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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 3,145	$ 1,543	$ 1,456	$ 1,574	$ 2,319	$ 2,175	$ 1,565	$ 11,460	$ 7,718	$ 17,519	$ 30,018
Total operating expenses	5,367	11,523	11,299	10,881	9,895	9,955	9,502	11,600	39,070	40,952	37,840
Net income (loss)	$ (2,192)	$ (9,883)	$ (9,881)	$ (9,282)	$ (7,549)	$ (7,726)	$ (7,868)	$ 244	$ (31,238)	$ (22,899)	$ (7,283)
Basic and diluted net loss per share	$ (0.03)	$ (0.11)	$ (0.11)	$ (0.11)	$ (0.09)	$ (0.09)	$ (0.09)	$ 0.00	$ (0.36)	$ (0.27)	$ (0.10)