Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of options, Outstanding beginning balance	2,829,301
Number of options, Granted	1,095,355
Number of options, Exercised	(138,425)
Number of options, Forfeited or expired	(90,597)
Number of options, Outstanding ending balance	3,695,634	2,829,301
Number of options, Exercisable	1,803,612
Number of options, Vested and expected to vest	3,646,249
Weighted average exercise price, Outstanding beginning balance	$ 6.89
Weighted average exercise price, Granted	2.34
Weighted average exercise price, Exercised	2.19
Weighted average exercise price, Forfeited or expired	8.04
Weighted average exercise price, Outstanding ending balance	5.69	$ 6.89
Weighted average exercise price, Exercisable	7.21
Weighted average exercise price, Vested and expected to vest	$ 5.69
Weighted average remaining contractual term, Outstanding ending balance	7 years 9 months 7 days	7 years 10 months 6 days
Weighted average remaining contractual term, Exercisable	6 years 8 months 23 days
Weighted average remaining contractual term, Vested and expected to vest	7 years 9 months 3 days
Aggregate intrinsic value, Outstanding	$ 0
Aggregate intrinsic value, Exercised	$ 0
Aggregate intrinsic value, Forfeited or expired	$ 0
Aggregate intrinsic value, Outstanding	$ 0
Aggregate intrinsic value, Exercisable	0
Aggregate intrinsic value, Vested and expected to vest	$ 0