| **Loans and Allowance - Composition of Impaired Loans by Loan Class (Detail) (USD $)In Thousands, unless otherwise specified** | 12 Months Ended |          |
|--------------------------------------------------------------------------------------------------------------------------------|-----------------|----------|
||
| **Financing Receivable, Impaired [Line Items]**                                                                                |            |     |   |
| Unpaid Principal Balance                                                                                                       | $ 109,594       | $ 98,922 | $ 120,104 |
| Recorded Investment                                                                                                            | 67,333          | 59,835   | 76,317 |
| Average Recorded Investment                                                                                                    | 76,517          | 69,330   |   |
| Interest Income Recognized                                                                                                     | 1,498           | 1,241    |   |
| Unpaid Principal Balance                                                                                                       | 12,918          | 24,234   | 42,488 |
| Recorded Investment                                                                                                            | 11,846          | 19,940   | 39,887 |
| Related Allowance                                                                                                              | 4,243           | 7,596    | 13,914 |
| Average Recorded Investment                                                                                                    | 12,097          | 21,462   |   |
| Interest Income Recognized                                                                                                     | 203             | 321      |   |
| Impaired Financing Receivable, Unpaid Principal Balance                                                                        | 122,512         | 123,156  | 162,592 |
| Impaired Financing Receivable, Recorded Investment                                                                             | 79,179          | 79,775   | 116,204 |
| Average Recorded Investment                                                                                                    | 88,614          | 90,792   |   |
| Interest Income Recognized                                                                                                     | 1,701           | 1,562    |   |
| Commercial and industrial                                                                                                      |            |     |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |            |     |   |
| Unpaid Principal Balance                                                                                                       | 28,532          | 23,364   | 30,006 |
| Recorded Investment                                                                                                            | 11,730          | 10,116   | 16,572 |
| Average Recorded Investment                                                                                                    | 15,089          | 13,399   |   |
| Interest Income Recognized                                                                                                     | 124             | 615      |   |
| Unpaid Principal Balance                                                                                                       | 4,415           | 8,691    | 11,477 |
| Recorded Investment                                                                                                            | 4,155           | 8,104    | 11,374 |
| Related Allowance                                                                                                              | 1,628           | 4,142    | 5,250 |
| Average Recorded Investment                                                                                                    | 4,225           | 8,196    |   |
| Interest Income Recognized                                                                                                     | 33              | 174      |   |
| Agriculture production financing and other loans                                                                               |            |     |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |            |     |   |
| Unpaid Principal Balance                                                                                                       |            |     | 966 |
| Recorded Investment                                                                                                            |            |     | 530 |
| Construction                                                                                                                   |            |     |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |            |     |   |
| Unpaid Principal Balance                                                                                                       | 9,787           | 14,301   | 12,598 |
| Recorded Investment                                                                                                            | 5,164           | 7,701    | 9,150 |
| Average Recorded Investment                                                                                                    | 6,471           | 8,836    |   |
| Interest Income Recognized                                                                                                     | 66              |     |   |
| Unpaid Principal Balance                                                                                                       | 1,202           | 961      | 9,353 |
| Recorded Investment                                                                                                            | 1,058           | 961      | 7,824 |
| Related Allowance                                                                                                              | 105             | 321      | 2,049 |
| Average Recorded Investment                                                                                                    | 1,175           | 961      |   |
| Commercial and farm land                                                                                                       |            |     |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |            |     |   |
| Unpaid Principal Balance                                                                                                       | 58,173          | 49,242   | 64,064 |
| Recorded Investment                                                                                                            | 43,204          | 34,571   | 43,653 |
| Average Recorded Investment                                                                                                    | 46,788          | 39,032   |   |
| Interest Income Recognized                                                                                                     | 1,211           | 591      |   |
| Unpaid Principal Balance                                                                                                       | 5,579           | 12,115   | 17,984 |
| Recorded Investment                                                                                                            | 5,182           | 8,748    | 17,076 |
| Related Allowance                                                                                                              | 2,460           | 2,183    | 5,496 |
| Average Recorded Investment                                                                                                    | 5,239           | 10,028   |   |
| Interest Income Recognized                                                                                                     | 95              | 140      |   |
| Residential                                                                                                                    |            |     |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |            |     |   |
| Unpaid Principal Balance                                                                                                       | 8,820           | 7,491    | 7,909 |
| Recorded Investment                                                                                                            | 6,215           | 6,185    | 5,153 |
| Average Recorded Investment                                                                                                    | 7,129           | 6,539    |   |
| Interest Income Recognized                                                                                                     | 83              | 20       |   |
| Unpaid Principal Balance                                                                                                       | 1,722           | 1,888    | 2,740 |
| Recorded Investment                                                                                                            | 1,451           | 1,575    | 2,691 |
| Related Allowance                                                                                                              | 50              | 391      | 465 |
| Average Recorded Investment                                                                                                    | 1,458           | 1,687    |   |
| Interest Income Recognized                                                                                                     | 75              | 7        |   |
| Home Equity                                                                                                                    |            |     |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |            |     |   |
| Unpaid Principal Balance                                                                                                       | 4,199           | 4,425    | 4,460 |
| Recorded Investment                                                                                                            | 1,006           | 1,241    | 1,245 |
| Average Recorded Investment                                                                                                    | 1,022           | 1,500    |   |
| Interest Income Recognized                                                                                                     | 13              | 15       |   |
| Unpaid Principal Balance                                                                                                       |            |     | 458 |
| Recorded Investment                                                                                                            |            |     | 446 |
| Related Allowance                                                                                                              |            |     | 178 |
| Other Loans                                                                                                                    |            |     |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |            |     |   |
| Unpaid Principal Balance                                                                                                       | 83              | 99       | 101 |
| Recorded Investment                                                                                                            | 14              | 21       | 14 |
| Average Recorded Investment                                                                                                    | 18              | 24       |   |
| Interest Income Recognized                                                                                                     | 1               |     |   |
| Unpaid Principal Balance                                                                                                       |            | 579      | 476 |
| Recorded Investment                                                                                                            |            | 552      | 476 |
| Related Allowance                                                                                                              |            | 559      | 476 |
| Average Recorded Investment                                                                                                    |            | $ 590    |   |

| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableImpairedLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5212-111524Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5258-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7512638&loc=SL6953550-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of allowance for credit losses related to recorded investment.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableRelatedAllowance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5212-111524Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5258-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of interest income, recognized on accrual basis, on financing receivables that are recognized as impaired, for which there is no related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7512638&loc=SL6953550-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5212-111524Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5258-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7512638&loc=SL6953550-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |