Condensed Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
Collaboration revenue	$ 114	$ 761	$ 358	$ 1,329
Operating expenses:
Research and development	6,269	6,754	11,507	12,573
General and administrative	2,247	2,052	4,850	3,990
Total operating expenses	8,516	8,806	16,357	16,563
Loss from operations	(8,402)	(8,045)	(15,999)	(15,234)
Other income (expense)
Other income, net	1,826	3	3,134	54
Interest expense	(330)	(79)	(565)	(163)
Total other income (expense), net	1,496	(76)	2,569	(109)
Net loss and comprehensive loss	(6,906)	(8,121)	(13,430)	(15,343)
Cumulative preferred stock dividends	0	0	0	(519)
Net loss applicable to common stockholders	$ (6,906)	$ (8,121)	$ (13,430)	$ (15,862)
Net loss per share applicable to common stockholders—basic and diluted (in USD per share)	$ (0.36)	$ (510.00)	$ (0.72)	$ (1.23)
Weighted-average number of common shares used in net loss per share applicable to common stockholders—basic and diluted	19,087	16,055	18,532	13