Amortized Cost and Estimated Fair Values of Investments Available-for-sale (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	$ 833,942	$ 951,201
Available-for-Sale Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	796,989	918,027
Gross Unrealized Gains	38,036	33,791
Gross Unrealized Losses	(360)	(517)
Estimated Fair Value	834,665	951,301
Available-for-Sale Securities \| Marketable Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	723	100
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	723	100
Available-for-Sale Securities \| Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	796,266	917,927
Gross Unrealized Gains	38,036	33,791
Gross Unrealized Losses	(360)	(517)
Estimated Fair Value	833,942	951,201
Available-for-Sale Securities \| Debt Securities \| U.S. Government Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	133,254	197,816
Gross Unrealized Gains	1,385	2,436
Gross Unrealized Losses	0	0
Estimated Fair Value	134,639	200,252
Available-for-Sale Securities \| Debt Securities \| State and Municipal
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	160,736	160,657
Gross Unrealized Gains	14,373	12,456
Gross Unrealized Losses	0	(2)
Estimated Fair Value	175,109	173,111
Available-for-Sale Securities \| Debt Securities \| Mortgage-Backed
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	498,560	551,518
Gross Unrealized Gains	22,278	18,639
Gross Unrealized Losses	(64)	(13)
Estimated Fair Value	520,774	570,144
Available-for-Sale Securities \| Debt Securities \| Corporate Debt
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,000	2,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(8)	(22)
Estimated Fair Value	1,992	1,978
Available-for-Sale Securities \| Debt Securities \| Trust Preferred
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,716	5,936
Gross Unrealized Gains	0	260
Gross Unrealized Losses	(288)	(480)
Estimated Fair Value	$ 1,428	$ 5,716

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.

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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- 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.

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