Stock-based compensation - Summary of stock-based compensation expense (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Employee Service Share-Based Compensation Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 1,004,781	$ 1,860,588	$ 3,266,879	$ 5,367,070
Research and development
Employee Service Share-Based Compensation Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	386,636	783,603	1,172,655	2,063,764
Selling, general and administrative
Employee Service Share-Based Compensation Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 618,145	$ 1,076,985	$ 2,094,224	$ 3,303,306