| **Credit Quality of Loan Portfolio by Loan Class (Detail) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2013 | Dec. 31, 2012 |
|-------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | $ 2,879,542   | $ 2,902,209   |
| Commercial and industrial                                                                                   |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 637,952       | 622,579       |
| Agriculture production financing and other loans                                                            |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 104,284       | 112,527       |
| Construction                                                                                                |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 75,877        | 98,639        |
| Commercial and farm land                                                                                    |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 1,280,611     | 1,266,682     |
| Residential                                                                                                 |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 467,629       | 473,537       |
| Home Equity                                                                                                 |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 201,767       | 203,406       |
| Consumer                                                                                                    |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 73,314        | 75,748        |
| Finance Lease                                                                                               |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 2,013         | 2,590         |
| Other Loans                                                                                                 |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 36,095        | 46,501        |
| Commercial Pass                                                                                             |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 2,109,099     | 2,107,317     |
| Commercial Pass | Commercial and industrial                                                                 |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 588,140       | 559,852       |
| Commercial Pass | Agriculture production financing and other loans                                          |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 103,894       | 112,209       |
| Commercial Pass | Construction                                                                              |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 64,904        | 85,728        |
| Commercial Pass | Commercial and farm land                                                                  |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 1,157,933     | 1,148,561     |
| Commercial Pass | Residential                                                                               |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 145,949       | 145,402       |
| Commercial Pass | Home Equity                                                                               |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 10,334        | 9,092         |
| Commercial Pass | Finance Lease                                                                             |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 1,866         |          |
| Commercial Pass | Other Loans                                                                               |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 36,079        | 46,473        |
| Commercial Special Mention                                                                                  |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 71,550        | 69,815        |
| Commercial Special Mention | Commercial and industrial                                                      |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 17,592        | 23,678        |
| Commercial Special Mention | Agriculture production financing and other loans                               |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 217           | 224           |
| Commercial Special Mention | Construction                                                                   |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 1,372         | 1,384         |
| Commercial Special Mention | Commercial and farm land                                                       |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 48,235        | 38,199        |
| Commercial Special Mention | Residential                                                                    |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 3,480         | 5,437         |
| Commercial Special Mention | Home Equity                                                                    |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 638           | 893           |
| Commercial Special Mention | Other Loans                                                                    |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 16            |     |
| Commercial Substandard                                                                                      |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 129,102       | 140,553       |
| Commercial Substandard | Commercial and industrial                                                          |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 30,027        | 34,460        |
| Commercial Substandard | Agriculture production financing and other loans                                   |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 173           | 94            |
| Commercial Substandard | Construction                                                                       |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 9,091         | 11,356        |
| Commercial Substandard | Commercial and farm land                                                           |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 73,845        | 79,078        |
| Commercial Substandard | Residential                                                                        |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 14,579        | 13,880        |
| Commercial Substandard | Home Equity                                                                        |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 1,240         | 1,657         |
| Commercial Substandard | Finance Lease                                                                      |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 147           |          |
| Commercial Substandard | Other Loans                                                                        |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       |     | 28            |
| Commercial Doubtful                                                                                         |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 2,687         | 6,064         |
| Commercial Doubtful | Commercial and industrial                                                             |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 2,193         | 4,589         |
| Commercial Doubtful | Commercial and farm land                                                              |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 350           | 553           |
| Commercial Doubtful | Residential                                                                           |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 144           | 922           |
| Commercial Doubtful | Other Loans                                                                           |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       |          |     |
| Consumer Performing                                                                                         |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 557,793       | 569,372       |
| Consumer Performing | Construction                                                                          |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       |          |     |
| Consumer Performing | Commercial and farm land                                                              |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       |          |     |
| Consumer Performing | Residential                                                                           |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 296,613       | 301,614       |
| Consumer Performing | Home Equity                                                                           |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 187,877       | 189,721       |
| Consumer Performing | Consumer                                                                              |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 73,303        | 75,748        |
| Consumer Performing | Finance Lease                                                                         |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       |     | 2,289         |
| Consumer Non-Performing                                                                                     |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 9,311         | 9,088         |
| Consumer Non-Performing | Construction                                                                      |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 510           | 171           |
| Consumer Non-Performing | Commercial and farm land                                                          |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 248           | 291           |
| Consumer Non-Performing | Residential                                                                       |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 6,864         | 6,282         |
| Consumer Non-Performing | Home Equity                                                                       |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 1,678         | 2,043         |
| Consumer Non-Performing | Consumer                                                                          |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 11            |     |
| Consumer Non-Performing | Finance Lease                                                                     |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       |     | $ 301         |

| X                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6375948&loc=d3e4428-111522Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 01-6 -Paragraph 13 -Subparagraph e -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.
+ Details**Name:**us-gaap_LoansAndLeasesReceivableGrossCarryingAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |