Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Total revenues	$ 72	$ 1,194	$ 20,759
Operating expenses:
Research and development	53,192	64,305	56,387
General and administrative	17,011	18,391	19,130
Total operating expenses	70,203	82,696	75,517
Loss from operations	(70,131)	(81,502)	(54,758)
Other income (expense):
Interest and other income	752	844	855
Interest and other expense	(2,723)	(1,182)	(52)
Loss from valuation of convertible note payable	0	0	(1,811)
Loss before income taxes	(72,102)	(81,840)	(55,766)
Income tax benefit	197	4	18
Net loss	(71,905)	(81,836)	(55,748)
Other comprehensive loss:
Unrealized (loss) gain on short-term investments, net	(11)	10	64
Comprehensive loss	$ (71,916)	$ (81,826)	$ (55,684)
Net loss per share - basic and diluted (in dollars per share)	$ (0.96)	$ (1.55)	$ (1.08)
Weighted average shares used to compute basic and diluted net loss per share (in shares)	75,230,762	52,813,474	51,411,353
Strategic Alliances and Collaborations
Revenues:
Total revenues	$ 72	$ 1,194	$ 20,759