| **Investment Securities - Amortized Cost and Approximate Fair Values of Securities (Details) - USD ($)** | Dec. 31, 2016 | Dec. 31, 2015 |
|----------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Total available for sale**                                                                             |          |          |
| Amortized Cost                                                                                           | $ 696,189,000 | $ 640,499,000 |
| Gross Unrealized Gains                                                                                   | 10,347,000    | 18,479,000    |
| Gross Unrealized Losses                                                                                  | 9,674,000     | 578,000       |
| Investment securities available for sale                                                                 | 696,862,000   | 658,400,000   |
| **Total held to maturity**                                                                               |          |          |
| Amortized Cost                                                                                           | 607,643,000   | 618,599,000   |
| Gross Unrealized Gains                                                                                   | 8,092,000     | 14,583,000    |
| Gross Unrealized Losses                                                                                  | 3,802,000     | 802,000       |
| Fair Value                                                                                               | 611,933,000   | 632,380,000   |
| **Total Investment Securities**                                                                          |          |          |
| Amortized Cost                                                                                           | 1,303,832,000 | 1,259,098,000 |
| Gross Unrealized Gains                                                                                   | 18,439,000    | 33,062,000    |
| Gross Unrealized Losses                                                                                  | 13,476,000    | 1,380,000     |
| Fair Value                                                                                               | 1,308,795,000 | 1,290,780,000 |
| U.S. Government-sponsored agency securities                                                              |          |          |
| **Total available for sale**                                                                             |          |          |
| Amortized Cost                                                                                           | 100,000       | 100,000       |
| Gross Unrealized Gains                                                                                   |               | 4,000         |
| Gross Unrealized Losses                                                                                  | 0             |               |
| Investment securities available for sale                                                                 | 100,000       | 104,000       |
| **Total held to maturity**                                                                               |          |          |
| Amortized Cost                                                                                           | 22,619,000    |          |
| Gross Unrealized Losses                                                                                  | 479,000       |          |
| Fair Value                                                                                               | 22,140,000    |          |
| State and municipal                                                                                      |          |          |
| **Total available for sale**                                                                             |          |          |
| Amortized Cost                                                                                           | 360,779,000   | 291,730,000   |
| Gross Unrealized Gains                                                                                   | 8,443,000     | 14,241,000    |
| Gross Unrealized Losses                                                                                  | 5,564,000     | 60,000        |
| Investment securities available for sale                                                                 | 363,658,000   | 305,911,000   |
| **Total held to maturity**                                                                               |          |          |
| Amortized Cost                                                                                           | 224,811,000   | 219,767,000   |
| Gross Unrealized Gains                                                                                   | 3,136,000     | 6,982,000     |
| Gross Unrealized Losses                                                                                  | 1,796,000     | 15,000        |
| Fair Value                                                                                               | 226,151,000   | 226,734,000   |
| U.S. Government-sponsored mortgage-backed securities                                                     |          |          |
| **Total available for sale**                                                                             |          |          |
| Amortized Cost                                                                                           | 313,459,000   | 342,550,000   |
| Gross Unrealized Gains                                                                                   | 1,904,000     | 4,234,000     |
| Gross Unrealized Losses                                                                                  | 3,071,000     | 518,000       |
| Investment securities available for sale                                                                 | 312,292,000   | 346,266,000   |
| **Total held to maturity**                                                                               |          |          |
| Amortized Cost                                                                                           | 360,213,000   | 398,832,000   |
| Gross Unrealized Gains                                                                                   | 4,956,000     | 7,601,000     |
| Gross Unrealized Losses                                                                                  | 1,527,000     | 787,000       |
| Fair Value                                                                                               | 363,642,000   | 405,646,000   |
| Corporate obligations                                                                                    |          |          |
| **Total available for sale**                                                                             |          |          |
| Amortized Cost                                                                                           | 31,000        | 31,000        |
| Gross Unrealized Gains                                                                                   |               |               |
| Gross Unrealized Losses                                                                                  |               |               |
| Investment securities available for sale                                                                 | 31,000        | 31,000        |
| Equity securities                                                                                        |          |          |
| **Total available for sale**                                                                             |          |          |
| Amortized Cost                                                                                           | 21,820,000    | 3,912,000     |
| Gross Unrealized Gains                                                                                   |               |               |
| Gross Unrealized Losses                                                                                  | 1,039,000     |               |
| Investment securities available for sale                                                                 | 20,781,000    | 3,912,000     |
| **Total held to maturity**                                                                               |          |          |
| Gross Unrealized Losses                                                                                  | $ 0           |          |
| Certificates of deposit                                                                                  |          |          |
| **Total available for sale**                                                                             |          |          |
| Amortized Cost                                                                                           |          | 2,176,000     |
| Investment securities available for sale                                                                 |          | $ 2,176,000   |

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.
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