| **Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2012 |
|------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|---------------|
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                               |          |          |          |
| 30 - 59 days past due                                                                                                                          | $ 22,010      | $ 14,069      |          |
| 60 - 89 days past due                                                                                                                          | 6,312         | 5,319         |          |
| 90 days and greater and still accruing                                                                                                         | 3,927         | 2,447         |          |
| Nonaccrual                                                                                                                                     | 64,141        | 94,518        |          |
| Total past due and nonaccrual                                                                                                                  | 96,390        | 116,353       |          |
| Current                                                                                                                                        | 4,122,420     | 4,088,351     |          |
| Total                                                                                                                                          | 4,218,810     | 4,204,704     | 4,128,588     |
| Commercial, financial, agricultural and other [Member]                                                                                         |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                               |          |          |          |
| 30 - 59 days past due                                                                                                                          | 3,054         | 991           |          |
| 60 - 89 days past due                                                                                                                          | 2,380         | 620           |          |
| 90 days and greater and still accruing                                                                                                         | 1,108         | 288           |          |
| Nonaccrual                                                                                                                                     | 28,666        | 29,258        |          |
| Total past due and nonaccrual                                                                                                                  | 35,208        | 31,157        |          |
| Current                                                                                                                                        | 1,022,455     | 988,665       |          |
| Total                                                                                                                                          | 1,057,663     | 1,019,822     |          |
| Real estate construction [Member]                                                                                                              |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                               |          |          |          |
| 30 - 59 days past due                                                                                                                          | 206           | 2             |          |
| 60 - 89 days past due                                                                                                                          | 15            | 19            |          |
| 90 days and greater and still accruing                                                                                                         | 500           | 15            |          |
| Nonaccrual                                                                                                                                     | 7,112         | 9,778         |          |
| Total past due and nonaccrual                                                                                                                  | 7,833         | 9,814         |          |
| Current                                                                                                                                        | 62,628        | 77,624        |          |
| Total                                                                                                                                          | 70,461        | 87,438        |          |
| Residential real estate [Member]                                                                                                               |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                               |          |          |          |
| 30 - 59 days past due                                                                                                                          | 6,282         | 6,597         |          |
| 60 - 89 days past due                                                                                                                          | 2,420         | 2,357         |          |
| 90 days and greater and still accruing                                                                                                         | 963           | 730           |          |
| Nonaccrual                                                                                                                                     | 9,740         | 9,283         |          |
| Total past due and nonaccrual                                                                                                                  | 19,405        | 18,967        |          |
| Current                                                                                                                                        | 1,236,110     | 1,222,598     |          |
| Total                                                                                                                                          | 1,255,515     | 1,241,565     |          |
| Commercial real estate [Member]                                                                                                                |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                               |          |          |          |
| 30 - 59 days past due                                                                                                                          | 9,903         | 3,339         |          |
| 60 - 89 days past due                                                                                                                          | 921           | 1,389         |          |
| 90 days and greater and still accruing                                                                                                         | 279           | 195           |          |
| Nonaccrual                                                                                                                                     | 18,456        | 46,023        |          |
| Total past due and nonaccrual                                                                                                                  | 29,559        | 50,946        |          |
| Current                                                                                                                                        | 1,214,117     | 1,222,715     |          |
| Total                                                                                                                                          | 1,243,676     | 1,273,661     |          |
| Loans to individuals [Member]                                                                                                                  |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                               |          |          |          |
| 30 - 59 days past due                                                                                                                          | 2,565         | 3,140         |          |
| 60 - 89 days past due                                                                                                                          | 576           | 934           |          |
| 90 days and greater and still accruing                                                                                                         | 1,077         | 1,219         |          |
| Nonaccrual                                                                                                                                     | 167           | 176           |          |
| Total past due and nonaccrual                                                                                                                  | 4,385         | 5,469         |          |
| Current                                                                                                                                        | 587,110       | 576,749       |          |
| Total                                                                                                                                          | $ 591,495     | $ 582,218     |          |

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| - Definition
Financing receivables that are less than 60 days past due but more than 29 days past due.
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Financing receivables that are less than 90 days past due but more than 59 days past due.
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| - Definition
Recorded investment in financing receivables that are 90 days or more past due and still accruing.
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| - Definition
Financing receivables that are current.
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| - Definition
Financing receivables that are equal to or greater than 90 days past due.
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| - Definition
Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.
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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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