| **Loans and Allowance - Composition of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands** | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 |
|----------------------------------------------------------------------------------------------------------|---------------|---------------|---------------|---------------|---------------|---------------|
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                         |          |          |          |          |          |          |
| Loans                                                                                                    | $ 4,791,429   |          | $ 4,693,822   |          |          |          |
| Allowance for loan losses                                                                                | (62,186)      | $ (62,086)    | (62,453)      | $ (62,550)    | $ (62,801)    | $ (63,964)    |
| Net Loans                                                                                                | 4,729,243     |          | 4,631,369     |          |          |          |
| Commercial and industrial loans                                                                          |          |          |          |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                         |          |          |          |          |          |          |
| Loans                                                                                                    | 1,084,890     |          | 1,057,075     |          |          |          |
| Agricultural production financing and other loans to farmers                                             |          |          |          |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                         |          |          |          |          |          |          |
| Loans                                                                                                    | 95,131        |          | 97,711        |          |          |          |
| Real estate loans - Construction                                                                         |          |          |          |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                         |          |          |          |          |          |          |
| Loans                                                                                                    | 352,980       |          | 366,704       |          |          |          |
| Real estate loans - Commercial and farmland                                                              |          |          |          |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                         |          |          |          |          |          |          |
| Loans                                                                                                    | 1,869,703     |          | 1,802,921     |          |          |          |
| Real estate loans - Residential                                                                          |          |          |          |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                         |          |          |          |          |          |          |
| Loans                                                                                                    | 758,870       |          | 786,105       |          |          |          |
| Allowance for loan losses                                                                                | (10,899)      | (10,856)      | (11,139)      | (12,311)      | (13,269)      | (13,152)      |
| Real estate loans - Home Equity                                                                          |          |          |          |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                         |          |          |          |          |          |          |
| Loans                                                                                                    | 374,159       |          | 348,613       |          |          |          |
| Individuals' loans for household and other personal expenditures                                         |          |          |          |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                         |          |          |          |          |          |          |
| Loans                                                                                                    | 75,205        |          | 74,717        |          |          |          |
| Allowance for loan losses                                                                                | (2,684)       | (2,647)       | (2,689)       | (2,927)       | (3,138)       | (2,658)       |
| Lease financing receivables, net of unearned income                                                      |          |          |          |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                         |          |          |          |          |          |          |
| Loans                                                                                                    | 388           |          | 588           |          |          |          |
| Allowance for loan losses                                                                                | (2)           | $ (2)         | (2)           | $ (5)         | $ (4)         | $ (3)         |
| Other commercial loans                                                                                   |          |          |          |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                         |          |          |          |          |          |          |
| Loans                                                                                                    | $ 180,103     |          | $ 159,388     |          |          |          |

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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.
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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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| - Definition
Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.
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