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Fair value of financial instruments and marketable securities (Details 3) - USD ($)
Dec. 31, 2016
Dec. 31, 2015
Aug. 31, 2015
Marketable securities on the balance sheet      
Less Than 12 Months $ 157,352,000 $ 186,782,000  
More Than 12 Months 15,993,000 94,121,000  
Commercial paper      
Marketable securities on the balance sheet      
Less Than 12 Months 12,966,000 26,957,000  
More Than 12 Months 0 0  
Corporate debt securities      
Marketable securities on the balance sheet      
Less Than 12 Months 137,196,000 140,831,000  
More Than 12 Months 15,993,000 85,488,000  
Government obligations      
Marketable securities on the balance sheet      
Less Than 12 Months 7,190,000 18,994,000  
More Than 12 Months 0 8,633,000  
3.00% Convertible senior notes due 2022 | Convertible debt      
Marketable securities on the balance sheet      
Principal 150,000,000 $ 150,000,000 $ 150,000,000
Interest rate     3.00%
Debt instrument $ 85,200,000