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Fair value of financial instruments and marketable securities (Details 3) - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months $ 195,021 $ 186,782
Total Marketable securities, More Than 12 Months 63,526 94,121
Commercial paper    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 28,969 26,957
Corporate debt securities    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 148,785 140,831
Total Marketable securities, More Than 12 Months 60,318 85,488
Government obligations    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 17,267 18,994
Total Marketable securities, More Than 12 Months $ 3,208 $ 8,633