Income Taxes - Effect of Temporary Differences and Carryforwards (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 49,662	$ 70,694
Research and development tax credit carryforwards	6,505	4,572
Deferred Tax Assets, Tax Credit Carryforwards, Foreign	448	0
Stock-based compensation	5,664	5,360
Other	6,382	7,827
Total deferred tax assets	68,661	88,453
Less: Valuation allowance	(68,661)	(88,453)	$ (70,629)	$ (53,923)
Net deferred tax assets	$ 0	$ 0