Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) Contract	Jun. 30, 2019 USD ($) Contract	Jun. 30, 2020 USD ($) Contract	Jun. 30, 2019 USD ($) Contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	5	10	16	20
Total Pre-Modification Outstanding Recorded Investment	$ 200	$ 6,233	$ 471	$ 7,164
Post- Modification Outstanding Recorded Investment	197	5,964	451	6,869
Troubled Debt Restructuring [Policy Text Block]	$ 0	$ 969	$ 0	$ 1,006
Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract				1
Total Pre-Modification Outstanding Recorded Investment				$ 61
Post- Modification Outstanding Recorded Investment				62
Troubled Debt Restructuring [Policy Text Block]				$ 0
Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	2	5	4	11
Total Pre-Modification Outstanding Recorded Investment	$ 146	$ 341	$ 264	$ 922
Post- Modification Outstanding Recorded Investment	142	337	256	897
Troubled Debt Restructuring [Policy Text Block]	$ 0	$ 969	$ 0	$ 37
Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract		2	2	3
Total Pre-Modification Outstanding Recorded Investment		$ 5,878	$ 12	$ 6,119
Post- Modification Outstanding Recorded Investment		5,613	9	5,854
Troubled Debt Restructuring [Policy Text Block]		$ 0	$ 0	$ 969
Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	3	3	10	5
Total Pre-Modification Outstanding Recorded Investment	$ 54	$ 14	$ 195	$ 62
Post- Modification Outstanding Recorded Investment	55	14	186	56
Troubled Debt Restructuring [Policy Text Block]	0	0	0	0
Extended Maturity [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	0	17
Extended Maturity [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment				0
Extended Maturity [Member] | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	0	17
Extended Maturity [Member] | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment		0	0	0
Extended Maturity [Member] | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	0	0
Contractual Interest Rate Reduction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	53	68	71	117
Contractual Interest Rate Reduction [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment				0
Contractual Interest Rate Reduction [Member] | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	68	0	117
Contractual Interest Rate Reduction [Member] | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment		0	0	0
Contractual Interest Rate Reduction [Member] | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	53	0	71	0
Contractual Payment Modification [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	147	6,165	400	7,030
Contractual Payment Modification [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment				61
Contractual Payment Modification [Member] | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	146	273	264	788
Contractual Payment Modification [Member] | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment		5,878	12	6,119
Contractual Payment Modification [Member] | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 1	$ 14	$ 124	$ 62