| **Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended | 6 Months Ended |           |
|----------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|-----------|
||
| **Allowance for Doubtful Accounts Receivable [Roll Forward]**                                                                    |           |           |      |   |   |
| Beginning Balance                                                                                                                | $ 54,506       | $ 62,262       | $ 54,225  | $ 67,187 |   |
| Charge-offs                                                                                                                      | (8,103)        | (16,136)       | (11,749)  | (26,612) |   |
| Recoveries                                                                                                                       | 1,005          | 526            | 1,701     | 1,580 |   |
| Provision for Loan, Lease, and Other Losses                                                                                      | 3,317          | 10,800         | 6,548     | 15,297 |   |
| Ending Balance                                                                                                                   | 50,725         | 57,452         | 50,725    | 57,452 |   |
| Ending balance: individually evaluated for impaired                                                                              | 6,072          | 9,655          | 6,072     | 9,655 |   |
| Ending balance: collectively evaluated for impaired                                                                              | 44,653         | 47,797         | 44,653    | 47,797 |   |
| Loans and Leases Receivable, Gross                                                                                               | 4,334,214      |           | 4,334,214 |   | 4,283,833 |
| Total                                                                                                                            | 4,334,214      | 4,229,752      | 4,334,214 | 4,229,752 | 4,283,833 |
| Ending balance: individually evaluated for impaired                                                                              | 39,648         | 66,826         | 39,648    | 66,826 |   |
| Ending balance: collectively evaluated for impaired                                                                              | 4,294,566      | 4,162,926      | 4,294,566 | 4,162,926 |   |
| Commercial, financial, agricultural and other [Member]                                                                           |           |           |      |   |   |
| **Allowance for Doubtful Accounts Receivable [Roll Forward]**                                                                    |           |           |      |   |   |
| Beginning Balance                                                                                                                | 26,125         | 20,275         | 22,663    | 19,852 |   |
| Charge-offs                                                                                                                      | (6,258)        | (13,683)       | (7,859)   | (14,221) |   |
| Recoveries                                                                                                                       | 336            | 136            | 421       | 264 |   |
| Provision for Loan, Lease, and Other Losses                                                                                      | 1,753          | 9,847          | 6,731     | 10,680 |   |
| Ending Balance                                                                                                                   | 21,956         | 16,575         | 21,956    | 16,575 |   |
| Ending balance: individually evaluated for impaired                                                                              | 4,995          | 6,627          | 4,995     | 6,627 |   |
| Ending balance: collectively evaluated for impaired                                                                              | 16,961         | 9,948          | 16,961    | 9,948 |   |
| Loans and Leases Receivable, Gross                                                                                               | 1,062,001      |           | 1,062,001 |   | 1,021,056 |
| Total                                                                                                                            |           | 1,012,315      |      | 1,012,315 |   |
| Ending balance: individually evaluated for impaired                                                                              | 24,573         | 24,592         | 24,573    | 24,592 |   |
| Ending balance: collectively evaluated for impaired                                                                              | 1,037,428      | 987,723        | 1,037,428 | 987,723 |   |
| Real estate construction [Member]                                                                                                |           |           |      |   |   |
| **Allowance for Doubtful Accounts Receivable [Roll Forward]**                                                                    |           |           |      |   |   |
| Beginning Balance                                                                                                                | 6,214          | 7,733          | 6,600     | 8,928 |   |
| Charge-offs                                                                                                                      | (296)          | (671)          | (296)     | (755) |   |
| Recoveries                                                                                                                       | 168            | 47             | 337       | 59 |   |
| Provision for Loan, Lease, and Other Losses                                                                                      | (187)          | 1,137          | (742)     | 14 |   |
| Ending Balance                                                                                                                   | 5,899          | 8,246          | 5,899     | 8,246 |   |
| Ending balance: individually evaluated for impaired                                                                              | 0              | 267            | 0         | 267 |   |
| Ending balance: collectively evaluated for impaired                                                                              | 5,899          | 7,979          | 5,899     | 7,979 |   |
| Loans and Leases Receivable, Gross                                                                                               | 100,709        |           | 100,709   |   | 93,289 |
| Total                                                                                                                            |           | 66,243         |      | 66,243 |   |
| Ending balance: individually evaluated for impaired                                                                              | 486            | 3,904          | 486       | 3,904 |   |
| Ending balance: collectively evaluated for impaired                                                                              | 100,223        | 62,339         | 100,223   | 62,339 |   |
| Residential real estate [Member]                                                                                                 |           |           |      |   |   |
| **Allowance for Doubtful Accounts Receivable [Roll Forward]**                                                                    |           |           |      |   |   |
| Beginning Balance                                                                                                                | 6,026          | 5,749          | 7,727     | 5,908 |   |
| Charge-offs                                                                                                                      | (640)          | (321)          | (1,735)   | (643) |   |
| Recoveries                                                                                                                       | 79             | 89             | 323       | 812 |   |
| Provision for Loan, Lease, and Other Losses                                                                                      | 660            | 922            | (190)     | 362 |   |
| Ending Balance                                                                                                                   | 6,125          | 6,439          | 6,125     | 6,439 |   |
| Ending balance: individually evaluated for impaired                                                                              | 546            | 1,943          | 546       | 1,943 |   |
| Ending balance: collectively evaluated for impaired                                                                              | 5,579          | 4,496          | 5,579     | 4,496 |   |
| Loans and Leases Receivable, Gross                                                                                               | 1,238,791      |           | 1,238,791 |   | 1,262,718 |
| Total                                                                                                                            |           | 1,269,830      |      | 1,269,830 |   |
| Ending balance: individually evaluated for impaired                                                                              | 7,624          | 10,468         | 7,624     | 10,468 |   |
| Ending balance: collectively evaluated for impaired                                                                              | 1,231,167      | 1,259,362      | 1,231,167 | 1,259,362 |   |
| Commercial Real Estate [Member]                                                                                                  |           |           |      |   |   |
| **Allowance for Doubtful Accounts Receivable [Roll Forward]**                                                                    |           |           |      |   |   |
| Beginning Balance                                                                                                                | 11,119         | 18,470         | 11,778    | 22,441 |   |
| Charge-offs                                                                                                                      | (157)          | (694)          | (297)     | (9,238) |   |
| Recoveries                                                                                                                       | 235            | 11             | 255       | 108 |   |
| Provision for Loan, Lease, and Other Losses                                                                                      | 464            | (1,659)        | (75)      | 2,817 |   |
| Ending Balance                                                                                                                   | 11,661         | 16,128         | 11,661    | 16,128 |   |
| Ending balance: individually evaluated for impaired                                                                              | 531            | 818            | 531       | 818 |   |
| Ending balance: collectively evaluated for impaired                                                                              | 11,130         | 15,310         | 11,130    | 15,310 |   |
| Loans and Leases Receivable, Gross                                                                                               | 1,306,752      |           | 1,306,752 |   | 1,296,472 |
| Total                                                                                                                            |           | 1,280,784      |      | 1,280,784 |   |
| Ending balance: individually evaluated for impaired                                                                              | 6,965          | 27,862         | 6,965     | 27,862 |   |
| Ending balance: collectively evaluated for impaired                                                                              | 1,299,787      | 1,252,922      | 1,299,787 | 1,252,922 |   |
| Loans to individuals [Member]                                                                                                    |           |           |      |   |   |
| **Allowance for Doubtful Accounts Receivable [Roll Forward]**                                                                    |           |           |      |   |   |
| Beginning Balance                                                                                                                | 5,022          | 4,139          | 5,457     | 4,132 |   |
| Charge-offs                                                                                                                      | (752)          | (767)          | (1,562)   | (1,755) |   |
| Recoveries                                                                                                                       | 187            | 243            | 365       | 337 |   |
| Provision for Loan, Lease, and Other Losses                                                                                      | 627            | 527            | 824       | 1,428 |   |
| Ending Balance                                                                                                                   | 5,084          | 4,142          | 5,084     | 4,142 |   |
| Ending balance: individually evaluated for impaired                                                                              | 0              | 0              | 0         | 0 |   |
| Ending balance: collectively evaluated for impaired                                                                              | 5,084          | 4,142          | 5,084     | 4,142 |   |
| Loans and Leases Receivable, Gross                                                                                               | 625,961        |           | 625,961   |   | 610,298 |
| Total                                                                                                                            |           | 600,580        |      | 600,580 |   |
| Ending balance: individually evaluated for impaired                                                                              | 0              | 0              | 0         | 0 |   |
| Ending balance: collectively evaluated for impaired                                                                              | 625,961        | 600,580        | 625,961   | 600,580 |   |
| Unallocated [Member]                                                                                                             |           |           |      |   |   |
| **Allowance for Doubtful Accounts Receivable [Roll Forward]**                                                                    |           |           |      |   |   |
| Beginning Balance                                                                                                                | 0              | 5,896          | 0         | 5,926 |   |
| Charge-offs                                                                                                                      | 0              | 0              | 0         | 0 |   |
| Recoveries                                                                                                                       | 0              | 0              | 0         | 0 |   |
| Provision for Loan, Lease, and Other Losses                                                                                      | 0              | 26             | 0         | (4) |   |
| Ending Balance                                                                                                                   | 0              | 5,922          | 0         | 5,922 |   |
| Ending balance: individually evaluated for impaired                                                                              | 0              | 0              | 0         | 0 |   |
| Ending balance: collectively evaluated for impaired                                                                              | $ 0            | $ 5,922        | $ 0       | $ 5,922 |   |

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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
A valuation allowance for financing receivables that are expected to be uncollectible.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of recovery of financing receivables doubtful of collection that were previously charged off.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of direct write-downs of financing receivables charged against the allowance.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The balance of financing receivables that were collectively evaluated for impairment.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The balance of financing receivables that were individually evaluated for impairment.
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| 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.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.
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