STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Collaboration revenue		$ 1,597	$ 146	$ 3,194
Operating expenses:
Research and development	$ 7,489	10,831	17,672	22,798
General and administrative	4,889	3,780	11,048	7,375
Total operating expenses	12,378	14,611	28,720	30,173
Loss from operations	(12,378)	(13,014)	(28,574)	(26,979)
Other income (expense), net	164	86	312	124
Change in fair value of derivative liabilities	183	66	211	(1,494)
Net loss	(12,031)	(12,862)	(28,051)	(28,349)
Preferred stock dividends	(5,462)	(4,865)	(10,582)	(9,036)
Net loss applicable to common stockholders	$ (17,493)	$ (17,727)	$ (38,633)	$ (37,385)
Basic and diluted net loss per share	$ (0.12)	$ (0.13)	$ (0.27)	$ (0.28)
Weighted average common shares outstanding used to compute basic and diluted net loss per share	141,017,898	135,630,210	140,935,964	133,176,934