Income Taxes - Reconciliation of difference between federal statutory rate and effective income tax rate (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effective income tax rate, Amount
Federal statutory rate	$ (28,224,013)	$ (13,081,107)	$ (5,339,964)
Permanent differences	3,990,847	6,077,476	1,110,536
State taxes	(2,453,391)	(1,275,192)
Research and development credit	(8,890,651)	(219,873)	(42,958)
Valuation allowance	35,502,582	8,471,572	4,271,416
Other	$ 74,626	$ 27,124	$ 970
Tax Rate (as a percent)
Federal statutory rate (as a percent)	34.00%	34.00%	34.00%
Permanent differences (as a percent)	(4.73%)	(15.80%)	(7.06%)
State taxes (as a percent)	2.97%	3.31%	0.00%
Research and development credit (as a percent)	10.55%	0.57%	0.27%
Valuation allowance (as a percent)	(42.70%)	(21.38%)	(27.20%)
Other (as a percent)	(0.09%)	(0.70%)	(0.01%)
Effective income tax rate (as a percent)	0.00%	0.00%	0.00%