Condensed Statements of Operations and Comprehensive Loss (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Statement Of Income And Comprehensive Income [Abstract]
Alliance revenue	$ 38	$ 29	$ 41	$ 36
Operating expenses:
Research and development	5,637	1,997	12,570	4,325
General and administrative	2,730	1,599	4,773	3,126
Total operating expenses	8,367	3,596	17,343	7,451
Loss from operations	(8,329)	(3,567)	(17,302)	(7,415)
Other income (expense):
Investment income, net	16	2	31	4
Foreign currency exchange gain (loss)	5	(26)	2	13
Net loss	(8,308)	(3,591)	(17,269)	(7,398)
Loss on extinguishment of convertible preferred stock and preferred stock dividends	118	2,030	303	2,309
Net loss applicable to common stockholders	(8,426)	(5,621)	(17,572)	(9,707)
Basic and diluted net loss per common share applicable to common stockholders (Note 13)	$ (0.10)	$ (0.15)	$ (0.22)	$ (0.30)
Shares used in computing basic and diluted net loss per common share applicable to common stockholders	82,961	38,048	79,509	32,875
Net loss	(8,308)	(3,591)	(17,269)	(7,398)
Other comprehensive (loss) gain:
Unrealized (loss) gain on available-for-sale securities	(1)		10
Comprehensive loss	$ (8,309)	$ (3,591)	$ (17,259)	$ (7,398)