Income Taxes Reconciliation between Federal Statutory Income Tax Rate and Effective Income Tax Rate (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 33.8
Federal statutory income tax rate	35.00%	35.00%	35.00%
State income taxes, net of the federal tax effect	5.80%	5.90%	5.70%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	4.30%	0.00%	0.00%
Meals and entertainment	0.30%	0.40%	0.30%
Disallowed officer's compensation	0.10%	0.10%	0.30%
Share-based compensation expense on incentive stock options and ESPP	(2.10%)	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Other, Percent	(0.40%)	(0.50%)	(0.50%)
Tax-exempt interest income	(0.30%)	(0.20%)	(0.20%)
Valuation allowance benefit	0.00%	(0.30%)	(0.40%)
Other, net	(0.70%)	(0.90%)	(0.30%)
Effective income tax rate	42.00%	39.50%	39.90%