Income Taxes - Significant Components of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Net operating loss carry forwards	$ 69,281	$ 32,500
Research and development tax credits	34,891	8,142
Stock based compensation and other	5,693	7,256
Depreciation and amortization		78
Total deferred tax assets	109,865	47,976
Less: Valuation allowance	(108,782)	$ (47,976)
Deferred tax liabilities	$ (1,083)