Equity Incentive Plans - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares Outstanding, Beginning Balance	1,328	2,333	3,423
Number of Shares, Exercised	(177)	(915)	(925)
Number of Shares, Forfeited or cancelled	(36)	(90)	(165)
Number of Shares Outstanding, Ending Balance	1,115	1,328	2,333
Number of Shares, Vested and Exercisable	1,096
Number of Shares, Vested and Expected to vest	984
Weighted-Average Exercise Price, Beginning Balance	$ 2.50	$ 2.26	$ 2.35
Weighted-Average Exercise Price, Exercised	2.42	1.89	2.36
Weighted-Average Exercise Price, Forfeited or cancelled	4.40	2.48	3.61
Weighted-Average Exercise Price, Ending Balance	2.45	$ 2.50	$ 2.26
Weighted-Average Exercise Price, Vested and Exercisable	2.38
Weighted-Average Exercise Price, Vested and Expected to Vest	$ 2.47
Weighted-Average Remaining Contractual Life (Years)	3 years 9 months
Weighted-Average Remaining Contractual Life (Years), Vested and Exercisable	3 years 8 months 15 days
Weighted-Average Remaining Contractual Life (Years), Vested and Expected to Vest	4 years 10 days
Aggregate Intrinsic Value, Exercised	$ 407	$ 3,689	$ 4,278
Aggregate Intrinsic Value, Outstanding	970	$ 6,175	$ 11,061
Aggregate Intrinsic Value, Vested and Exercisable	970
Aggregate Intrinsic Value, Vested and Expected to Vest	$ 892