STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Collaboration revenue	$ 6,389	$ 6,861	$ 4,332
Operating expenses:
Research and development	45,084	157,791	106,785
General and administrative	14,798	14,377	17,647
Total operating expenses	59,882	172,168	124,432
Loss from operations	(53,493)	(165,307)	(120,100)
Other income (expense), net	465	134	12
Change in fair value of derivative liabilities	(1,295)	(124)
Net loss	(54,323)	(165,297)	(120,088)
Preferred stock dividends	(18,938)	(7,123)
Net loss applicable to common stockholders	$ (73,261)	$ (172,420)	$ (120,088)
Basic and diluted net loss per share	$ (0.53)	$ (1.32)	$ (0.96)
Weighted average common shares outstanding used to compute basic and diluted net loss per share	136,938,264	130,391,463	125,416,084