Condensed Statements of Operations and Comprehensive Loss (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Statement [Abstract]
Collaboration revenue	$ 539	$ 622	$ 1,868	$ 824
Operating expenses:
Research and development	8,872	3,376	21,445	10,576
General and administrative	2,269	786	6,259	2,606
Total operating expenses	11,141	4,162	27,704	13,182
Loss from operations	(10,602)	(3,540)	(25,836)	(12,358)
Other income (expense):
Other income (expense), net	2	16	56	(96)
Interest expense	(74)	(289)	(237)	(505)
Total other expense, net	(72)	(273)	(181)	(601)
Net loss and comprehensive loss	(10,674)	(3,813)	(26,017)	(12,959)
Cumulative preferred stock dividends		(905)	(519)	(2,715)
Net loss applicable to common stockholders	$ (10,674)	$ (4,718)	$ (26,536)	$ (15,674)
Net loss per share applicable to common stockholders-basic and diluted	$ (0.66)	$ (3.39)	$ (1.90)	$ (11.87)
Weighted-average number of common shares used in net loss per share applicable to common stockholders-basic and diluted	16,098	1,390	13,954	1,320