Amortized Cost and Estimated Fair Values of Investments Available-for-sale (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	$ 765,357	$ 824,859
Available-for-Sale Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	735,378	791,889
Gross Unrealized Gains	31,425	34,160
Gross Unrealized Losses	(723)	(467)
Estimated Fair Value	766,080	825,582
Available-for-Sale Securities \| Marketable Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	723	723
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	723	723
Available-for-Sale Securities \| Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	734,655	791,166
Gross Unrealized Gains	31,425	34,160
Gross Unrealized Losses	(723)	(467)
Estimated Fair Value	765,357	824,859
Available-for-Sale Securities \| Debt Securities \| U.S. Government Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	130,412	155,442
Gross Unrealized Gains	643	1,084
Gross Unrealized Losses	(158)	(98)
Estimated Fair Value	130,897	156,428
Available-for-Sale Securities \| Debt Securities \| State and Municipal
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	160,255	160,496
Gross Unrealized Gains	13,168	13,996
Gross Unrealized Losses	0	(1)
Estimated Fair Value	173,423	174,491
Available-for-Sale Securities \| Debt Securities \| Mortgage-Backed
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	440,287	471,527
Gross Unrealized Gains	17,607	19,080
Gross Unrealized Losses	(350)	(128)
Estimated Fair Value	457,544	490,479
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	7	0
Gross Unrealized Losses	0	(4)
Estimated Fair Value	2,007	1,996
Available-for-Sale Securities \| Debt Securities \| Trust Preferred
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,701	1,701
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(215)	(236)
Estimated Fair Value	$ 1,486	$ 1,465

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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