Income Taxes - Components of Net Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Net deferred income tax assets (liability):
Net operating loss carryforwards	$ 18,677	$ 17,443
Research and development credit carryforwards	3,576	3,337
Share-based compensation	645	1,006
Accrued expenses and reserves	127	47
Depreciation and amortization	17	48
Deferred income tax (liability), Deferred revenue		(63)
Deferred income tax assets, Deferred revenue	275
Other	14	19
Gross deferred tax assets	23,331	21,837
Less: valuation allowance	(23,324)	(21,830)
Net deferred tax assets	$ 7	$ 7