Summary of Amortized Costs and Estimated Fair Values of Available-For-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Investments [Line Items]
Available-for-sale, Amortized Cost	$ 658,675	$ 572,917
Available-for-sale, Unrealized Gains	20,285	20,225
Available-for-sale, Unrealized Losses	(1,698)	(3,106)
Available-for-sale, Fair Value	677,262	590,036
Obligations of states and political subdivisions
Schedule of Investments [Line Items]
Available-for-sale, Amortized Cost	35,161	37,138
Available-for-sale, Unrealized Gains	2,073	2,620
Available-for-sale, Fair Value	37,234	39,758
Mortgage-backed securities issued or guaranteed by U.S. government sponsored enterprises
Schedule of Investments [Line Items]
Available-for-sale, Amortized Cost	611,806	488,226
Available-for-sale, Unrealized Gains	18,212	17,489
Available-for-sale, Unrealized Losses	(84)	(331)
Available-for-sale, Fair Value	629,934	505,384
Private issue collateralized mortgage obligations
Schedule of Investments [Line Items]
Available-for-sale, Amortized Cost	11,708	12,557
Available-for-sale, Unrealized Losses	(1,614)	(1,916)
Available-for-sale, Fair Value	10,094	10,641
Obligations of U.S. government sponsored enterprises
Schedule of Investments [Line Items]
Available-for-sale, Amortized Cost		29,996
Available-for-sale, Unrealized Gains		116
Available-for-sale, Unrealized Losses		(5)
Available-for-sale, Fair Value		30,107
Total debt securities
Schedule of Investments [Line Items]
Available-for-sale, Amortized Cost		567,917
Available-for-sale, Unrealized Gains		20,225
Available-for-sale, Unrealized Losses		(2,252)
Available-for-sale, Fair Value		585,890
Equity Securities
Schedule of Investments [Line Items]
Available-for-sale, Amortized Cost		5,000
Available-for-sale, Unrealized Losses		(854)
Available-for-sale, Fair Value		$ 4,146

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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+ Details

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References+ Details

- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References+ Details