STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Collaboration revenue	$ 146	$ 6,389	$ 6,861
Operating expenses:
Research and development	34,134	45,084	157,791
General and administrative	19,918	14,798	14,377
Total operating expenses	54,052	59,882	172,168
Loss from operations	(53,906)	(53,493)	(165,307)
Other income (expense), net	631	465	134
Change in fair value of derivative liabilities	158	(1,295)	(124)
Net loss	(53,117)	(54,323)	(165,297)
Preferred stock dividends	(16,998)	(18,938)	(7,123)
Settlement of a related party relationship	207,361
Net income (loss) applicable to common stockholders	$ 137,246	$ (73,261)	$ (172,420)
Net income (loss) per share - basic	$ 0.96	$ (0.53)	$ (1.32)
Net income (loss) per share - diluted	$ 0.96	$ (0.53)	$ (1.32)
Weighted average common shares outstanding used to compute basic net income (loss) per share	143,508,674	136,938,264	130,391,463
Weighted average common shares outstanding used to compute diluted net income (loss) per share	143,710,160	136,938,264	130,391,463