Income Taxes - Effect of Temporary Differences and Carryforwards (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 63,442	$ 49,662
Research and development tax credit carryforwards	8,491	6,505
Deferred Tax Assets, Tax Credit Carryforwards, Foreign	613	448
Stock-based compensation	7,703	5,664
Other	8,347	6,382
Total deferred tax assets	88,596	68,661
Less: Valuation allowance	(88,596)	(68,661)	$ (88,453)	$ (70,629)
Net deferred tax assets	$ 0	$ 0