| **Composition of Loan Portfolio by Loan Class (Detail) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2012 | Dec. 31, 2011 |
|----------------------------------------------------------------------------------------------------------|---------------|---------------|---------------|---------------|
| **Loans:**                                                                                               |          |          |          |          |
| Loans                                                                                                    | $ 2,879,542   | $ 2,902,209   |          |          |
| Allowance for loan losses                                                                                | (68,537)      | (69,366)      | (70,369)      | (70,898)      |
| Total Loans                                                                                              | 2,811,005     | 2,832,843     |          |          |
| Commercial and industrial                                                                                |          |          |          |          |
| **Loans:**                                                                                               |          |          |          |          |
| Loans                                                                                                    | 637,952       | 622,579       |          |          |
| Agriculture production financing and other loans                                                         |          |          |          |          |
| **Loans:**                                                                                               |          |          |          |          |
| Loans                                                                                                    | 104,284       | 112,527       |          |          |
| Construction                                                                                             |          |          |          |          |
| **Loans:**                                                                                               |          |          |          |          |
| Loans                                                                                                    | 75,877        | 98,639        |          |          |
| Commercial and farm land                                                                                 |          |          |          |          |
| **Loans:**                                                                                               |          |          |          |          |
| Loans                                                                                                    | 1,280,611     | 1,266,682     |          |          |
| Residential                                                                                              |          |          |          |          |
| **Loans:**                                                                                               |          |          |          |          |
| Loans                                                                                                    | 467,629       | 473,537       |          |          |
| Allowance for loan losses                                                                                | (15,479)      | (14,157)      | (14,083)      | (12,343)      |
| Home Equity                                                                                              |          |          |          |          |
| **Loans:**                                                                                               |          |          |          |          |
| Loans                                                                                                    | 201,767       | 203,406       |          |          |
| Consumer                                                                                                 |          |          |          |          |
| **Loans:**                                                                                               |          |          |          |          |
| Loans                                                                                                    | 73,314        | 75,748        |          |          |
| Allowance for loan losses                                                                                | (2,689)       | (2,593)       | (2,805)       | (2,902)       |
| Finance Lease                                                                                            |          |          |          |          |
| **Loans:**                                                                                               |          |          |          |          |
| Loans                                                                                                    | 2,013         | 2,590         |          |          |
| Allowance for loan losses                                                                                | (20)          |          | 0             | (3)           |
| Other Loans                                                                                              |          |          |          |          |
| **Loans:**                                                                                               |          |          |          |          |
| Loans                                                                                                    | $ 36,095      | $ 46,501      |          |          |

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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.
+ References
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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.
+ References
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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.
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