Summary of (Unaudited) Quarterly Financial Information - Summary of Company's (Unaudited) Quarterly Results of Operations (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 4,823	$ 3,442	$ 4,505	$ 2,447	$ 3,145	$ 1,543	$ 1,456	$ 1,574	$ 15,217	$ 7,718	$ 17,519
Total operating expenses	9,505	7,838	8,570	5,930	5,367	11,523	11,299	10,881	31,843	39,070	40,952
Net income (loss)	$ (4,633)	$ (4,364)	$ (4,040)	$ (3,455)	$ (2,192)	$ (9,883)	$ (9,881)	$ (9,282)	$ (16,492)	$ (31,238)	$ (22,899)
Basic and diluted net loss per share	$ (0.05)	$ (0.05)	$ (0.05)	$ (0.04)	$ (0.03)	$ (0.11)	$ (0.11)	$ (0.11)	$ (0.19)	$ (0.36)	$ (0.27)