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Fair value of financial instruments and marketable securities (Details 3) - USD ($)
$ in Thousands
Sep. 30, 2015
Dec. 31, 2014
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months $ 164,120 $ 163,761
Total Marketable securities, More Than 12 Months 66,689 101,732
Corporate debt securities    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 144,846 157,758
Total Marketable securities, More Than 12 Months 52,321 72,273
Government obligations    
Marketable securities on the balance sheet    
Total Marketable securities, Less Than 12 Months 19,274 6,003
Total Marketable securities, More Than 12 Months $ 14,368 $ 29,459