Income Taxes Effect of Each Type of Temporary Difference Comprising Net Differed Tax Asset (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating losses	$ 110,715	$ 95,181
Stock-based compensation	12,446	16,894
Tax credits	6,378	6,893
Other reserves and accrued liabilities	3,587	3,193
Gross deferred tax assets	133,126	122,161
Less: valuation allowance	(131,389)	(109,274)	$ (95,999)
Total deferred tax assets	1,737	12,887
Deferred tax liabilities:
Depreciation and amortization	(1,145)	(1,050)
Other liabilities	(592)	(755)
Total deferred tax liabilities	$ (1,737)	(1,805)
Net deferred tax assets		$ 11,082