Stock-based compensation - Summary of stock-based compensation expense (Details) - USD ($)	10 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
Employee Service Share-Based Compensation Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 1,462,061	$ 6,572,963	$ 6,597,359
Research and development
Employee Service Share-Based Compensation Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	648,001	2,435,565	3,192,063
Selling, general and administrative
Employee Service Share-Based Compensation Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 814,060	$ 4,137,398	$ 3,405,296