STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Collaboration revenue	$ 1,597	$ 1,697	$ 3,194	$ 3,666
Operating expenses:
Research and development	10,831	129,228	22,798	139,427
General and administrative	3,780	3,711	7,375	7,521
Total operating expenses	14,611	132,939	30,173	146,948
Loss from operations	(13,014)	(131,242)	(26,979)	(143,282)
Other income (expense), net	86	42	124	63
Change in fair value of derivative liabilities	66		(1,494)
Net loss	(12,862)	(131,200)	(28,349)	(143,219)
Preferred stock dividends	(4,865)		(9,036)
Net loss applicable to common stockholders	$ (17,727)	$ (131,200)	$ (37,385)	$ (143,219)
Basic and diluted net loss per share	$ (0.13)	$ (1.01)	$ (0.28)	$ (1.10)
Weighted average common shares outstanding used to compute basic and diluted net loss per share	135,630,210	130,385,077	133,176,934	130,271,806