| **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                                                                                                       | $ 189,779       | $ 109,594 | $ 98,922 |
| Recorded Investment                                                                                                            | 114,253         | 67,333 | 59,835 |
| Average Recorded Investment                                                                                                    | 119,561         | 76,517 | 69,330 |
| Interest Income Recognized                                                                                                     | 2,048           | 1,498 | 1,241 |
| Unpaid Principal Balance                                                                                                       | 6,121           | 12,918 | 24,234 |
| Recorded Investment                                                                                                            | 5,502           | 11,846 | 19,940 |
| Related Allowance                                                                                                              | 1,583           | 4,243 | 7,596 |
| Average Recorded Investment                                                                                                    | 5,607           | 12,097 | 21,462 |
| Interest Income Recognized                                                                                                     | 18              | 203 | 321 |
| Impaired Financing Receivable, Unpaid Principal Balance                                                                        | 195,900         | 122,512 | 123,156 |
| Impaired Financing Receivable, Recorded Investment                                                                             | 119,755         | 79,179 | 79,775 |
| Average Recorded Investment                                                                                                    | 125,168         | 88,614 | 90,792 |
| Interest Income Recognized                                                                                                     | 2,066           | 1,701 | 1,562 |
| Commercial and industrial loans                                                                                                |            |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |            |   |   |
| Unpaid Principal Balance                                                                                                       | 35,066          | 28,532 | 23,364 |
| Recorded Investment                                                                                                            | 16,371          | 11,730 | 10,116 |
| Average Recorded Investment                                                                                                    | 19,209          | 15,089 | 13,399 |
| Interest Income Recognized                                                                                                     | 192             | 124 | 615 |
| Unpaid Principal Balance                                                                                                       | 1,390           | 4,415 | 8,691 |
| Recorded Investment                                                                                                            | 1,216           | 4,155 | 8,104 |
| Related Allowance                                                                                                              | 683             | 1,628 | 4,142 |
| Average Recorded Investment                                                                                                    | 1,240           | 4,225 | 8,196 |
| Interest Income Recognized                                                                                                     | 9               | 33 | 174 |
| Agriculture Production financing and other loans to farmers                                                                    |            |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |            |   |   |
| Unpaid Principal Balance                                                                                                       | 32              |   |   |
| Recorded Investment                                                                                                            | 30              |   |   |
| Average Recorded Investment                                                                                                    | 32              |   |   |
| Construction                                                                                                                   |            |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |            |   |   |
| Unpaid Principal Balance                                                                                                       | 16,109          | 9,787 | 14,301 |
| Recorded Investment                                                                                                            | 10,625          | 5,164 | 7,701 |
| Average Recorded Investment                                                                                                    | 11,621          | 6,471 | 8,836 |
| Interest Income Recognized                                                                                                     | 117             | 66 |   |
| Unpaid Principal Balance                                                                                                       |       | 1,202 | 961 |
| Recorded Investment                                                                                                            |       | 1,058 | 961 |
| Related Allowance                                                                                                              |       | 105 | 321 |
| Average Recorded Investment                                                                                                    |       | 1,175 | 961 |
| Commercial and farmland                                                                                                        |            |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |            |   |   |
| Unpaid Principal Balance                                                                                                       | 128,073         | 58,173 | 49,242 |
| Recorded Investment                                                                                                            | 83,033          | 43,204 | 34,571 |
| Average Recorded Investment                                                                                                    | 84,057          | 46,788 | 39,032 |
| Interest Income Recognized                                                                                                     | 1,663           | 1,211 | 591 |
| Unpaid Principal Balance                                                                                                       | 4,657           | 5,579 | 12,115 |
| Recorded Investment                                                                                                            | 4,215           | 5,182 | 8,748 |
| Related Allowance                                                                                                              | 894             | 2,460 | 2,183 |
| Average Recorded Investment                                                                                                    | 4,291           | 5,239 | 10,028 |
| Interest Income Recognized                                                                                                     | 9               | 95 | 140 |
| Residential                                                                                                                    |            |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |            |   |   |
| Unpaid Principal Balance                                                                                                       | 6,746           | 8,820 | 7,491 |
| Recorded Investment                                                                                                            | 3,910           | 6,215 | 6,185 |
| Average Recorded Investment                                                                                                    | 4,236           | 7,129 | 6,539 |
| Interest Income Recognized                                                                                                     | 75              | 83 | 20 |
| Unpaid Principal Balance                                                                                                       | 74              | 1,722 | 1,888 |
| Recorded Investment                                                                                                            | 71              | 1,451 | 1,575 |
| Related Allowance                                                                                                              | 6               | 50 | 391 |
| Average Recorded Investment                                                                                                    | 76              | 1,458 | 1,687 |
| Interest Income Recognized                                                                                                     |       | 75 | 7 |
| Home Equity                                                                                                                    |            |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |            |   |   |
| Unpaid Principal Balance                                                                                                       | 3,299           | 4,199 | 4,425 |
| Recorded Investment                                                                                                            | 112             | 1,006 | 1,241 |
| Average Recorded Investment                                                                                                    | 225             | 1,022 | 1,500 |
| Interest Income Recognized                                                                                                     |       | 13 | 15 |
| Other loans                                                                                                                    |            |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |            |   |   |
| Unpaid Principal Balance                                                                                                       | 454             | 83 | 99 |
| Recorded Investment                                                                                                            | 172             | 14 | 21 |
| Average Recorded Investment                                                                                                    | 181             | 18 | 24 |
| Interest Income Recognized                                                                                                     | 1               | 1 |   |
| Unpaid Principal Balance                                                                                                       |            |   | 579 |
| Recorded Investment                                                                                                            |            |   | 552 |
| Related Allowance                                                                                                              |            |   | 559 |
| Average Recorded Investment                                                                                                    |            |   | $ 590 |

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