Consolidated Statements of Operations - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
License revenue	$ 152,381	$ 152,381	$ 152,381
Research and development:
Noncash stock expense associated with in-licensing agreements		0	5,350,094
Noncash compensation	2,742,354	4,261,406	8,731,566
Other research and development	66,489,820	43,445,817	26,004,687
Total research and development	69,232,174	47,707,223	40,086,347
General and administrative:
Noncash compensation	4,767,645	11,435,686	12,373,726
Other general and administrative	5,121,690	4,189,488	3,413,400
Total general and administrative	9,889,335	15,625,174	15,787,126
Total costs and expenses	79,121,509	63,332,397	55,873,473
Operating loss	(78,969,128)	(63,180,016)	(55,721,092)
Other (income) expense:
Interest income	(323,032)	(174,653)	(55,049)
Other (income) expense	(393,202)	(56,717)	115,234
Total other (income) expense, net	(716,234)	(231,370)	60,185
Net loss	$ (78,252,894)	$ (62,948,646)	$ (55,781,277)
Basic and diluted net loss per common share (in dollars per share)	$ (1.60)	$ (1.38)	$ (1.64)
Weighted average shares used in computing basic and diluted net loss per common share (in shares)	49,041,354	45,646,414	34,068,926