Income Taxes - Reconciliation from Federal Statutory Income Tax Rate to Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Federal statutory tax rate	35.00%	35.00%	35.00%
Net tax-exempt interest income on securities of state and political subdivisions	(7.00%)	(6.80%)	(6.40%)
State income taxes, net of federal tax effect	1.40%	1.60%	1.40%
Bank-owned life insurance	(1.20%)	(1.60%)	(1.70%)
General business credits	(2.10%)	(2.10%)	(3.10%)
All other - net	0.30%	(0.10%)	0.10%
Effective tax rate	26.40%	26.00%	25.30%