Income Taxes - Summary of Reconciliation Between the Statutory Tax Rate and Actual Effective Tax Rate (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Statutory tax rate	35.00%	35.00%	35.00%
State taxes, net of federal benefit	0.50%	0.70%	0.80%
Valuation allowance reversal	0.00%	0.00%	(0.80%)
Tax-exempt interest	(3.30%)	(2.90%)	(2.20%)
Cash surrender value of life insurance	1.10%	1.50%	1.30%
Tax credits	(2.60%)	(0.90%)	(1.10%)
TCJA deferred tax corporate rate change	0.20%	0.00%	0.00%
Tax Cuts and Jobs Act revaluation of net deferred tax assets	15.20%	0.00%	0.00%
Other items	0.20%	0.20%	0.10%
Actual effective tax rate	44.10%	30.60%	30.50%
Income tax expense	$ 54.0