| **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** | Sep. 30, 2017 | Dec. 31, 2016 |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | $ 230,302     | $ 330,902     |
| 12 Months or Longer                                                                                                                                                                                            | 29,842        |               |
| Total                                                                                                                                                                                                          | 260,144       | 330,902       |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 2,753         | 9,674         |
| 12 Months or Longer                                                                                                                                                                                            | 1,210         |               |
| Total                                                                                                                                                                                                          | 3,963         | 9,674         |
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | 103,645       | 179,395       |
| 12 Months or Longer                                                                                                                                                                                            | 18,483        |               |
| Total                                                                                                                                                                                                          | 122,128       | 179,395       |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 1,292         | 3,802         |
| 12 Months or Longer                                                                                                                                                                                            | 406           |               |
| Total                                                                                                                                                                                                          | 1,698         | 3,802         |
| **Available-for-sale Securities and Held-to-maturity Securities [Abstract]**                                                                                                                                   |          |          |
| Less than 12 Months, Fair Value                                                                                                                                                                                | 333,947       | 510,297       |
| 12 Months or Longer, Fair Value                                                                                                                                                                                | 48,325        |               |
| Total, Fair Value                                                                                                                                                                                              | 382,272       | 510,297       |
| Less than 12 Months, Gross Unrealized Losses                                                                                                                                                                   | 4,045         | 13,476        |
| 12 Months or Longer, Gross Unrealized Losses                                                                                                                                                                   | 1,616         |               |
| Total, Gross Unrealized Losses                                                                                                                                                                                 | 5,661         | 13,476        |
| State and municipal                                                                                                                                                                                            |          |          |
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | 76,469        | 126,593       |
| 12 Months or Longer                                                                                                                                                                                            | 21,693        |               |
| Total                                                                                                                                                                                                          | 98,162        | 126,593       |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 959           | 5,564         |
| 12 Months or Longer                                                                                                                                                                                            | 1,047         |               |
| Total                                                                                                                                                                                                          | 2,006         | 5,564         |
| U.S. Government-sponsored mortgage-backed securities                                                                                                                                                           |          |          |
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | 153,833       | 185,544       |
| 12 Months or Longer                                                                                                                                                                                            | 8,149         |               |
| Total                                                                                                                                                                                                          | 161,982       | 185,544       |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 1,794         | 3,071         |
| 12 Months or Longer                                                                                                                                                                                            | 163           |               |
| Total                                                                                                                                                                                                          | 1,957         | 3,071         |
| Equity securities                                                                                                                                                                                              |          |          |
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            |          | 18,765        |
| Total                                                                                                                                                                                                          |          | 18,765        |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            |          | 1,039         |
| Total                                                                                                                                                                                                          |          | 1,039         |
| U.S. Government-sponsored agency securities                                                                                                                                                                    |          |          |
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | 9,941         | 19,121        |
| 12 Months or Longer                                                                                                                                                                                            | 12,267        |          |
| Total                                                                                                                                                                                                          | 22,208        | 19,121        |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 78            | 479           |
| 12 Months or Longer                                                                                                                                                                                            | 233           |          |
| Total                                                                                                                                                                                                          | 311           | 479           |
| State and municipal                                                                                                                                                                                            |          |          |
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | 18,754        | 50,897        |
| 12 Months or Longer                                                                                                                                                                                            | 6,216         |               |
| Total                                                                                                                                                                                                          | 24,970        | 50,897        |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 402           | 1,796         |
| 12 Months or Longer                                                                                                                                                                                            | 173           |               |
| Total                                                                                                                                                                                                          | 575           | 1,796         |
| U.S. Government-sponsored mortgage-backed securities                                                                                                                                                           |          |          |
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | 74,950        | 109,377       |
| 12 Months or Longer                                                                                                                                                                                            |               |               |
| Total                                                                                                                                                                                                          | 74,950        | 109,377       |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 812           | 1,527         |
| 12 Months or Longer                                                                                                                                                                                            |               |               |
| Total                                                                                                                                                                                                          | $ 812         | $ 1,527       |

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