| **Investment Securities - Investments' Gross Unrealized Losses and Fair Value Aggregated by Investment Category and Length of Time in Continuous Unrealized Loss Position (Detail) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2013 | Dec. 31, 2012 |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                                               |          |          |
| Less than 12 Months Fair Value                                                                                                                                                                                                       | $ 468,331     | $ 16,844      |
| Less than 12 Months Gross Unrealized Losses                                                                                                                                                                                          | 9,132         | 80            |
| 12 Months or Longer Fair Value                                                                                                                                                                                                       | 8,801         | 194           |
| 12 Months or Longer Gross Unrealized Losses                                                                                                                                                                                          | 4,224         | 5,881         |
| Total Fair Value                                                                                                                                                                                                                     | 477,132       | 17,038        |
| Total Gross Unrealized Losses                                                                                                                                                                                                        | 13,356        | 5,961         |
| U.S. Treasury                                                                                                                                                                                                                        |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                                               |          |          |
| Less than 12 Months Fair Value                                                                                                                                                                                                       | 4,875         |          |
| Less than 12 Months Gross Unrealized Losses                                                                                                                                                                                          | 21            |          |
| Total Fair Value                                                                                                                                                                                                                     | 4,875         |          |
| Total Gross Unrealized Losses                                                                                                                                                                                                        | 21            |          |
| U.S. Government-sponsored agency securities                                                                                                                                                                                          |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                                               |          |          |
| Less than 12 Months Fair Value                                                                                                                                                                                                       | 3,433         |          |
| Less than 12 Months Gross Unrealized Losses                                                                                                                                                                                          | 17            |          |
| Total Fair Value                                                                                                                                                                                                                     | 3,433         |          |
| Total Gross Unrealized Losses                                                                                                                                                                                                        | 17            |          |
| State and municipal                                                                                                                                                                                                                  |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                                               |          |          |
| Less than 12 Months Fair Value                                                                                                                                                                                                       | 129,109       | 4,524         |
| Less than 12 Months Gross Unrealized Losses                                                                                                                                                                                          | 3,931         | 19            |
| 12 Months or Longer Fair Value                                                                                                                                                                                                       | 767           |          |
| 12 Months or Longer Gross Unrealized Losses                                                                                                                                                                                          | 56            |          |
| Total Fair Value                                                                                                                                                                                                                     | 129,876       | 4,524         |
| Total Gross Unrealized Losses                                                                                                                                                                                                        | 3,987         | 19            |
| U.S. Government-sponsored mortgage-backed securities                                                                                                                                                                                 |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                                               |          |          |
| Less than 12 Months Fair Value                                                                                                                                                                                                       | 330,914       | 12,320        |
| Less than 12 Months Gross Unrealized Losses                                                                                                                                                                                          | 5,163         | 61            |
| 12 Months or Longer Fair Value                                                                                                                                                                                                       | 5,323         |          |
| 12 Months or Longer Gross Unrealized Losses                                                                                                                                                                                          | 532           |          |
| Total Fair Value                                                                                                                                                                                                                     | 336,237       | 12,320        |
| Total Gross Unrealized Losses                                                                                                                                                                                                        | 5,695         | 61            |
| Corporate obligations                                                                                                                                                                                                                |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                                               |          |          |
| Less than 12 Months Fair Value                                                                                                                                                                                                       |     |          |
| Less than 12 Months Gross Unrealized Losses                                                                                                                                                                                          |     |          |
| 12 Months or Longer Fair Value                                                                                                                                                                                                       | 2,711         | 194           |
| 12 Months or Longer Gross Unrealized Losses                                                                                                                                                                                          | 3,636         | 5,881         |
| Total Fair Value                                                                                                                                                                                                                     | 2,711         | 194           |
| Total Gross Unrealized Losses                                                                                                                                                                                                        | $ 3,636       | $ 5,881       |

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