Income Taxes - Reconciliation of Effective Tax Rate and Statutory Federal Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	35.00%	35.00%
State income taxes, net of federal tax benefit	8.60%	6.90%	6.80%
Tax Cuts and Jobs Act impact of federal rate change		9.60%
Tax-exempt interest on municipal obligations	(1.00%)	(1.90%)	(1.80%)
Tax-exempt life insurance related income	(0.60%)	(1.30%)	(1.30%)
Low income housing tax credits	(2.20%)	(2.30%)	(1.30%)
Low income housing tax credit amortization	2.00%	2.10%	0.80%
Equity compensation	(0.50%)	(1.20%)
Non-deductible joint beneficiary agreement expense	0.10%	0.10%	0.10%
Non-deductible merger expenses	0.20%	0.20%
Other	(0.80%)	0.50%	(0.10%)
Effective Tax Rate	26.80%	47.70%	38.20%