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Stock-based compensation - Summary of stock-based compensation expense (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Employee Service Share-Based Compensation Allocation of Recognized Period Costs [Line Items]        
Stock-based compensation expense $ 1,860,588 $ 1,506,679 $ 5,367,070 $ 4,892,998
Research and development        
Employee Service Share-Based Compensation Allocation of Recognized Period Costs [Line Items]        
Stock-based compensation expense 783,603 617,476 2,063,764 2,429,769
Selling, general and administrative        
Employee Service Share-Based Compensation Allocation of Recognized Period Costs [Line Items]        
Stock-based compensation expense $ 1,076,985 $ 889,203 $ 3,303,306 $ 2,463,229