Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
Total revenues	$ 18	$ 18	$ 36	$ 36
Operating expenses:
Research and development	10,013	14,278	21,841	30,030
General and administrative	3,349	7,057	7,122	11,016
Total operating expenses	13,362	21,335	28,963	41,046
Loss from operations	(13,344)	(21,317)	(28,927)	(41,010)
Other income (expense):
Interest and other income	118	184	282	398
Interest and other expense	(621)	(603)	(1,227)	(1,149)
Loss before income taxes	(13,847)	(21,736)	(29,872)	(41,761)
Income tax benefit (expense)	0	128	(1)	132
Net loss	(13,847)	(21,608)	(29,873)	(41,629)
Other comprehensive loss:
Unrealized gain on short-term investments, net	47	10	54	36
Comprehensive loss	$ (13,800)	$ (21,598)	$ (29,819)	$ (41,593)
Net loss per share, basic and diluted (in dollars per share)	$ (0.13)	$ (0.41)	$ (0.29)	$ (0.78)
Weighted average shares used to compute basic and diluted net loss per share (in shares)	104,319,555	53,182,330	104,169,746	53,086,887
Revenue under strategic alliances and collaborations
Revenues:
Total revenues	$ 18	$ 18	$ 36	$ 36