| **Loans and Allowance for Credit Losses - Credit Risk Profile by Creditworthiness (Detail) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2012 |
|----------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|---------------|
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | $ 4,218,810   | $ 4,204,704   | $ 4,128,588   |
| Pass [Member]                                                                                                                                |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 3,963,944     | 3,916,202     |          |
| OAEM [Member]                                                                                                                                |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 109,791       | 120,205       |          |
| Substandard [Member]                                                                                                                         |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 141,553       | 164,775       |          |
| Doubtful [Member]                                                                                                                            |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 3,522         | 3,522         |          |
| Total Non-Pass [Member]                                                                                                                      |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 254,866       | 288,502       |          |
| Commercial, financial, agricultural and other [Member]                                                                                       |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 1,057,663     | 1,019,822     |          |
| Commercial, financial, agricultural and other [Member] | Pass [Member]                                                                       |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 942,673       | 925,868       |          |
| Commercial, financial, agricultural and other [Member] | OAEM [Member]                                                                       |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 43,909        | 31,049        |          |
| Commercial, financial, agricultural and other [Member] | Substandard [Member]                                                                |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 71,081        | 62,905        |          |
| Commercial, financial, agricultural and other [Member] | Doubtful [Member]                                                                   |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 0             | 0             |          |
| Commercial, financial, agricultural and other [Member] | Total Non-Pass [Member]                                                             |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 114,990       | 93,954        |          |
| Real estate construction [Member]                                                                                                            |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 70,461        | 87,438        |          |
| Real estate construction [Member] | Pass [Member]                                                                                            |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 50,664        | 64,353        |          |
| Real estate construction [Member] | OAEM [Member]                                                                                            |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 911           | 925           |          |
| Real estate construction [Member] | Substandard [Member]                                                                                     |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 15,364        | 18,638        |          |
| Real estate construction [Member] | Doubtful [Member]                                                                                        |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 3,522         | 3,522         |          |
| Real estate construction [Member] | Total Non-Pass [Member]                                                                                  |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 19,797        | 23,085        |          |
| Residential real estate [Member]                                                                                                             |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 1,255,515     | 1,241,565     |          |
| Residential real estate [Member] | Pass [Member]                                                                                             |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 1,238,633     | 1,224,849     |          |
| Residential real estate [Member] | OAEM [Member]                                                                                             |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 5,075         | 5,647         |          |
| Residential real estate [Member] | Substandard [Member]                                                                                      |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 11,807        | 11,069        |          |
| Residential real estate [Member] | Doubtful [Member]                                                                                         |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 0             | 0             |          |
| Residential real estate [Member] | Total Non-Pass [Member]                                                                                   |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 16,882        | 16,716        |          |
| Commercial real estate [Member]                                                                                                              |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 1,243,676     | 1,273,661     |          |
| Commercial real estate [Member] | Pass [Member]                                                                                              |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 1,140,648     | 1,119,093     |          |
| Commercial real estate [Member] | OAEM [Member]                                                                                              |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 59,894        | 82,581        |          |
| Commercial real estate [Member] | Substandard [Member]                                                                                       |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 43,134        | 71,987        |          |
| Commercial real estate [Member] | Doubtful [Member]                                                                                          |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 0             | 0             |          |
| Commercial real estate [Member] | Total Non-Pass [Member]                                                                                    |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 103,028       | 154,568       |          |
| Loans to individuals [Member]                                                                                                                |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 591,495       | 582,218       |          |
| Loans to individuals [Member] | Pass [Member]                                                                                                |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 591,326       | 582,039       |          |
| Loans to individuals [Member] | OAEM [Member]                                                                                                |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 2             | 3             |          |
| Loans to individuals [Member] | Substandard [Member]                                                                                         |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 167           | 176           |          |
| Loans to individuals [Member] | Doubtful [Member]                                                                                            |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | 0             | 0             |          |
| Loans to individuals [Member] | Total Non-Pass [Member]                                                                                      |          |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                             |          |          |          |
| Loans                                                                                                                                        | $ 169         | $ 179         |          |

| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AccountsNotesAndLoansReceivableLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher OTS -Name Federal Regulation (FR) -Number Title 12 -Section 563c.102 -Paragraph 8 -Subparagraph e -Chapter V -Subsection I -LegacyDoc This is a non-GAAP reference that was included in the 2009 taxonomy.  It will be removed from future versions of this taxonomy.Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-DEP -Paragraph 116 -IssueDate 2006-05-01 -Chapter 8 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5074-111524Reference 4: 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=6375948&loc=d3e4428-111522Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 01-6 -Paragraph 13 -Subparagraph e -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.
+ Details**Name:**us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |