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Income Taxes - Summary of Reconciliation Between the Statutory Tax Rate and Actual Effective Tax Rate (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Tax Disclosure [Abstract]      
Statutory tax rate 21.00% 35.00% 35.00%
State taxes, net of federal benefit 1.10% 0.50% 0.70%
Tax-exempt interest (2.10%) (3.30%) (2.90%)
Cash surrender value of life insurance (0.50%) (1.10%) (1.50%)
Tax credits (2.80%) (2.60%) (0.90%)
TCJA deferred tax corporate rate change 0.70% 0.20% 0.00%
Tax Cuts and Jobs Act revaluation of net deferred tax assets (0.40%) 15.20% 0.00%
Other items 0.60% 0.20% 0.20%
Actual effective tax rate 17.60% 44.10% 30.60%
Income tax expense   $ 54.0