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Fair value of financial instruments and marketable securities (Details 3) - USD ($)
$ in Thousands
Dec. 31, 2015
Aug. 31, 2015
Dec. 31, 2014
Marketable securities on the balance sheet      
Total Marketable securities, Less Than 12 Months $ 186,782   $ 163,761
Total Marketable securities, More Than 12 Months 94,121   101,732
Commercial paper      
Marketable securities on the balance sheet      
Total Marketable securities, Less Than 12 Months 26,957    
Corporate debt securities      
Marketable securities on the balance sheet      
Total Marketable securities, Less Than 12 Months 140,831   157,758
Total Marketable securities, More Than 12 Months 85,488   72,273
Government obligations      
Marketable securities on the balance sheet      
Total Marketable securities, Less Than 12 Months 18,994   6,003
Total Marketable securities, More Than 12 Months 8,633   $ 29,459
3.00% Convertible senior notes due 2022 | Convertible debt      
Marketable securities on the balance sheet      
Principal amount of Notes $ 150,000 $ 150,000  
Interest rate   3.00%