Equity Incentive Plans - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares Outstanding, Beginning Balance	2,333	3,423	5,555
Number of Shares, Granted			75
Number of Shares, Exercised	(915)	(925)	(966)
Number of Shares, Forfeited or cancelled	(90)	(165)	(1,241)
Number of Shares Outstanding, Ending Balance	1,328	2,333	3,423
Number of Shares, Vested and Exercisable	1,252
Number of Shares, Vested and Expected to vest	1,082
Weighted-Average Exercise Price, Beginning Balance	$ 2.26	$ 2.35	$ 3.33
Weighted-Average Exercise Price, Granted			6.45
Weighted-Average Exercise Price, Exercised	1.89	2.36	2.23
Weighted-Average Exercise Price, Forfeited or cancelled	2.48	3.61	7.07
Weighted-Average Exercise Price, Ending Balance	2.50	$ 2.26	$ 2.35
Weighted-Average Exercise Price, Vested and Exercisable	2.32
Weighted-Average Exercise Price, Vested and Expected to Vest	$ 2.52
Weighted-Average Remaining Contractual Life (Years)	4 years 7 months 28 days
Weighted-Average Remaining Contractual Life (Years), Vested and Exercisable	4 years 6 months 7 days
Weighted-Average Remaining Contractual Life (Years), Vested and Expected to Vest	5 years 1 month 6 days
Aggregate Intrinsic Value, Exercised	$ 3,689	$ 4,278	$ 4,954
Aggregate Intrinsic Value, Outstanding	6,175	$ 11,061	$ 18,341
Aggregate Intrinsic Value, Vested and Exercisable	6,045
Aggregate Intrinsic Value, Vested and Expected to Vest	$ 5,008