Income Taxes - Schedule of Significant Components of Company's Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 54,831	$ 57,887
Federal and state tax credits	8,401	6,221
Accelerated depreciation		259
Stock-based compensation expense	2,382	1,783
Other	582	641
Gross deferred tax asset	66,196	66,791
Deferred tax liabilities:
Accelerated depreciation	(23)
Valuation allowance	(66,173)	(66,791)
Net deferred tax asset	$ 0	$ 0