Difference Between Provision for Income Taxes and Amounts Computed by Applying Federal Statutory Income Tax Rate to Loss before Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal statutory tax	$ (12,510)	$ (14,906)	$ (12,040)
Federal research credits	(1,630)	(1,857)	(1,390)
State research credits	749	821	655
Expiration of federal carryovers	9,200	5,472	4,154
Expiration of state carryovers			1,344
Change in valuation allowance	6,738	13,059	9,913
Compensation related items	978	158	(361)
State taxes	(1,921)	(1,111)	(1,141)
Other	178	269	405
Provision for income taxes	$ 284	$ 263	$ 229