| **Loans and Allowance for Credit Losses - Outstanding Balances of Loan (Detail) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended | 6 Months Ended |           |
|-----------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|-----------|
||
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                  |           |           |      |   |   |
| Charge-offs                                                                                                                       | $ 8,103        | $ 16,136       | $ 11,749  | $ 26,612 |   |
| Loans and Leases Receivable, Gross                                                                                                | 4,334,214      |           | 4,334,214 |   | 4,283,833 |
| Commercial, financial, agricultural and other [Member]                                                                            |           |           |      |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                  |           |           |      |   |   |
| Charge-offs                                                                                                                       | 6,258          | 13,683         | 7,859     | 14,221 |   |
| Loans and Leases Receivable, Gross                                                                                                | 1,062,001      |           | 1,062,001 |   | 1,021,056 |
| Real estate construction [Member]                                                                                                 |           |           |      |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                  |           |           |      |   |   |
| Charge-offs                                                                                                                       | 296            | 671            | 296       | 755 |   |
| Loans and Leases Receivable, Gross                                                                                                | 100,709        |           | 100,709   |   | 93,289 |
| Residential real estate [Member]                                                                                                  |           |           |      |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                  |           |           |      |   |   |
| Charge-offs                                                                                                                       | 640            | 321            | 1,735     | 643 |   |
| Loans and Leases Receivable, Gross                                                                                                | 1,238,791      |           | 1,238,791 |   | 1,262,718 |
| Commercial Real Estate [Member]                                                                                                   |           |           |      |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                  |           |           |      |   |   |
| Charge-offs                                                                                                                       | 157            | 694            | 297       | 9,238 |   |
| Loans and Leases Receivable, Gross                                                                                                | 1,306,752      |           | 1,306,752 |   | 1,296,472 |
| Loans to individuals [Member]                                                                                                     |           |           |      |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                  |           |           |      |   |   |
| Charge-offs                                                                                                                       | 752            | 767            | 1,562     | 1,755 |   |
| Loans and Leases Receivable, Gross                                                                                                | $ 625,961      |           | $ 625,961 |   | $ 610,298 |

| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AccountsNotesAndLoansReceivableLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of direct write-downs of financing receivables charged against the allowance.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=28368275&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522
+ Details**Name:**us-gaap_LoansAndLeasesReceivableGrossCarryingAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |