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Fair value of financial instruments and marketable securities - Balance Sheet (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months $ 144,476 $ 157,352
Total Marketable securities, More Than 12 Months 3,995 15,993
Commercial paper    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 19,197 12,966
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 122,077 137,196
Total Marketable securities, More Than 12 Months 3,995 15,993
Government obligations    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 3,202 7,190
Total Marketable securities, More Than 12 Months $ 0 $ 0