Basic and Diluted Net Loss Per Share - Potential Shares of Common Stock Excluded from Diluted Weighted-Average Common Shares Outstanding (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from diluted net loss per share	10,806	11,726	11,477
Repurchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from diluted net loss per share			2
Equity Incentive Plans
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential common shares excluded from diluted net loss per share	10,806	11,726	11,475