Condensed Statements of Operations (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Collaboration and license revenue	$ 2,415	$ 2,601	$ 4,786	$ 5,709
Operating expenses:
Research and development	28,983	20,833	57,138	38,556
General and administrative	4,937	3,708	10,177	6,747
Total operating expenses	33,920	24,541	67,315	45,303
Loss from operations	(31,505)	(21,940)	(62,529)	(39,594)
Interest and other income (expense), net	155	342	453	(147)
Net loss	$ (31,350)	$ (21,598)	$ (62,076)	$ (39,741)
Net loss per share attributable to common stockholders:
Basic and diluted	$ (0.76)	$ (1.47)	$ (1.51)	$ (4.92)
Shares used to compute net loss per share attributable to common stockholders:
Basic and diluted	41,228,885	14,681,570	41,119,310	8,078,308