Income Taxes - Reconciliation of Effective Tax Rate and Statutory Federal Income Tax Rate (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	35.00%	35.00%	35.00%	35.00%
State income taxes, net of federal tax benefit	6.90%	6.90%	6.80%	6.70%
Tax-exempt interest on municipal obligations	(1.90%)	(1.70%)	(1.80%)	(1.90%)
Increase in cash value of insurance policies	(1.30%)	(1.20%)	(1.30%)	(1.60%)
Low income housing tax credits	(0.50%)	(0.30%)	(0.40%)	(0.30%)
Equity compensation	(0.80%)		(1.40%)
Other	0.10%		0.20%	0.10%
Effective Tax Rate	37.50%	38.70%	37.10%	38.00%