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Stock-based compensation - Summary of stock-based compensation expense (Details) - USD ($)
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Employee Service Share-Based Compensation Allocation of Recognized Period Costs [Line Items]    
Stock-based compensation expense $ 1,188,752 $ 1,518,161
Research and development    
Employee Service Share-Based Compensation Allocation of Recognized Period Costs [Line Items]    
Stock-based compensation expense 394,417 595,466
Selling, general and administrative    
Employee Service Share-Based Compensation Allocation of Recognized Period Costs [Line Items]    
Stock-based compensation expense $ 794,335 $ 922,695