| **Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) (USD $)In Thousands, unless otherwise specified** | Jun. 30, 2014 | Dec. 31, 2013 |
|------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |          |          |
| Current                                                                                                                            | $ 3,648,461   | $ 3,549,125   |
| 30-59 Days Past Due                                                                                                                | 14,772        | 16,864        |
| 60-89 Days Past Due                                                                                                                | 7,076         | 8,668         |
| Loans 90 Days And Accruing                                                                                                         | 1,135         | 1,350         |
| Non-Accrual                                                                                                                        | 51,289        | 56,402        |
| Total Past Due & Non-Accrual                                                                                                   | 74,272        | 83,284        |
| Total                                                                                                                              | 3,722,733     | 3,632,409     |
| Commercial and industrial loans                                                                                                    |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |          |          |
| Current                                                                                                                            | 842,996       | 749,020       |
| 30-59 Days Past Due                                                                                                                | 5,403         | 2,628         |
| 60-89 Days Past Due                                                                                                                | 2,045         | 774           |
| Non-Accrual                                                                                                                        | 7,400         | 9,283         |
| Total Past Due & Non-Accrual                                                                                                   | 14,848        | 12,685        |
| Total                                                                                                                              | 857,844       | 761,705       |
| Agricultural production financing and other loans to farmers                                                                       |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |          |          |
| Current                                                                                                                            | 101,864       | 114,305       |
| 30-59 Days Past Due                                                                                                                | 380           | 13            |
| Non-Accrual                                                                                                                        | 26            | 30            |
| Total Past Due & Non-Accrual                                                                                                   | 406           | 43            |
| Total                                                                                                                              | 102,270       | 114,348       |
| Real estate loans - Construction                                                                                                   |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |          |          |
| Current                                                                                                                            | 161,867       | 171,046       |
| 30-59 Days Past Due                                                                                                                | 473           | 1,058         |
| Loans 90 Days And Accruing                                                                                                         | 134           |          |
| Non-Accrual                                                                                                                        | 2,914         | 4,978         |
| Total Past Due & Non-Accrual                                                                                                   | 3,521         | 6,036         |
| Total                                                                                                                              | 165,388       | 177,082       |
| Real estate loans - Commercial and farm land                                                                                       |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |          |          |
| Current                                                                                                                            | 1,589,583     | 1,573,403     |
| 30-59 Days Past Due                                                                                                                | 3,968         | 3,807         |
| 60-89 Days Past Due                                                                                                                | 2,792         | 5,801         |
| Loans 90 Days And Accruing                                                                                                         | 90            | 703           |
| Non-Accrual                                                                                                                        | 25,003        | 28,095        |
| Total Past Due & Non-Accrual                                                                                                   | 31,853        | 38,406        |
| Total                                                                                                                              | 1,621,436     | 1,611,809     |
| Real estate loans - Residential                                                                                                    |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |          |          |
| Current                                                                                                                            | 610,191       | 595,192       |
| 30-59 Days Past Due                                                                                                                | 3,392         | 7,156         |
| 60-89 Days Past Due                                                                                                                | 1,449         | 1,475         |
| Loans 90 Days And Accruing                                                                                                         | 787           | 494           |
| Non-Accrual                                                                                                                        | 13,343        | 12,068        |
| Total Past Due & Non-Accrual                                                                                                   | 18,971        | 21,193        |
| Total                                                                                                                              | 629,162       | 616,385       |
| Real estate loans - Home equity                                                                                                    |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |          |          |
| Current                                                                                                                            | 257,791       | 251,188       |
| 30-59 Days Past Due                                                                                                                | 733           | 1,652         |
| 60-89 Days Past Due                                                                                                                | 736           | 563           |
| Loans 90 Days And Accruing                                                                                                         | 118           | 153           |
| Non-Accrual                                                                                                                        | 2,433         | 1,667         |
| Total Past Due & Non-Accrual                                                                                                   | 4,020         | 4,035         |
| Total                                                                                                                              | 261,811       | 255,223       |
| Individuals' loans for household and other personal expenditures                                                                   |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |          |          |
| Current                                                                                                                            | 60,880        | 69,061        |
| 30-59 Days Past Due                                                                                                                | 423           | 550           |
| 60-89 Days Past Due                                                                                                                | 54            | 55            |
| Loans 90 Days And Accruing                                                                                                         | 6             |          |
| Non-Accrual                                                                                                                        | 170           | 117           |
| Total Past Due & Non-Accrual                                                                                                   | 653           | 722           |
| Total                                                                                                                              | 61,533        | 69,783        |
| Lease financing receivables, net of unearned income                                                                                |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |          |          |
| Current                                                                                                                            | 1,231         | 1,545         |
| Total                                                                                                                              | 1,231         | 1,545         |
| Other loans                                                                                                                        |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |          |          |
| Current                                                                                                                            | 22,058        | 24,365        |
| 30-59 Days Past Due                                                                                                                |     |          |
| Non-Accrual                                                                                                                        |     | 164           |
| Total Past Due & Non-Accrual                                                                                                   |     | 164           |
| Total                                                                                                                              | $ 22,058      | $ 24,529      |

| X                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Recorded Investment, More Than Ninety Days Past Due and Still Accruing
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing receivables that are less than 60 days past due but more than 29 days past due.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953401-111524
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing receivables that are less than 90 days past due but more than 59 days past due.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing receivables that are current.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
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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