| **Loans and Allowance for Credit Losses - Recorded Investment and Unpaid Principal Balance for Impaired Loans with Associated Allowance (Detail) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended | 9 Months Ended |          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|----------|
||
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                                    |           |           |     |   |   |
| Impaired Financing Receivable, with No Related Allowance, Recorded Investment                                                                                                                      | $ 43,905       |           | $ 43,905 |   | $ 58,014 |
| Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance                                                                                                                 | 50,433         |           | 50,433   |   | 88,596 |
| Recorded investment                                                                                                                                                                                | 71,219         |           | 71,219   |   | 107,555 |
| Unpaid principal balance                                                                                                                                                                           | 82,042         |           | 82,042   |   | 139,687 |
| Related allowance                                                                                                                                                                                  | 8,239          |           | 8,239    |   | 17,778 |
| Impaired Financing Receivable, with Related Allowance, Recorded Investment                                                                                                                         | 27,314         |           | 27,314   |   | 49,541 |
| Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance                                                                                                                    | 31,609         |           | 31,609   |   | 51,091 |
| Average recorded investment                                                                                                                                                                        | 20,631         | 41,201         | 26,029   | 32,651 |   |
| Impaired Financing Receivable, with Related Allowance, Interest Income, Accrual Method                                                                                                             | 35             | 19             | 106      | 36 |   |
| Impaired Financing Receivable, Average Recorded Investment                                                                                                                                         | 70,627         | 87,099         | 81,915   | 84,055 |   |
| Interest Income Recognized                                                                                                                                                                         | 171            | 133            | 504      | 219 |   |
| Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment                                                                                                              | 49,996         | 45,898         | 55,886   | 51,404 |   |
| Impaired Financing Receivable, with No Related Allowance, Interest Income, Accrual Method                                                                                                          | 136            | 114            | 398      | 183 |   |
| Commercial, financial, agricultural and other [Member]                                                                                                                                             |           |           |     |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                                    |           |           |     |   |   |
| Impaired Financing Receivable, with No Related Allowance, Recorded Investment                                                                                                                      | 10,209         |           | 10,209   |   | 8,080 |
| Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance                                                                                                                 | 11,137         |           | 11,137   |   | 8,983 |
| Related allowance                                                                                                                                                                                  | 4,889          |           | 4,889    |   | 10,331 |
| Impaired Financing Receivable, with Related Allowance, Recorded Investment                                                                                                                         | 16,530         |           | 16,530   |   | 26,532 |
| Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance                                                                                                                    | 17,064         |           | 17,064   |   | 27,412 |
| Average recorded investment                                                                                                                                                                        | 10,370         | 25,469         | 15,552   | 21,025 |   |
| Impaired Financing Receivable, with Related Allowance, Interest Income, Accrual Method                                                                                                             | 16             | 2              | 46       | 5 |   |
| Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment                                                                                                              | 14,000         | 6,664          | 14,482   | 9,281 |   |
| Impaired Financing Receivable, with No Related Allowance, Interest Income, Accrual Method                                                                                                          | 41             | 94             | 147      | 116 |   |
| Real estate construction [Member]                                                                                                                                                                  |           |           |     |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                                    |           |           |     |   |   |
| Impaired Financing Receivable, with No Related Allowance, Recorded Investment                                                                                                                      | 2,251          |           | 2,251    |   | 8,491 |
| Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance                                                                                                                 | 5,795          |           | 5,795    |   | 35,555 |
| Related allowance                                                                                                                                                                                  | 165            |           | 165      |   | 300 |
| Impaired Financing Receivable, with Related Allowance, Recorded Investment                                                                                                                         | 1,866          |           | 1,866    |   | 2,756 |
| Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance                                                                                                                    | 2,181          |           | 2,181    |   | 3,087 |
| Average recorded investment                                                                                                                                                                        | 1,541          | 8,848          | 1,848    | 7,381 |   |
| Impaired Financing Receivable, with Related Allowance, Interest Income, Accrual Method                                                                                                             | 8              | 0              | 35       | 0 |   |
| Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment                                                                                                              | 2,340          | 4,954          | 5,390    | 6,641 |   |
| Impaired Financing Receivable, with No Related Allowance, Interest Income, Accrual Method                                                                                                          | 0              | 0              | 0        | 0 |   |
| Loans to individuals [Member]                                                                                                                                                                      |           |           |     |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                                    |           |           |     |   |   |
| Impaired Financing Receivable, with No Related Allowance, Recorded Investment                                                                                                                      | 257            |           | 257      |   | 256 |
| Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance                                                                                                                 | 272            |           | 272      |   | 256 |
| Related allowance                                                                                                                                                                                  | 0              |           | 0        |   | 0 |
| Impaired Financing Receivable, with Related Allowance, Recorded Investment                                                                                                                         | 0              |           | 0        |   | 0 |
| Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance                                                                                                                    | 0              |           | 0        |   | 0 |
| Average recorded investment                                                                                                                                                                        | 0              | 0              | 0        | 0 |   |
| Impaired Financing Receivable, with Related Allowance, Interest Income, Accrual Method                                                                                                             | 0              | 0              | 0        | 0 |   |
| Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment                                                                                                              | 251            | 28             | 248      | 9 |   |
| Impaired Financing Receivable, with No Related Allowance, Interest Income, Accrual Method                                                                                                          | 1              | 0              | 3        | 0 |   |
| Residential real estate [Member]                                                                                                                                                                   |           |           |     |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                                    |           |           |     |   |   |
| Impaired Financing Receivable, with No Related Allowance, Recorded Investment                                                                                                                      | 9,493          |           | 9,493    |   | 7,928 |
| Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance                                                                                                                 | 10,118         |           | 10,118   |   | 8,401 |
| Related allowance                                                                                                                                                                                  | 1,377          |           | 1,377    |   | 780 |
| Impaired Financing Receivable, with Related Allowance, Recorded Investment                                                                                                                         | 3,406          |           | 3,406    |   | 2,695 |
| Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance                                                                                                                    | 3,440          |           | 3,440    |   | 2,696 |
| Average recorded investment                                                                                                                                                                        | 3,349          | 2,130          | 3,001    | 1,532 |   |
| Impaired Financing Receivable, with Related Allowance, Interest Income, Accrual Method                                                                                                             | 10             | 6              | 24       | 20 |   |
| Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment                                                                                                              | 9,977          | 4,374          | 9,376    | 7,604 |   |
| Impaired Financing Receivable, with No Related Allowance, Interest Income, Accrual Method                                                                                                          | 52             | 6              | 129      | 17 |   |
| Commercial Real Estate [Member]                                                                                                                                                                    |           |           |     |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                                    |           |           |     |   |   |
| Impaired Financing Receivable, with No Related Allowance, Recorded Investment                                                                                                                      | 21,695         |           | 21,695   |   | 33,259 |
| Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance                                                                                                                 | 23,111         |           | 23,111   |   | 35,401 |
| Related allowance                                                                                                                                                                                  | 1,808          |           | 1,808    |   | 6,367 |
| Impaired Financing Receivable, with Related Allowance, Recorded Investment                                                                                                                         | 5,512          |           | 5,512    |   | 17,558 |
| Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance                                                                                                                    | 8,924          |           | 8,924    |   | 17,896 |
| Average recorded investment                                                                                                                                                                        | 5,371          | 4,754          | 5,628    | 2,713 |   |
| Impaired Financing Receivable, with Related Allowance, Interest Income, Accrual Method                                                                                                             | 1              | 11             | 1        | 11 |   |
| Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment                                                                                                              | 23,428         | 29,878         | 26,390   | 27,869 |   |
| Impaired Financing Receivable, with No Related Allowance, Interest Income, Accrual Method                                                                                                          | $ 42           | $ 14           | $ 119    | $ 50 |   |

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Amount of allowance for credit losses related to recorded investment.
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Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.
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Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.
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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.
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Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.
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Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.
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Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.
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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.
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Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.
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Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.
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