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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 3,953	$ 3,240	$ 3,421	$ 3,205	$ 3,163	$ 2,642	$ 4,122	$ 4,604	$ 13,819	$ 14,531	$ 20,950
Total operating expenses	7,834	6,421	6,832	6,118	5,938	5,213	5,443	7,114	27,205	23,708	30,354
Net income (loss)	$ (3,728)	$ (3,088)	$ (3,320)	$ (2,824)	$ (2,772)	$ (2,523)	$ (1,255)	$ (2,423)	$ (12,960)	$ (8,973)	$ (9,238)
Basic and diluted net loss per share	$ (0.21)	$ (0.27)	$ (0.30)	$ (0.25)	$ (0.25)	$ (0.24)	$ (0.14)	$ (0.26)	$ (1.01)	$ (0.90)	$ (1.01)