Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Investment Securities [Line Items]
Amortized Cost	$ 1,297,923	$ 1,169,613
Gross Unrealized Gains	28,142	30,650
Gross Unrealized Losses	(28,862)	(28,960)
Estimated Fair Value of Available for Sale	1,297,203	1,171,303
Obligations of U.S. Government Agencies [Member]
Investment Securities [Line Items]
Amortized Cost	26,500
Estimated Fair Value of Available for Sale	30,500
Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	969,700
Estimated Fair Value of Available for Sale	990,800
Obligations of States and Political Subdivisions [Member]
Investment Securities [Line Items]
Amortized Cost	82	82
Gross Unrealized Gains	3	4
Gross Unrealized Losses	0	0
Estimated Fair Value of Available for Sale	85	86
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Investment Securities [Line Items]
Amortized Cost	51,077	51,866
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(26,566)	(28,496)
Estimated Fair Value of Available for Sale	24,512	23,373
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,296,103	1,167,754
Gross Unrealized Gains	28,029	30,534
Gross Unrealized Losses	(28,862)	(28,960)
Estimated Fair Value of Available for Sale	1,295,270	1,169,328
Mortgage Backed Securities - Residential [Member] \| Obligations of U.S. Government Agencies [Member]
Investment Securities [Line Items]
Amortized Cost	26,533	27,883
Gross Unrealized Gains	3,929	3,781
Gross Unrealized Losses	0	0
Estimated Fair Value of Available for Sale	30,462	31,664
Mortgage Backed Securities - Residential [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	969,600	839,102
Gross Unrealized Gains	23,213	25,691
Gross Unrealized Losses	(2,153)	(392)
Estimated Fair Value of Available for Sale	990,660	864,401
Mortgage Backed Securities - Commercial [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	139	148
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Estimated Fair Value of Available for Sale	140	149
Other Government Sponsored Enterprises [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	241,972	241,970
Gross Unrealized Gains	579	766
Gross Unrealized Losses	(113)	(72)
Estimated Fair Value of Available for Sale	242,438	242,664
Corporate Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	6,700	6,703
Gross Unrealized Gains	303	288
Gross Unrealized Losses	(30)
Estimated Fair Value of Available for Sale	6,973	6,991
Corporate Debt Securities [Member] \| Corporate Debt Securities [Member]
Investment Securities [Line Items]
Gross Unrealized Losses		0
Equity Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,820	1,859
Gross Unrealized Gains	113	116
Gross Unrealized Losses	0	0
Estimated Fair Value of Available for Sale	$ 1,933	$ 1,975

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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