Income Taxes - Schedule of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 42,773	$ 35,162
Research and development tax credit carryforwards	3,602	2,925
Capitalized research and development expenses	68	74
Accrued expenses and reserves	396	396
Depreciation and amortization	(1,398)	88
Construction financing liability	1,454
Stock compensation	504
Total deferred tax assets	47,399	38,645
Valuation allowance	$ (47,399)	$ (38,645)	$ (45,676)