Summary of (Unaudited) Quarterly Financial Information - Summary of Company's (Unaudited) Quarterly Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 6,813	$ 5,017	$ 4,176	$ 4,944	$ 4,823	$ 3,442	$ 4,505	$ 2,447	$ 20,950	$ 15,217	$ 7,718
Total operating expenses	9,235	5,350	6,968	8,801	9,505	7,838	8,570	5,930	30,354	31,843	39,070
Net income (loss)	$ (2,355)	$ (300)	$ (2,762)	$ (3,821)	$ (4,633)	$ (4,364)	$ (4,040)	$ (3,455)	$ (9,238)	$ (16,492)	$ (31,238)
Basic and diluted net loss per share	$ (0.03)	$ 0.00	$ (0.03)	$ (0.04)	$ (0.05)	$ (0.05)	$ (0.05)	$ (0.04)	$ (0.10)	$ (0.19)	$ (0.36)