Income Taxes - Schedule of Reconciliation of Effective Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Federal income tax benefit at statutory rate	$ (21,556)	$ (7,756)	$ (1,089)
State income tax benefit, net of federal benefit	(1,617)	(1,511)	(264)
Change in income tax rates upon conversion to C-corporation			427
Federal deferred tax assets upon conversion to C-corporation			(105)
Step-up in assets upon conversion to C-corporation		(709)	(448)
Research and development credits	(7,275)
Stock-based compensation expense for incentive stock options and employee stock purchase plan	1,805	283	98
Other non-deductible expenses and reconciling items	212	240	129
Change in valuation allowance	27,996	9,453	1,252
Total income tax expense (benefit)	$ (435)	$ 0	$ 0