Stock-based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Outstanding number of shares, beginning balance | shares	7,669,093
Number of shares, granted | shares	120,200
Number of shares, exercised | shares	(24,819)
Number of shares, forfeited | shares	(41,512)
Number of shares, expired | shares	(167,847)
Outstanding number of shares, ending balance | shares	7,555,115
Outstanding number of shares vested and expected to vest | shares	7,410,324
Outstanding number of shares vested and exercisable | shares	5,652,596
Outstanding, weighted average exercise price, beginning balance | $ / shares	$ 2.51
Weighted average exercise price, granted | $ / shares	1.68
Weighted average exercise price, exercised | $ / shares	1.49
Weighted average exercise price, forfeited | $ / shares	2.15
Weighted average exercise price, expired | $ / shares	2.61
Outstanding, weighted average exercise price, ending balance | $ / shares	2.50
Vested and expected to vest, weighted average exercise price, ending balance | $ / shares	2.50
Vested and exercisable, weighted average exercise price, ending balance | $ / shares	$ 2.54
Weighted average remaining contractual term (in years), outstanding	6 years 3 months 3 days
Weighted average remaining contractual term (in years), vested and expected to vest	6 years 2 months 15 days
Weighted average remaining contractual term (in years), vested and exercisable	5 years 6 months 3 days
Aggregate intrinsic value, outstanding | $	$ 58
Aggregate intrinsic value, vested and expected to vest | $	58
Aggregate intrinsic value, vested and exercisable | $	$ 49