| **Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands** | Jun. 30, 2019 | Dec. 31, 2018 |
|------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Available for sale securities**                                                                                      |          |          |
| Total Investment Securities                                                                                            | $ 1,360,946   | $ 1,149,727   |
| Gross Unrealized Gains                                                                                                 | 39,820        | 6,721         |
| Gross Unrealized Losses                                                                                                | 807           | 14,253        |
| Investment securities available for sale                                                                               | 1,399,959     | 1,142,195     |
| **Held to maturity securities**                                                                                        |          |          |
| Total Investment Securities                                                                                            | 692,965       | 490,387       |
| Gross Unrealized Gains                                                                                                 | 16,264        | 3,571         |
| Gross Unrealized Losses                                                                                                | 314           | 4,741         |
| Fair Value                                                                                                             | 708,915       | 489,217       |
| Amortized Cost                                                                                                         | 2,053,911     | 1,640,114     |
| Gross Unrealized Gains                                                                                                 | 56,084        | 10,292        |
| Gross Unrealized Losses                                                                                                | 1,121         | 18,994        |
| Fair Value                                                                                                             | 2,108,874     | 1,631,412     |
| U.S. Government-sponsored agency securities                                                                            |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Total Investment Securities                                                                                            | 33,494        | 13,493        |
| Gross Unrealized Gains                                                                                                 | 453           | 92            |
| Gross Unrealized Losses                                                                                                | 0             | 3             |
| Investment securities available for sale                                                                               | 33,947        | 13,582        |
| **Held to maturity securities**                                                                                        |          |          |
| Total Investment Securities                                                                                            | 33,865        | 22,618        |
| Gross Unrealized Gains                                                                                                 | 20            | 0             |
| Gross Unrealized Losses                                                                                                | 66            | 545           |
| Fair Value                                                                                                             | 33,819        | 22,073        |
| State and municipal                                                                                                    |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Total Investment Securities                                                                                            | 685,123       | 605,994       |
| Gross Unrealized Gains                                                                                                 | 30,835        | 5,995         |
| Gross Unrealized Losses                                                                                                | 200           | 5,854         |
| Investment securities available for sale                                                                               | 715,758       | 606,135       |
| **Held to maturity securities**                                                                                        |          |          |
| Total Investment Securities                                                                                            | 310,076       | 197,909       |
| Gross Unrealized Gains                                                                                                 | 10,987        | 2,858         |
| Gross Unrealized Losses                                                                                                | 22            | 872           |
| Fair Value                                                                                                             | 321,041       | 199,895       |
| U.S. Government-sponsored mortgage-backed securities                                                                   |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Total Investment Securities                                                                                            | 642,298       | 530,209       |
| Gross Unrealized Gains                                                                                                 | 8,532         | 634           |
| Gross Unrealized Losses                                                                                                | 607           | 8,396         |
| Investment securities available for sale                                                                               | 650,223       | 522,447       |
| **Held to maturity securities**                                                                                        |          |          |
| Total Investment Securities                                                                                            | 347,524       | 268,860       |
| Gross Unrealized Gains                                                                                                 | 5,257         | 713           |
| Gross Unrealized Losses                                                                                                | 226           | 3,323         |
| Fair Value                                                                                                             | 352,555       | 266,250       |
| Corporate obligations                                                                                                  |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Total Investment Securities                                                                                            | 31            | 31            |
| Gross Unrealized Gains                                                                                                 | 0             | 0             |
| Gross Unrealized Losses                                                                                                | 0             | 0             |
| Investment securities available for sale                                                                               | 31            | 31            |
| Foreign investment                                                                                                     |          |          |
| **Held to maturity securities**                                                                                        |          |          |
| Total Investment Securities                                                                                            | 1,500         | 1,000         |
| Gross Unrealized Gains                                                                                                 | 0             | 0             |
| Gross Unrealized Losses                                                                                                | 0             | 1             |
| Fair Value                                                                                                             | $ 1,500       | $ 999         |

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