Income Taxes - Reconciliation of Federal Income Tax Rate to Losses Before Income Tax Benefit (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income tax reconciliation
Income tax benefit at federal statutory rate	$ (20,298)	$ (9,540)	$ (3,872)
State income taxes, net of federal benefit	(1,204)	(741)	(376)
Non-deductible expenses	229	608	547
Distribution of intellectual property rights		657
Research and development tax credits	(1,692)	(767)	(478)
Other	(124)	283	(17)
Change in valuation allowance	23,089	9,500	4,196
Total income tax provision	$ 0	$ 0	$ 0