Income Taxes - Tax Effects of Temporary Differences and Carryforwards That Give Rise to Significant Portions of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 68,552	$ 60,999
Tax credits	12,451	10,198
Depreciation	339	133
Stock-based compensation	10,770	6,983
Accruals and reserves	7,507	7,280
Other	2,057	2,525
Deferred tax assets	101,676	88,118
Valuation allowance	(100,747)	$ (88,118)
Net deferred tax assets	$ 929