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Net loss per share (Details 2)
 1 Months Ended 9 Months Ended
Aug. 31, 2015
USD ($)
$ / shares
Sep. 30, 2015
USD ($)
shares
Sep. 30, 2014
shares
Net loss per share      
Total shares excluded from calculation   5,141,399 4,173,098
Stock options      
Net loss per share      
Total shares excluded from calculation   4,788,264 3,443,778
Unvested restricted stock      
Net loss per share      
Total shares excluded from calculation   353,135 729,320
Convertible debt | 3.0% Convertible senior notes due 2022      
Convertible Senior Notes.      
Principal amount of Notes | $ $ 150,000,000 $ 150,000,000  
Interest rate 3.00%    
Conversion ratio 17.7487    
Common stock per principal amount | $ $ 1,000    
Conversion price per share | $ / shares $ 56.34