Income Taxes - Summary of Reconciliation Between the Statutory Tax Rate and Actual Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Statutory tax rate	35.00%	35.00%	35.00%
State taxes, net of federal benefit	0.80%	0.10%	0.20%
Valuation allowance reversal	(0.80%)
Tax-exempt interest	(2.20%)	(1.80%)	(2.90%)
Cash surrender value of life insurance	(1.30%)	(1.30%)	(1.80%)
Tax credits	(1.10%)	(1.40%)	(2.40%)
Other items	0.10%	(0.30%)	(0.60%)
Actual effective tax rate	30.50%	30.30%	27.50%