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Fair value of financial instruments and marketable securities - Marketable securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months $ 161,132 $ 58,088
Total Marketable securities, More Than 12 Months 66,315 0
Commercial paper    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 70,301 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 70,192 26,389
Total Marketable securities, More Than 12 Months 59,514 $ 0
Asset-backed securities    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 20,639  
Total Marketable securities, More Than 12 Months $ 6,801