| **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** | Mar. 31, 2014 | Dec. 31, 2013 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                                                |          |          |
| Less than 12 Months Fair Value                                                                                                                                                                                                        | $ 319,120     | $ 468,331     |
| Less than 12 Months Gross Unrealized Losses                                                                                                                                                                                           | 3,186         | 9,132         |
| 12 Months or Longer Fair Value                                                                                                                                                                                                        | 18,316        | 8,801         |
| 12 Months or Longer Gross Unrealized Losses                                                                                                                                                                                           | 2,638         | 4,224         |
| Total Fair Value                                                                                                                                                                                                                      | 337,436       | 477,132       |
| Total Gross Unrealized Losses                                                                                                                                                                                                         | 5,824         | 13,356        |
| U.S. Treasury                                                                                                                                                                                                                         |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                                                |          |          |
| Less than 12 Months Fair Value                                                                                                                                                                                                        | 4,889         | 4,875         |
| Less than 12 Months Gross Unrealized Losses                                                                                                                                                                                           | 14            | 21            |
| Total Fair Value                                                                                                                                                                                                                      | 4,889         | 4,875         |
| Total Gross Unrealized Losses                                                                                                                                                                                                         | 14            | 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                                                                                                                                                                                                        | 59,027        | 129,109       |
| Less than 12 Months Gross Unrealized Losses                                                                                                                                                                                           | 1,410         | 3,931         |
| 12 Months or Longer Fair Value                                                                                                                                                                                                        | 1,028         | 767           |
| 12 Months or Longer Gross Unrealized Losses                                                                                                                                                                                           | 69            | 56            |
| Total Fair Value                                                                                                                                                                                                                      | 60,055        | 129,876       |
| Total Gross Unrealized Losses                                                                                                                                                                                                         | 1,479         | 3,987         |
| U.S. Government-sponsored mortgage-backed securities                                                                                                                                                                                  |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                                                |          |          |
| Less than 12 Months Fair Value                                                                                                                                                                                                        | 255,204       | 330,914       |
| Less than 12 Months Gross Unrealized Losses                                                                                                                                                                                           | 1,762         | 5,163         |
| 12 Months or Longer Fair Value                                                                                                                                                                                                        | 14,098        | 5,323         |
| 12 Months or Longer Gross Unrealized Losses                                                                                                                                                                                           | 541           | 532           |
| Total Fair Value                                                                                                                                                                                                                      | 269,302       | 336,237       |
| Total Gross Unrealized Losses                                                                                                                                                                                                         | 2,303         | 5,695         |
| Corporate obligations                                                                                                                                                                                                                 |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                                                |          |          |
| 12 Months or Longer Fair Value                                                                                                                                                                                                        | 3,190         | 2,711         |
| 12 Months or Longer Gross Unrealized Losses                                                                                                                                                                                           | 2,028         | 3,636         |
| Total Fair Value                                                                                                                                                                                                                      | 3,190         | 2,711         |
| Total Gross Unrealized Losses                                                                                                                                                                                                         | $ 2,028       | $ 3,636       |

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