Stockholders' Equity - Summary of the Company's Stock Option Activity under Equity Incentive Plans (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Options Available for Grant, Beginning Balance	1,846
Options Available for Grant, Options forfeited	175
Options Available for Grant, Ending Balance	2,021
Options Outstanding, Beginning Balance	4,880
Options Outstanding, Options forfeited	(175)
Options Outstanding, Ending Balance	4,705
Options Outstanding, Vested and exercisable	2,252
Options Outstanding, Vested and expected to vest, Ending Balance	4,077
Options Outstanding, Unvested	2,453
Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 1.05
Weighted Average Exercise Price, Options forfeited | $ / shares	0.81
Weighted Average Exercise Price, Ending Balance | $ / shares	1.06
Weighted Average Exercise Price, Vested and exercisable | $ / shares	1.09
Weighted Average Exercise Price, Vested and expected to vest, Ending Balance | $ / shares	0.87
Weighted Average Exercise Price, Unvested | $ / shares	$ 1.03
Weighted Average Remaining Contractual Life, Beginning Balance	7 years 7 months 17 days
Weighted Average Remaining Contractual Life, Ending Balance	7 years 4 months 9 days
Weighted Average Remaining Contractual Life, Vested and exercisable	7 years 1 month 24 days
Weighted Average Remaining Contractual Life, Vested and expected to vest, Ending Balance	7 years 3 months 8 days
Aggregate Intrinsic Value | $	$ 0
Aggregate Intrinsic Value, Vested and exercisable | $	0
Aggregate Intrinsic Value, Vested and expected to vest, Ending Balance | $	$ 0