Summary of Amortized Costs and Estimated Fair Values of Available-For-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Investments [Line Items]
Available-for-sale, Amortized Cost	$ 705,536	$ 572,917
Available-for-sale, Unrealized Gains	26,059	20,225
Available-for-sale, Unrealized Losses	(965)	(3,106)
Available-for-sale, Fair Value	730,630	590,036
Obligations of states and political subdivisions
Schedule of Investments [Line Items]
Available-for-sale, Amortized Cost	34,272	37,138
Available-for-sale, Unrealized Gains	2,045	2,620
Available-for-sale, Fair Value	36,317	39,758
Mortgage-backed securities issued or guaranteed by U.S. government sponsored enterprises
Schedule of Investments [Line Items]
Available-for-sale, Amortized Cost	369,595	361,073
Available-for-sale, Unrealized Gains	17,353	15,861
Available-for-sale, Unrealized Losses	(1)
Available-for-sale, Fair Value	386,947	376,934
Collateralized mortgage obligations issued or guaranteed by U.S. government sponsored enterprises
Schedule of Investments [Line Items]
Available-for-sale, Amortized Cost	290,322	127,153
Available-for-sale, Unrealized Gains	6,661	1,628
Available-for-sale, Unrealized Losses	(1)	(331)
Available-for-sale, Fair Value	296,982	128,450
Private issue collateralized mortgage obligations
Schedule of Investments [Line Items]
Available-for-sale, Amortized Cost	11,347	12,557
Available-for-sale, Unrealized Losses	(963)	(1,916)
Available-for-sale, Fair Value	10,384	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