Income Taxes - Reconciliation of Statutory Income Tax Rate to Consolidated Effective Income Tax Rate (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Federal income tax at statutory rate, rate	35.00%	35.00%	35.00%
State franchise taxes, net of federal benefit, rate	6.90%	6.10%	7.00%
Tax-exempt income, rate	(5.30%)	(7.10%)	(9.30%)
Tax credits, rate	(0.60%)	(0.70%)	(1.10%)
Other, net, rate	0.10%	0.40%	1.00%
Provision for income taxes, rate	36.10%	33.70%	32.60%
Federal income tax at statutory rate	$ 56,979	$ 50,496	$ 40,143
State franchise taxes, net of federal benefit	11,233	8,734	8,081
Tax-exempt income	(8,622)	(10,189)	(10,654)
Tax credits	(1,014)	(940)	(1,279)
Other, net	200	566	1,122
Provision for income taxes	$ 58,776	$ 48,667	$ 37,413