| **Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands**    | Jun. 30, 2019 | Dec. 31, 2018 |
|----------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Current                                                                                                        | $ 7,470,923   | $ 7,186,609   |
| Non-Accrual                                                                                                    | 25,635        | 26,148        |
| Total Past Due & Non-Accrual                                                                               | 40,447        | 37,858        |
| Total                                                                                                          | 7,511,370     | 7,224,467     |
| Commercial and industrial loans                                                                                |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Current                                                                                                        | 1,874,004     | 1,723,337     |
| Non-Accrual                                                                                                    | 2,770         | 1,803         |
| Total Past Due & Non-Accrual                                                                               | 3,038         | 3,327         |
| Total                                                                                                          | 1,877,042     | 1,726,664     |
| Agricultural production financing and other loans to farmers                                                   |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Current                                                                                                        | 80,634        | 89,440        |
| Non-Accrual                                                                                                    | 2,785         | 679           |
| Total Past Due & Non-Accrual                                                                               | 2,818         | 2,964         |
| Total                                                                                                          | 83,452        | 92,404        |
| Construction                                                                                                   |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Current                                                                                                        | 617,519       | 535,520       |
| Non-Accrual                                                                                                    | 6,918         | 8,667         |
| Total Past Due & Non-Accrual                                                                               | 7,029         | 10,209        |
| Total                                                                                                          | 624,548       | 545,729       |
| Commercial and farmland                                                                                        |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Current                                                                                                        | 2,808,422     | 2,822,515     |
| Non-Accrual                                                                                                    | 6,340         | 8,156         |
| Total Past Due & Non-Accrual                                                                               | 13,267        | 9,587         |
| Total                                                                                                          | 2,821,689     | 2,832,102     |
| Residential                                                                                                    |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Current                                                                                                        | 984,086       | 959,252       |
| Non-Accrual                                                                                                    | 4,579         | 4,966         |
| Total Past Due & Non-Accrual                                                                               | 9,716         | 7,169         |
| Total                                                                                                          | 993,802       | 966,421       |
| Home equity                                                                                                    |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Current                                                                                                        | 544,338       | 524,198       |
| Non-Accrual                                                                                                    | 1,826         | 1,481         |
| Total Past Due & Non-Accrual                                                                               | 3,668         | 3,959         |
| Total                                                                                                          | 548,006       | 528,157       |
| Individuals' loans for household and other personal expenditures                                               |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Current                                                                                                        | 97,826        | 99,499        |
| Non-Accrual                                                                                                    | 64            | 42            |
| Total Past Due & Non-Accrual                                                                               | 558           | 289           |
| Total                                                                                                          | 98,384        | 99,788        |
| Public finance and other commercial loans                                                                      |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Current                                                                                                        | 464,094       | 432,848       |
| Non-Accrual                                                                                                    | 353           | 354           |
| Total Past Due & Non-Accrual                                                                               | 353           | 354           |
| Total                                                                                                          | 464,447       | 433,202       |
| 30-59 Days Past Due                                                                                            |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 13,398        | 8,794         |
| 30-59 Days Past Due | Commercial and industrial loans                                                          |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 248           | 1,093         |
| 30-59 Days Past Due | Agricultural production financing and other loans to farmers                             |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 33            | 2,285         |
| 30-59 Days Past Due | Construction                                                                             |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 111           | 64            |
| 30-59 Days Past Due | Commercial and farmland                                                                  |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 6,751         | 1,253         |
| 30-59 Days Past Due | Residential                                                                              |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 4,540         | 1,756         |
| 30-59 Days Past Due | Home equity                                                                              |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 1,308         | 2,164         |
| 30-59 Days Past Due | Individuals' loans for household and other personal expenditures                         |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 407           | 179           |
| 30-59 Days Past Due | Public finance and other commercial loans                                                |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 0             | 0             |
| 60-89 Days Past Due                                                                                            |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 1,205         | 1,061         |
| 60-89 Days Past Due | Commercial and industrial loans                                                          |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 20            | 182           |
| 60-89 Days Past Due | Agricultural production financing and other loans to farmers                             |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 0             | 0             |
| 60-89 Days Past Due | Construction                                                                             |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 0             | 0             |
| 60-89 Days Past Due | Commercial and farmland                                                                  |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 176           | 178           |
| 60-89 Days Past Due | Residential                                                                              |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 422           | 430           |
| 60-89 Days Past Due | Home equity                                                                              |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 500           | 207           |
| 60-89 Days Past Due | Individuals' loans for household and other personal expenditures                         |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 87            | 64            |
| 60-89 Days Past Due | Public finance and other commercial loans                                                |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 0             | 0             |
| Loans 90 Days or More Past Due And Accruing                                                                    |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 209           | 1,855         |
| Loans 90 Days or More Past Due And Accruing | Commercial and industrial loans                                  |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 0             | 249           |
| Loans 90 Days or More Past Due And Accruing | Agricultural production financing and other loans to farmers     |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 0             | 0             |
| Loans 90 Days or More Past Due And Accruing | Construction                                                     |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 0             | 1,478         |
| Loans 90 Days or More Past Due And Accruing | Commercial and farmland                                          |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 0             | 0             |
| Loans 90 Days or More Past Due And Accruing | Residential                                                      |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 175           | 17            |
| Loans 90 Days or More Past Due And Accruing | Home equity                                                      |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 34            | 107           |
| Loans 90 Days or More Past Due And Accruing | Individuals' loans for household and other personal expenditures |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | 0             | 4             |
| Loans 90 Days or More Past Due And Accruing | Public finance and other commercial loans                        |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                                |          |          |
| Past Due                                                                                                       | $ 0           | $ 0           |

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| - Definition
Amount of financing receivable not past due.
+ References
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Amount of financing receivable on nonaccrual status.
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Amount of financing receivable past due.
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| - Definition
Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.
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