Statements of Operations and Comprehensive Loss - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Revenue:
Collaboration revenue	$ 1,875,000		$ 4,375,000
Total revenue	1,875,000		4,375,000
Operating expenses:
General and administrative	2,780,115	$ 2,536,807	8,977,000	$ 7,033,492
Research and development	9,650,138	13,006,568	30,524,601	29,671,114
Total operating expenses	12,430,253	15,543,375	39,501,601	36,704,606
Loss from operations	(10,555,253)	(15,543,375)	(35,126,601)	(36,704,606)
Other income (expense):
Change in fair value of warrant liability	(68,037)	11,181	(71,102)	109,522
Gain on asset disposal			2,656
Miscellaneous income	731		174,266	184,015
Interest income	79,407	1,809	172,095	11,589
Interest expense	(72,331)	(4,487)	(215,293)	(4,487)
Total other income (expense)	(60,230)	8,503	62,622	300,639
Net loss and comprehensive loss	(10,615,483)	(15,534,872)	(35,063,979)	(36,403,967)
Accretion of redeemable convertible preferred stock				(28,521)
Net loss attributable to common stockholders	$ (10,615,483)	$ (15,534,872)	$ (35,063,979)	$ (36,432,488)
Per share information:
Net loss per share of common stock, basic and diluted (in dollars per share)	$ (0.24)	$ (0.59)	$ (0.85)	$ (1.58)
Weighted average shares outstanding, basic and diluted (in shares)	44,214,428	26,366,300	41,443,362	23,036,366