Income Taxes - Reconciliation Between Income Taxes Computed at the Federal Statutory Income Tax Rate and the Provision for Income Taxes (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal statutory rate	34.00%	34.00%	34.00%
Effect of ownership change on deferred tax assets	(84.80%)
Change in valuation allowance	69.50%	(33.50%)	(27.80%)
US tax reform deferred impact of tax rate change	(17.30%)
Federal tax credit	(0.90%)	1.60%	2.60%
State income tax benefit, net of federal benefit	0.10%	0.10%	2.90%
Change in fair value of preferred stock warrants			(11.10%)
Other permanent items	(1.00%)	(2.50%)	(0.70%)
Total provision for income taxes	(0.40%)	(0.30%)	(0.10%)