Income Taxes - Tax Effects of Temporary Differences that Give Rise to Deferred Tax Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Tax credit carryforwards	$ 62,096	$ 63,985
Net operating loss carryforwards	9,066	10,055
Reserves and accruals not currently deductible for tax purposes	8,785	14,079
Stock-based compensation	3,432	8,487
Deferred revenue	1,332	1,642
Other	6,374	6,971
Gross deferred tax assets	91,085	105,219
Depreciation and amortization	(11,075)	(17,845)
State Taxes	(1,073)	(2,092)
Gross deferred tax liabilities	(12,148)	(19,937)
Deferred tax valuation allowance	(45,506)	(42,406)
Net deferred tax assets	$ 33,431	$ 42,876