| **Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands** | Jun. 30, 2016 | Dec. 31, 2015 |
|------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | $ 625,877     | $ 640,499     |
| Gross Unrealized Gains                                                                                                 | 32,972        | 18,479        |
| Gross Unrealized Losses                                                                                                | 20            | 578           |
| Fair Value                                                                                                             | 658,829       | 658,400       |
| **Held to maturity securities**                                                                                        |          |          |
| Amortized Cost                                                                                                         | 638,972       | 618,599       |
| Gross Unrealized Gains                                                                                                 | 26,099        | 14,583        |
| Gross Unrealized Losses                                                                                                | 5             | 802           |
| Fair Value                                                                                                             | 665,066       | 632,380       |
| Amortized Cost                                                                                                         | 1,264,849     | 1,259,098     |
| Gross Unrealized Gains                                                                                                 | 59,071        | 33,062        |
| Gross Unrealized Losses                                                                                                | 25            | 1,380         |
| Fair Value                                                                                                             | 1,323,895     | 1,290,780     |
| U.S. Government-sponsored agency securities                                                                            |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | 100           | 100           |
| Gross Unrealized Gains                                                                                                 | 2             | 4             |
| Gross Unrealized Losses                                                                                                |               |               |
| Fair Value                                                                                                             | 102           | 104           |
| State and municipal                                                                                                    |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | 329,042       | 291,730       |
| Gross Unrealized Gains                                                                                                 | 24,438        | 14,241        |
| Gross Unrealized Losses                                                                                                |               | 60            |
| Fair Value                                                                                                             | 353,480       | 305,911       |
| **Held to maturity securities**                                                                                        |          |          |
| Amortized Cost                                                                                                         | 207,731       | 219,767       |
| Gross Unrealized Gains                                                                                                 | 11,621        | 6,982         |
| Gross Unrealized Losses                                                                                                |               | 15            |
| Fair Value                                                                                                             | 219,352       | 226,734       |
| U.S. Government-sponsored mortgage-backed securities                                                                   |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | 292,822       | 342,550       |
| Gross Unrealized Gains                                                                                                 | 8,469         | 4,234         |
| Gross Unrealized Losses                                                                                                | 20            | 518           |
| Fair Value                                                                                                             | 301,271       | 346,266       |
| **Held to maturity securities**                                                                                        |          |          |
| Amortized Cost                                                                                                         | 402,261       | 398,832       |
| Gross Unrealized Gains                                                                                                 | 14,444        | 7,601         |
| Gross Unrealized Losses                                                                                                |               | 787           |
| Fair Value                                                                                                             | 416,705       | 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                                                                                                         | 3,882         | 3,912         |
| Gross Unrealized Gains                                                                                                 | 63            |               |
| Gross Unrealized Losses                                                                                                |               |               |
| Fair Value                                                                                                             | 3,945         | 3,912         |
| Federal agencies                                                                                                       |          |          |
| **Held to maturity securities**                                                                                        |          |          |
| Amortized Cost                                                                                                         | 28,980        |          |
| Gross Unrealized Gains                                                                                                 | 34            |          |
| Gross Unrealized Losses                                                                                                | 5             |          |
| Fair Value                                                                                                             | $ 29,009      |          |
| 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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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.
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Fair value of investments in debt securities classified as held-to-maturity.
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