Income Taxes - Deferred Income Tax (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Tax Assets, Net [Abstract]
Net operating loss carryforwards	$ 76,753	$ 61,049
Research and development tax credits	4,699	3,731
Reserves and accruals	1,444	1,168
Other	9,109	6,611
Total deferred tax assets	92,005	72,559
Valuation allowance	(92,005)	(72,559)
Net deferred tax assets	$ 0	$ 0