| **Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands** | Sep. 30, 2016 | Dec. 31, 2015 |
|------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | $ 648,261     | $ 640,499     |
| Gross Unrealized Gains                                                                                                 | 27,893        | 18,479        |
| Gross Unrealized Losses                                                                                                | 341           | 578           |
| Fair Value                                                                                                             | 675,813       | 658,400       |
| **Held to maturity securities**                                                                                        |          |          |
| Total Investment Securities                                                                                            | 624,615       | 618,599       |
| Gross Unrealized Gains                                                                                                 | 22,496        | 14,583        |
| Gross Unrealized Losses                                                                                                | 65            | 802           |
| Fair Value                                                                                                             | 647,046       | 632,380       |
| Amortized Cost                                                                                                         | 1,272,876     | 1,259,098     |
| Gross Unrealized Gains                                                                                                 | 50,389        | 33,062        |
| Gross Unrealized Losses                                                                                                | 406           | 1,380         |
| Fair Value                                                                                                             | 1,322,859     | 1,290,780     |
| U.S. Government-sponsored agency securities                                                                            |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | 100           | 100           |
| Gross Unrealized Gains                                                                                                 | 1             | 4             |
| Gross Unrealized Losses                                                                                                |               |               |
| Fair Value                                                                                                             | 101           | 104           |
| State and municipal                                                                                                    |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | 348,557       | 291,730       |
| Gross Unrealized Gains                                                                                                 | 20,852        | 14,241        |
| Gross Unrealized Losses                                                                                                | 324           | 60            |
| Fair Value                                                                                                             | 369,085       | 305,911       |
| **Held to maturity securities**                                                                                        |          |          |
| Total Investment Securities                                                                                            | 209,857       | 219,767       |
| Gross Unrealized Gains                                                                                                 | 9,682         | 6,982         |
| Gross Unrealized Losses                                                                                                | 14            | 15            |
| Fair Value                                                                                                             | 219,525       | 226,734       |
| U.S. Government-sponsored mortgage-backed securities                                                                   |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | 297,557       | 342,550       |
| Gross Unrealized Gains                                                                                                 | 7,040         | 4,234         |
| Gross Unrealized Losses                                                                                                | 17            | 518           |
| Fair Value                                                                                                             | 304,580       | 346,266       |
| **Held to maturity securities**                                                                                        |          |          |
| Total Investment Securities                                                                                            | 385,259       | 398,832       |
| Gross Unrealized Gains                                                                                                 | 12,804        | 7,601         |
| Gross Unrealized Losses                                                                                                |               | 787           |
| Fair Value                                                                                                             | 398,063       | 405,646       |
| Corporate obligations                                                                                                  |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | 31            | 31            |
| Gross Unrealized Gains                                                                                                 |               |               |
| Gross Unrealized Losses                                                                                                |               |               |
| Fair Value                                                                                                             | 31            | 31            |
| Equity securities                                                                                                      |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | 2,016         | 3,912         |
| Gross Unrealized Gains                                                                                                 |               |               |
| Gross Unrealized Losses                                                                                                |               |               |
| Fair Value                                                                                                             | 2,016         | 3,912         |
| Federal agencies                                                                                                       |          |          |
| **Held to maturity securities**                                                                                        |          |          |
| Total Investment Securities                                                                                            | 29,499        |          |
| Gross Unrealized Gains                                                                                                 | 10            |          |
| Gross Unrealized Losses                                                                                                | 51            |          |
| Fair Value                                                                                                             | $ 29,458      |          |
| Certificates of deposit                                                                                                |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         |          | 2,176         |
| Gross Unrealized Gains                                                                                                 |          |               |
| Gross Unrealized Losses                                                                                                |          |               |
| Fair Value                                                                                                             |          | $ 2,176       |

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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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