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Fair value of financial instruments and marketable securities (Details 3) - USD ($)
$ in Thousands
Jun. 30, 2015
Dec. 31, 2014
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months $ 166,747 $ 163,761
Total Marketable securities, More Than 12 Months 59,252 101,732
Commercial paper    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 19,100  
Corporate debt securities    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 129,763 157,758
Total Marketable securities, More Than 12 Months 41,629 72,273
Government obligations    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 17,884 6,003
Total Marketable securities, More Than 12 Months $ 17,623 $ 29,459