Income Taxes - Significant Components of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Net operating loss carry forwards	$ 2,335	$ 69,281
Research and development tax credits	168	34,891
Stock based compensation and other	8,567	5,693
Total deferred tax assets	11,070	109,865
Less: Valuation allowance	(10,336)	(108,782)
Deferred tax liabilities	$ (734)	$ (1,083)