Stockholders' Equity - Summary of the Company's stock option activity under its equity incentive plans (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Options Available for Grant, Beginning Balance	2,465
Options Available for Grant, Options forfeited	11
Options Available for Grant, Ending Balance	2,476
Options Outstanding, Beginning Balance	6,785
Options Outstanding, Options forfeited	(11)
Options Outstanding, Ending Balance	6,774
Options Outstanding, Vested and exercisable	4,335
Options Outstanding, Vested and expected to vest, Ending Balance	6,461
Options Outstanding, Unvested	2,439
Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 0.75
Weighted Average Exercise Price, Options forfeited | $ / shares	4.78
Weighted Average Exercise Price, Ending Balance | $ / shares	0.74
Weighted Average Exercise Price, Vested and exercisable | $ / shares	0.72
Weighted Average Exercise Price, Vested and expected to vest, Ending Balance | $ / shares	0.59
Weighted Average Exercise Price, Unvested | $ / shares	$ 0.78
Weighted Average Remaining Contractual Life, Balance	7 years 1 month 6 days
Weighted Average Remaining Contractual Life, Ending Balance	6 years 10 months 24 days
Weighted Average Remaining Contractual Life, Vested and exercisable	6 years 10 months 24 days
Weighted Average Remaining Contractual Life, Vested and expected to vest, Ending Balance	6 years 10 months 24 days
Aggregate Intrinsic Value | $	$ 0
Aggregate Intrinsic Value, Vested and exercisable | $	0
Aggregate Intrinsic Value, Vested and expected to vest, Ending Balance | $	$ 0