Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenues:
Total revenues	$ 18	$ 483	$ 36	$ 972
Operating expenses:
Research and development	14,278	18,007	30,030	34,772
General and administrative	7,057	3,664	11,016	8,767
Total operating expenses	21,335	21,671	41,046	43,539
Loss from operations	(21,317)	(21,188)	(41,010)	(42,567)
Other income (expense):
Interest and other income	184	180	398	372
Interest and other expense	(603)	(90)	(1,149)	(114)
Loss before income taxes	(21,736)	(21,098)	(41,761)	(42,309)
Income tax benefit	128	8	132	13
Net loss	(21,608)	(21,090)	(41,629)	(42,296)
Other comprehensive loss:
Unrealized gain on short-term investments, net	10	9	36	50
Comprehensive loss	$ (21,598)	$ (21,081)	$ (41,593)	$ (42,246)
Net loss per share, basic and diluted (in dollars per share)	$ (0.41)	$ (0.40)	$ (0.78)	$ (0.80)
Weighted average shares used to compute basic and diluted net loss per share (in shares)	53,182,330	52,782,643	53,086,887	52,746,657
Strategic Alliances and Collaborations
Revenues:
Total revenues	$ 18	$ 483	$ 36	$ 972