Income Taxes - Schedule of Significant Components of Company's Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 73,578	$ 54,831
Federal and state tax credits	11,108	8,401
Deferred revenue	1,111
Stock-based compensation expense	3,605	2,382
Other	420	582
Gross deferred tax asset	89,822	66,196
Deferred tax liabilities:
Accelerated depreciation	(7)	(23)
Valuation allowance	(89,815)	(66,173)
Net deferred tax asset	$ 0	$ 0