Quarterly Financial Information (Unaudited) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly financial information
Loss from operations	$ (25,547,384)	$ (21,180,396)	$ (15,797,787)	$ (20,889,002)	$ (13,165,526)	$ (10,029,427)	$ (9,923,360)	$ (5,454,659)	$ (83,414,569)	$ (38,572,972)	$ (15,420,321)
Interest income	172,453	98,588	78,829	52,895					402,765	14,570	(131,527)
Other income					4,719	88,769	4,185	1,455		84,558
Net loss	(25,374,931)	(21,081,808)	(15,718,958)	(20,836,107)	$ (13,160,807)	$ (9,940,658)	$ (9,919,175)	$ (5,453,204)	(83,011,804)	(38,473,844)	(15,705,776)
Comprehensive loss	$ (25,374,931)	$ (21,081,808)	$ (15,718,958)	$ (20,836,107)					$ (83,011,804)	$ (38,473,844)	$ (15,705,776)
Basic and diluted net loss per common share (in dollars per share)	$ (0.92)	$ (0.87)	$ (0.68)	$ (1.24)	$ (1.82)	$ (1.32)	$ (1.41)	$ (0.79)	$ (3.67)	$ (5.43)	$ (2.39)
Weighted-average basic and diluted common shares outstanding (in shares)	27,678,348	24,166,113	23,013,838	16,774,718	7,226,122	7,504,148	7,030,976	6,895,185	22,647,583	7,087,618	6,916,404