Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 Contract	Mar. 31, 2014 Contract
Financing Receivable, Modifications [Line Items]
Number of Contracts	8	21
Total Pre-Modification Outstanding Recorded Investment	$ 2,249	$ 812
Post- Modification Outstanding Recorded Investment	2,139	791
Specific Reserve	0	34
Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts	1	1
Total Pre-Modification Outstanding Recorded Investment	1,498	60
Post- Modification Outstanding Recorded Investment	1,476	60
Specific Reserve	0	29
Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts	5	13
Total Pre-Modification Outstanding Recorded Investment	269	689
Post- Modification Outstanding Recorded Investment	203	676
Specific Reserve	0	5
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts	1	1
Total Pre-Modification Outstanding Recorded Investment	464	12
Post- Modification Outstanding Recorded Investment	449	8
Specific Reserve	0	0
Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts	1	6
Total Pre-Modification Outstanding Recorded Investment	18	51
Post- Modification Outstanding Recorded Investment	11	47
Specific Reserve	0	0
Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	1,498	60
Extended Maturity [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	1,498	60
Extended Maturity [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0
Extended Maturity [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0
Extended Maturity [Member] | Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0
Contractual Interest Rate Reduction [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	252	203
Contractual Interest Rate Reduction [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0
Contractual Interest Rate Reduction [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	252	172
Contractual Interest Rate Reduction [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0
Contractual Interest Rate Reduction [Member] | Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	31
Contractual Payment Modification [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	499	549
Contractual Payment Modification [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0
Contractual Payment Modification [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	17	517
Contractual Payment Modification [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	464	12
Contractual Payment Modification [Member] | Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 18	$ 20