STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Collaboration Revenue	$ 6,861	$ 4,332	$ 1,373
Operating expenses:
Research and development	157,791	106,785	32,706
General and administrative	14,377	17,647	12,166
Total operating expenses	172,168	124,432	44,872
Loss from operations	(165,307)	(120,100)	(43,499)
Other income (expense), net	134	12	(5)
Change in fair value of warrants			11,723
Change in fair value of derivative liabilities	(124)
Net loss	(165,297)	(120,088)	(31,781)
Preferred stock dividends	(7,123)
Net loss applicable to common stockholders	$ (172,420)	$ (120,088)	$ (31,781)
Basic and diluted net loss per share	$ (1.32)	$ (0.96)	$ (0.31)
Weighted average common shares outstanding used to compute basic and diluted net loss per share	130,391,463	125,416,084	101,130,710