Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) Contract	Sep. 30, 2020 USD ($) Contract	Sep. 30, 2021 USD ($) Contract	Sep. 30, 2020 USD ($) Contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	15	17	28	33
Total Pre-Modification Outstanding Recorded Investment	$ 1,160	$ 1,309	$ 8,140	$ 1,781
Post- Modification Outstanding Recorded Investment	1,144	1,165	4,191	1,607
Troubled Debt Restructuring [Policy Text Block]	$ 0	$ 489	$ 0	$ 489
Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	5		6
Total Pre-Modification Outstanding Recorded Investment	$ 206		$ 6,803
Post- Modification Outstanding Recorded Investment	202		2,895
Troubled Debt Restructuring [Policy Text Block]	$ 0		$ 0
Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	5		13	16
Total Pre-Modification Outstanding Recorded Investment	$ 230		$ 520	$ 877
Post- Modification Outstanding Recorded Investment	221		500	729
Troubled Debt Restructuring [Policy Text Block]	$ 0		$ 0	$ 0
Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract			7	14
Total Pre-Modification Outstanding Recorded Investment			$ 173	$ 263
Post- Modification Outstanding Recorded Investment			151	245
Troubled Debt Restructuring [Policy Text Block]			$ 0	$ 0
Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	2			2
Total Pre-Modification Outstanding Recorded Investment	$ 644			$ 12
Post- Modification Outstanding Recorded Investment	645			8
Troubled Debt Restructuring [Policy Text Block]	$ 0			$ 0
Time and Demand Loans | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	5		6
Total Pre-Modification Outstanding Recorded Investment	$ 206		$ 6,803
Post- Modification Outstanding Recorded Investment	202		2,895
Troubled Debt Restructuring [Policy Text Block]	$ 0		$ 0
Senior Lien | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	3		10
Total Pre-Modification Outstanding Recorded Investment	$ 113		$ 390
Post- Modification Outstanding Recorded Investment	109		375
Troubled Debt Restructuring [Policy Text Block]	$ 0		$ 0
Junior Lien | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	2		3
Total Pre-Modification Outstanding Recorded Investment	$ 117		$ 130
Post- Modification Outstanding Recorded Investment	112		125
Troubled Debt Restructuring [Policy Text Block]	$ 0		$ 0
Automobiles | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	3		7
Total Pre-Modification Outstanding Recorded Investment	$ 80		$ 173
Post- Modification Outstanding Recorded Investment	76		151
Troubled Debt Restructuring [Policy Text Block]	$ 0		$ 0
Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract		12
Total Pre-Modification Outstanding Recorded Investment		$ 613
Post- Modification Outstanding Recorded Investment		477
Troubled Debt Restructuring [Policy Text Block]		$ 0
Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	3	4
Total Pre-Modification Outstanding Recorded Investment	$ 80	$ 67
Post- Modification Outstanding Recorded Investment	76	63
Troubled Debt Restructuring [Policy Text Block]	$ 0	$ 0
Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract		1		1
Total Pre-Modification Outstanding Recorded Investment		$ 629		$ 629
Post- Modification Outstanding Recorded Investment		625		625
Troubled Debt Restructuring [Policy Text Block]		489		489
Nonowner Occupied Commercial Real Estate | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	2		2
Total Pre-Modification Outstanding Recorded Investment	$ 644		$ 644
Post- Modification Outstanding Recorded Investment	645		645
Troubled Debt Restructuring [Policy Text Block]	0		$ 0
Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract			2
Total Pre-Modification Outstanding Recorded Investment			$ 644
Post- Modification Outstanding Recorded Investment			645
Troubled Debt Restructuring [Policy Text Block]			0
Extended Maturity [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	0	0
Extended Maturity [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0		0
Extended Maturity [Member] | Residential 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
Extended Maturity [Member] | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0			0
Extended Maturity [Member] | Time and Demand Loans | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0		0
Extended Maturity [Member] | Senior Lien | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0		0
Extended Maturity [Member] | Junior Lien | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0		0
Extended Maturity [Member] | Automobiles | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0		0
Extended Maturity [Member] | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment		0
Extended Maturity [Member] | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0
Extended Maturity [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment		0		0
Extended Maturity [Member] | Nonowner Occupied Commercial Real Estate | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0		0
Extended Maturity [Member] | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment			0
Contractual Interest Rate Reduction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	130	705	329	776
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		0
Contractual Interest Rate Reduction [Member] | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	113		219	33
Contractual Interest Rate Reduction [Member] | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment			110	114
Contractual Interest Rate Reduction [Member] | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0			0
Contractual Interest Rate Reduction [Member] | Time and Demand Loans | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0		0
Contractual Interest Rate Reduction [Member] | Senior Lien | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	113		219
Contractual Interest Rate Reduction [Member] | Junior Lien | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0		0
Contractual Interest Rate Reduction [Member] | Automobiles | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	17		110
Contractual Interest Rate Reduction [Member] | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment		33
Contractual Interest Rate Reduction [Member] | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	17	43
Contractual Interest Rate Reduction [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment		629		629
Contractual Interest Rate Reduction [Member] | Nonowner Occupied Commercial Real Estate | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0		0
Contractual Interest Rate Reduction [Member] | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment			0
Contractual Payment Modification [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	1,030	604	7,811	1,005
Contractual Payment Modification [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	206		6,803
Contractual Payment Modification [Member] | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	117		301	844
Contractual Payment Modification [Member] | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment			63	149
Contractual Payment Modification [Member] | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	644			12
Contractual Payment Modification [Member] | Time and Demand Loans | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	206		6,803
Contractual Payment Modification [Member] | Senior Lien | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0		171
Contractual Payment Modification [Member] | Junior Lien | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	117		130
Contractual Payment Modification [Member] | Automobiles | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	63		63
Contractual Payment Modification [Member] | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment		580
Contractual Payment Modification [Member] | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	63	24
Contractual Payment Modification [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment		$ 0		$ 0
Contractual Payment Modification [Member] | Nonowner Occupied Commercial Real Estate | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 644		644
Contractual Payment Modification [Member] | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment			$ 644