| **Investment Securities - Amortized Cost and Approximate Fair Values of Securities (Detail) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2012 | Dec. 31, 2011 |
|-----------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |          |          |
| Amortized Cost                                                                                                                                | $ 498,104     | $ 503,479     |
| Gross Unrealized Gains                                                                                                                        | 21,184        | 20,600        |
| Gross Unrealized losses                                                                                                                       | 5,945         | 5,588         |
| Fair Value                                                                                                                                    | 513,343       | 518,491       |
| Amortized Cost                                                                                                                                | 361,020       | 427,909       |
| Gross Unrealized Gains                                                                                                                        | 17,170        | 14,560        |
| Gross Unrealized Losses                                                                                                                       | 16            |          |
| Fair Value                                                                                                                                    | 378,174       | 442,469       |
| Amortized Cost                                                                                                                                | 859,124       | 931,388       |
| Gross Unrealized Gains                                                                                                                        | 38,354        | 35,160        |
| Gross Unrealized Losses                                                                                                                       | 5,961         | 5,588         |
| Fair Value                                                                                                                                    | 891,517       | 960,960       |
| Government Agency and Municipal Bonds and Notes                                                                                               |          |          |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |          |          |
| Amortized Cost                                                                                                                                | 4,475         | 99            |
| Gross Unrealized Gains                                                                                                                        | 165           | 18            |
| Gross Unrealized losses                                                                                                                       |     |          |
| Fair Value                                                                                                                                    | 4,640         | 117           |
| State and municipal                                                                                                                           |          |          |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |          |          |
| Amortized Cost                                                                                                                                | 148,187       | 136,857       |
| Gross Unrealized Gains                                                                                                                        | 10,025        | 10,496        |
| Gross Unrealized losses                                                                                                                       | 18            |          |
| Fair Value                                                                                                                                    | 158,194       | 147,353       |
| Amortized Cost                                                                                                                                | 117,227       | 120,171       |
| Gross Unrealized Gains                                                                                                                        | 5,489         | 3,785         |
| Gross Unrealized Losses                                                                                                                       | 1             |          |
| Fair Value                                                                                                                                    | 122,715       | 123,956       |
| U.S. Government-sponsored mortgage-backed securities                                                                                          |          |          |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |          |          |
| Amortized Cost                                                                                                                                | 337,631       | 358,928       |
| Gross Unrealized Gains                                                                                                                        | 10,994        | 10,086        |
| Gross Unrealized losses                                                                                                                       | 46            | 16            |
| Fair Value                                                                                                                                    | 348,579       | 368,998       |
| Amortized Cost                                                                                                                                | 243,793       | 307,738       |
| Gross Unrealized Gains                                                                                                                        | 11,681        | 10,775        |
| Gross Unrealized Losses                                                                                                                       | 15            |          |
| Fair Value                                                                                                                                    | 255,459       | 318,513       |
| Corporate Bonds and Notes                                                                                                                     |          |          |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |          |          |
| Amortized Cost                                                                                                                                | 6,105         | 5,765         |
| Gross Unrealized Gains                                                                                                                        |     |          |
| Gross Unrealized losses                                                                                                                       | 5,881         | 5,572         |
| Fair Value                                                                                                                                    | 224           | 193           |
| Equity securities                                                                                                                             |          |          |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |          |          |
| Amortized Cost                                                                                                                                | 1,706         | 1,830         |
| Gross Unrealized Gains                                                                                                                        |     |          |
| Gross Unrealized losses                                                                                                                       |     |          |
| Fair Value                                                                                                                                    | $ 1,706       | $ 1,830       |

| X                                                                                                                                                                                                                                       |
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
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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.
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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
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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.
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No definition available.
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| - Definition
For an unclassified balance sheet, this item represents investments in debt securities which are categorized as held-to-maturity. The held-to-maturity category is for those securities that the entity has the positive intent and ability to hold until maturity.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt securities as of the balance sheet date and which are categorized as held-to-maturity.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
As of the balance sheet date, the excess of fair value over carrying value (amortized cost) of a debt security categorized as held-to-maturity, exclusive of dividend or interest income recognized but not yet received and exclusive of any write-downs for other-than-temporary impairment. Such item represents the gross unrecognized holding gain.
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
As of the balance sheet date, the excess of carrying value (amortized cost) over fair value of a debt security categorized as held-to-maturity, exclusive of dividend or interest income recognized but not yet received and exclusive of any write-downs for other-than-temporary impairment. Such item represents the gross unrecognized holding loss.
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