| **Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands** | Dec. 31, 2018 | Dec. 31, 2017 |
|-------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | $ 7,186,609   | $ 6,703,256   |
| Loans 90 Days or More Past Due And Accruing                                                                 | 1,855         | 924           |
| Non-Accrual                                                                                                 | 26,148        | 28,724        |
| Total Past Due & Non-Accrual                                                                            | 37,858        | 47,943        |
| Total                                                                                                       | 7,224,467     | 6,751,199     |
| 30-59 Days Past Due                                                                                         |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 8,794         | 16,773        |
| 60-89 Days Past Due                                                                                         |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 1,061         | 1,522         |
| Commercial and industrial loans                                                                             |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 1,723,337     | 1,487,221     |
| Loans 90 Days or More Past Due And Accruing                                                                 | 249           |               |
| Non-Accrual                                                                                                 | 1,803         | 3,275         |
| Total Past Due & Non-Accrual                                                                            | 3,327         | 6,272         |
| Total                                                                                                       | 1,726,664     | 1,493,493     |
| Commercial and industrial loans | 30-59 Days Past Due                                                       |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 1,093         | 2,967         |
| Commercial and industrial loans | 60-89 Days Past Due                                                       |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 182           | 30            |
| Agricultural production financing and other loans to farmers                                                |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 89,440        | 120,720       |
| Loans 90 Days or More Past Due And Accruing                                                                 |          |               |
| Non-Accrual                                                                                                 | 679           | 1,027         |
| Total Past Due & Non-Accrual                                                                            | 2,964         | 1,037         |
| Total                                                                                                       | 92,404        | 121,757       |
| Agricultural production financing and other loans to farmers | 30-59 Days Past Due                          |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 2,285         | 10            |
| Agricultural production financing and other loans to farmers | 60-89 Days Past Due                          |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    |          |               |
| Construction                                                                                                |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 535,520       | 610,896       |
| Loans 90 Days or More Past Due And Accruing                                                                 | 1,478         | 65            |
| Non-Accrual                                                                                                 | 8,667         | 65            |
| Total Past Due & Non-Accrual                                                                            | 10,209        | 1,323         |
| Total                                                                                                       | 545,729       | 612,219       |
| Construction | 30-59 Days Past Due                                                                          |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 64            | 1,193         |
| Construction | 60-89 Days Past Due                                                                          |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    |               |               |
| Commercial and farmland                                                                                     |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 2,822,515     | 2,542,048     |
| Loans 90 Days or More Past Due And Accruing                                                                 |               | 603           |
| Non-Accrual                                                                                                 | 8,156         | 12,951        |
| Total Past Due & Non-Accrual                                                                            | 9,587         | 20,643        |
| Total                                                                                                       | 2,832,102     | 2,562,691     |
| Commercial and farmland | 30-59 Days Past Due                                                               |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 1,253         | 6,923         |
| Commercial and farmland | 60-89 Days Past Due                                                               |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 178           | 166           |
| Residential                                                                                                 |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 959,252       | 948,947       |
| Loans 90 Days or More Past Due And Accruing                                                                 | 17            | 56            |
| Non-Accrual                                                                                                 | 4,966         | 9,444         |
| Total Past Due & Non-Accrual                                                                            | 7,169         | 13,818        |
| Total                                                                                                       | 966,421       | 962,765       |
| Residential | 30-59 Days Past Due                                                                           |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 1,756         | 4,010         |
| Residential | 60-89 Days Past Due                                                                           |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 430           | 308           |
| Home equity                                                                                                 |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 524,198       | 510,362       |
| Loans 90 Days or More Past Due And Accruing                                                                 | 107           | 175           |
| Non-Accrual                                                                                                 | 1,481         | 1,928         |
| Total Past Due & Non-Accrual                                                                            | 3,959         | 3,659         |
| Total                                                                                                       | 528,157       | 514,021       |
| Home equity | 30-59 Days Past Due                                                                           |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 2,164         | 1,372         |
| Home equity | 60-89 Days Past Due                                                                           |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 207           | 184           |
| Consumer                                                                                                    |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 99,499        | 85,744        |
| Loans 90 Days or More Past Due And Accruing                                                                 | 4             | 25            |
| Non-Accrual                                                                                                 | 42            | 34            |
| Total Past Due & Non-Accrual                                                                            | 289           | 1,191         |
| Total                                                                                                       | 99,788        | 86,935        |
| Consumer | 30-59 Days Past Due                                                                              |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 179           | 298           |
| Consumer | 60-89 Days Past Due                                                                              |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 64            | 834           |
| Lease financing receivables, net of unearned income                                                         |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 1,600         | 2,527         |
| Non-Accrual                                                                                                 |          |               |
| Total Past Due & Non-Accrual                                                                            |          |               |
| Total                                                                                                       | 1,600         | 2,527         |
| Other commercial loans                                                                                      |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 431,248       | 394,791       |
| Non-Accrual                                                                                                 | 354           |               |
| Total Past Due & Non-Accrual                                                                            | 354           |               |
| Total                                                                                                       | 431,602       | 394,791       |
| Other commercial loans | 60-89 Days Past Due                                                                |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    |               |               |

| X                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Recorded Investment, More Than Ninety Days Past Due and Still Accruing
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No definition available.
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| - Definition
Financing Receivable, Recorded Investment, Past Due And Non-Accrual
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No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
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| - Definition
Financing receivables that are current.
+ References
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| - Definition
Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded Investment in financing receivables that are past due at the balance sheet date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
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