| **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       |

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| - 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
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| - Definition
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.
+ References
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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.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 22 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27161-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 13, 19 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.
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| - Definition
This item represents the gross unrealized losses for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.
+ References
No definition available.
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