STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Collaboration revenue	$ 146	$ 1,597
Operating expenses:
Research and development	10,183	11,967
General and administrative	6,159	3,595
Total operating expenses	16,342	15,562
Loss from operations	(16,196)	(13,965)
Other income (expense), net	148	38
Change in fair value of derivative liabilities	28	(1,560)
Net loss	(16,020)	(15,487)
Preferred stock dividends	(5,120)	(4,171)
Net loss applicable to common stockholders	$ (21,140)	$ (19,658)
Basic and diluted net loss per share	$ (0.15)	$ (0.15)
Weighted average common shares outstanding used to compute basic and diluted net loss per share	140,853,120	130,696,400