 0 10,077
Commercial paper    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 18,435 13,827
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 24,056 55,550
Total Marketable securities, More Than 12 Months $ 0 $ 10,077