Income Taxes - Reconciliation of Effective Tax Rate and Statutory Federal Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	35.00%	35.00%	35.00%
State income taxes, net of federal tax benefit	6.90%	6.80%	6.60%
Tax Cuts and Jobs Act impact on deferred re-measurement	9.60%
Tax-exempt interest on municipal obligations	(1.90%)	(1.80%)	(0.70%)
Tax-exempt life insurance related income	(1.30%)	(1.30%)	(1.30%)
Equity compensation	(1.20%)
Low income housing tax credit benefits	(2.30%)	(1.30%)	(0.40%)
Low income housing tax credit amortization	2.10%	0.80%
Non-deductible joint beneficiary agreement expense	0.10%	0.10%	0.10%
Non-deductible merger expense	0.20%
Other	0.50%	(0.10%)	0.40%
Effective Tax Rate	47.70%	38.20%	39.70%