Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) Contract	Mar. 31, 2016 USD ($) Contract
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	13	16
Total Pre-Modification Outstanding Recorded Investment	$ 817	$ 4,978
Post- Modification Outstanding Recorded Investment	$ 778	$ 4,844
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	0
Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	1	2
Total Pre-Modification Outstanding Recorded Investment	$ 42	$ 3,769
Post- Modification Outstanding Recorded Investment	$ 38	$ 3,749
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	0
Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	7	8
Total Pre-Modification Outstanding Recorded Investment	$ 536	$ 988
Post- Modification Outstanding Recorded Investment	$ 504	$ 910
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	0
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	2	3
Total Pre-Modification Outstanding Recorded Investment	$ 195	$ 198
Post- Modification Outstanding Recorded Investment	$ 193	$ 169
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	0
Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	3	3
Total Pre-Modification Outstanding Recorded Investment	$ 44	$ 23
Post- Modification Outstanding Recorded Investment	$ 43	$ 16
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	0
Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 308	$ 65
Extended Maturity [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0
Extended Maturity [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	129	0
Extended Maturity [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	179	65
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	157	3,901
Contractual Interest Rate Reduction [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	42	3,769
Contractual Interest Rate Reduction [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	101	114
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	14	18
Contractual Payment Modification [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	352	1,012
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	306	874
Contractual Payment Modification [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	16	133
Contractual Payment Modification [Member] | Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 30	$ 5