Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 Contract	Mar. 31, 2012 Contract
Financing Receivable, Modifications [Line Items]
Number of Contracts	16
Extend Maturity	$ 433
Modify Rate	481
Modify Payments	530
Total Pre-Modification Outstanding Recorded Investment	1,444
Post- Modification Outstanding Recorded Investment	1,321
Specific Reserve	0
Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts	2
Extend Maturity	426
Modify Rate	0
Modify Payments	12
Total Pre-Modification Outstanding Recorded Investment	438
Post- Modification Outstanding Recorded Investment	377
Specific Reserve	0
Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts	9	2
Extend Maturity	7	0
Modify Rate	214	97
Modify Payments	514	0
Total Pre-Modification Outstanding Recorded Investment	735	97
Post- Modification Outstanding Recorded Investment	677	53
Specific Reserve	0	0
Commercial real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts	1
Extend Maturity	0
Modify Rate	244
Modify Payments	0
Total Pre-Modification Outstanding Recorded Investment	244
Post- Modification Outstanding Recorded Investment	242
Specific Reserve	0
Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts	4
Extend Maturity	0
Modify Rate	23
Modify Payments	4
Total Pre-Modification Outstanding Recorded Investment	27
Post- Modification Outstanding Recorded Investment	$ 25