Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating expenses:
Research and development expenses	$ 17,839,951	$ 8,473,522	$ 4,484,367
General and administrative expenses	8,658,867	4,720,661	2,003,992
Total operating expenses	26,498,818	13,194,183	6,488,359
Loss from operations	(26,498,818)	(13,194,183)	(6,488,359)
Other income (expense):
Warrant remeasurement	(1,879,823)	582,360	(854,336)
Interest income	9,084	2,941	3,951
Interest expense	(816,494)	(183,179)	(3,354)
Loss before income taxes	(29,186,051)	(12,792,061)	(7,342,098)
Benefit for income taxes	1,107,405	590,675	459,018
Net loss	(28,078,646)	(12,201,386)	(6,883,080)
Comprehensive loss	(28,078,646)	(12,201,386)	(6,883,080)
Cumulative dividends on Series C, C-1 and C-2 convertible preferred stock	(4,356,408)	(1,580,701)	(1,076,256)
Net loss attributable to common stockholders	$ (32,435,054)	$ (13,782,087)	$ (7,959,336)
Loss per share attributable to common stockholders basic and diluted (in dollars per share)	$ (4.01)	$ (8.16)	$ (4.71)
Weighted average common shares outstanding basic and diluted (in shares)	8,087,924	1,688,475	1,688,475