Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) Contract	Sep. 30, 2014 USD ($) Contract	Sep. 30, 2015 USD ($) Contract	Sep. 30, 2014 USD ($) Contract
Financing Receivable, Modifications [Line Items]
Financing Receivable Modification Number Of Contracts | Contract	11	27
Number of Contracts | Contract			37	60
Total Pre-Modification Outstanding Recorded Investment			$ 4,217	$ 3,871
Post- Modification Outstanding Recorded Investment			$ 3,963	$ 3,846
Financing Receivable Modifications Extend Maturity	$ 0	$ 0
Financing Receivable Modifications Modify Rate	0	172
Financing Receivable Modifications Modify Payments	1,016	1,131
Financing Receivable, Modifications, Pre-Mod Recorded Investment	1,016	1,303
Financing Receivable, Modification, Post-Modification Recorded Investment	$ 996	$ 1,253
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	2000	52000	49000
Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable Modification Number Of Contracts | Contract	1
Number of Contracts | Contract			4	2
Total Pre-Modification Outstanding Recorded Investment			$ 2,403	$ 1,505
Post- Modification Outstanding Recorded Investment			$ 2,314	$ 1,648
Financing Receivable Modifications Extend Maturity	$ 0
Financing Receivable Modifications Modify Rate	0
Financing Receivable Modifications Modify Payments	543
Financing Receivable, Modifications, Pre-Mod Recorded Investment	543
Financing Receivable, Modification, Post-Modification Recorded Investment	$ 525
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0		52	27
Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable Modification Number Of Contracts | Contract	8	24
Number of Contracts | Contract			24	44
Total Pre-Modification Outstanding Recorded Investment			$ 1,254	$ 2,235
Post- Modification Outstanding Recorded Investment			$ 1,165	$ 2,091
Financing Receivable Modifications Extend Maturity	$ 0	$ 0
Financing Receivable Modifications Modify Rate	0	164
Financing Receivable Modifications Modify Payments	455	1,116
Financing Receivable, Modifications, Pre-Mod Recorded Investment	455	1,280
Financing Receivable, Modification, Post-Modification Recorded Investment	$ 455	$ 1,233
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	2	0	22
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract			1	1
Total Pre-Modification Outstanding Recorded Investment			$ 464	$ 8
Post- Modification Outstanding Recorded Investment			$ 407	$ 6
Financing Receivable, Modifications, Determination of Allowance for Credit Losses			0	0
Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable Modification Number Of Contracts | Contract	2	3
Number of Contracts | Contract			8	13
Total Pre-Modification Outstanding Recorded Investment			$ 96	$ 123
Post- Modification Outstanding Recorded Investment			$ 77	$ 101
Financing Receivable Modifications Extend Maturity	$ 0	$ 0
Financing Receivable Modifications Modify Rate	0	8
Financing Receivable Modifications Modify Payments	18	15
Financing Receivable, Modifications, Pre-Mod Recorded Investment	18	23
Financing Receivable, Modification, Post-Modification Recorded Investment	$ 16	$ 20
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	0	0	0
Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment			$ 1,751	$ 1,505
Extended Maturity [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment			1,751	1,505
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			357	549
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			296	468
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			61	81
Contractual Payment Modification [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment			2,109	1,817
Contractual Payment Modification [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment			652	0
Contractual Payment Modification [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment			958	1,767
Contractual Payment Modification [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment			464	8
Contractual Payment Modification [Member] | Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment			$ 35	$ 42