| **Investment Securities - Investments' Gross Unrealized Losses and Fair Value Aggregated by Investment Category and Length of Time in Continuous Unrealized Loss Position (Details) (USD $)In Thousands, unless otherwise specified** | Sep. 30, 2013 | Dec. 31, 2012 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                                                |          |          |
| Less than 12 Months Fair Value                                                                                                                                                                                                        | $ 282,863     | $ 16,844      |
| Less than 12 Months Gross Unrealized Losses                                                                                                                                                                                           | 9,633         | 80            |
| 12 Months or Longer Fair Value                                                                                                                                                                                                        | 692           | 194           |
| 12 Months or Longer Gross Unrealized Losses                                                                                                                                                                                           | 5,853         | 5,881         |
| Total Fair Value                                                                                                                                                                                                                      | 283,555       | 17,038        |
| Total Gross Unrealized Losses                                                                                                                                                                                                         | 15,486        | 5,961         |
| U.S. Government-sponsored agency securities                                                                                                                                                                                           |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                                                |          |          |
| Less than 12 Months Fair Value                                                                                                                                                                                                        | 1,439         |          |
| Less than 12 Months Gross Unrealized Losses                                                                                                                                                                                           | 9             |          |
| Total Fair Value                                                                                                                                                                                                                      | 1,439         |          |
| Total Gross Unrealized Losses                                                                                                                                                                                                         | 9             |          |
| State and municipal                                                                                                                                                                                                                   |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                                                |          |          |
| Less than 12 Months Fair Value                                                                                                                                                                                                        | 76,681        | 4,524         |
| Less than 12 Months Gross Unrealized Losses                                                                                                                                                                                           | 3,467         | 19            |
| 12 Months or Longer Fair Value                                                                                                                                                                                                        | 239           |          |
| 12 Months or Longer Gross Unrealized Losses                                                                                                                                                                                           | 2             |          |
| Total Fair Value                                                                                                                                                                                                                      | 76,920        | 4,524         |
| Total Gross Unrealized Losses                                                                                                                                                                                                         | 3,469         | 19            |
| U.S. Government-sponsored mortgage-backed securities                                                                                                                                                                                  |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                                                |          |          |
| Less than 12 Months Fair Value                                                                                                                                                                                                        | 204,743       | 12,320        |
| Less than 12 Months Gross Unrealized Losses                                                                                                                                                                                           | 6,157         | 61            |
| Total Fair Value                                                                                                                                                                                                                      | 204,743       | 12,320        |
| Total Gross Unrealized Losses                                                                                                                                                                                                         | 6,157         | 61            |
| Corporate obligations                                                                                                                                                                                                                 |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                                                |          |          |
| 12 Months or Longer Fair Value                                                                                                                                                                                                        | 453           | 194           |
| 12 Months or Longer Gross Unrealized Losses                                                                                                                                                                                           | 5,851         | 5,881         |
| Total Fair Value                                                                                                                                                                                                                      | 453           | 194           |
| Total Gross Unrealized Losses                                                                                                                                                                                                         | $ 5,851       | $ 5,881       |

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