| **Carrying Amount of Loans Included in Balance Sheet Amounts of Loans Receivable (Detail) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2012 |
|---------------------------------------------------------------------------------------------------------------------------------------------|---------------|
| **Business Acquisition [Line Items]**                                                                                                       |          |
| Carrying amount of loans acquired                                                                                                           | $ 65,600      |
| Commercial and industrial                                                                                                                   |          |
| **Business Acquisition [Line Items]**                                                                                                       |          |
| Carrying amount of loans acquired                                                                                                           | 8,542         |
| Agriculture production financing and other loans                                                                                            |          |
| **Business Acquisition [Line Items]**                                                                                                       |          |
| Carrying amount of loans acquired                                                                                                           | 1,127         |
| Construction                                                                                                                                |          |
| **Business Acquisition [Line Items]**                                                                                                       |          |
| Carrying amount of loans acquired                                                                                                           | 58            |
| Commercial and farm land                                                                                                                    |          |
| **Business Acquisition [Line Items]**                                                                                                       |          |
| Carrying amount of loans acquired                                                                                                           | 24,259        |
| Residential                                                                                                                                 |          |
| **Business Acquisition [Line Items]**                                                                                                       |          |
| Carrying amount of loans acquired                                                                                                           | 12,118        |
| Home Equity                                                                                                                                 |          |
| **Business Acquisition [Line Items]**                                                                                                       |          |
| Carrying amount of loans acquired                                                                                                           | 18,805        |
| Consumer                                                                                                                                    |          |
| **Business Acquisition [Line Items]**                                                                                                       |          |
| Carrying amount of loans acquired                                                                                                           | $ 691         |

| X                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_BusinessAcquisitionLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The recorded balance, net of any applicable reductions (for example, unamortized accretable yield and allowance for loan losses).  This relates to loans not accounted for debt securities with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 15 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=7514592&loc=d3e13503-111538Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 03-3 -Paragraph 16 -Subparagraph a(1) -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.
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