Income Taxes - Income Tax Expense (Benefit) Differed from Amounts Computed by Applying Statutory Federal Income Tax Rate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income tax provision at statutory rate	$ (15,076)	$ (16,010)	$ (15,662)
Effective Income Tax Rate Reconciliation, Repatriation of Foreign Earnings and Other Nondeductible Amounts	179	135	401
Change in tax credits	(2,361)	(1,449)	(792)
Change in valuation allowance	(19,792)	17,824	16,706
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	37,690	0
Foreign tax and other	(436)	(384)	(487)
Total provision for income taxes	$ 204	$ 116	$ 166