Income Taxes - Income Tax Provision Differences from Expected Statutory Provision (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Expected provision at statutory rate	35.00%	35.00%	35.00%
Difference resulting from:
Tax exempt interest income	(9.50%)	(11.20%)	(11.70%)
Increase in value of bank owned life insurance assets	(1.20%)	(1.90%)	(3.90%)
Effective income tax rate reconciliation retirement plans	1.90%	0.00%	2.60%
Other, including state income taxes, valuation allowances and rate differentials	0.20%	3.20%	0.80%
Provision for income taxes - effective income tax rate	26.40%	25.10%	22.80%