Income Taxes - Components of Net Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Net deferred income tax assets (liability):
Depreciation and amortization	$ 238	$ 365
Accrued expenses and reserves	148	345
Deferred revenue	508	66
Net operating loss carryforwards	14,561	20,448
Research and development credit carryforwards	3,037	2,740
Stock-based compensation	940	840
Other	10	91
Gross deferred tax assets	19,442	24,895
Less: valuation allowance	$ (19,442)	(24,911)
Net deferred tax liability		$ (16)