Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) Contract	Sep. 30, 2019 USD ($) Contract	Sep. 30, 2020 USD ($) Contract	Sep. 30, 2019 USD ($) Contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	17	6	33	26
Total Pre-Modification Outstanding Recorded Investment	$ 1,309	$ 217	$ 1,781	$ 7,381
Post- Modification Outstanding Recorded Investment	1,165	215	1,607	6,917
Troubled Debt Restructuring [Policy Text Block]	$ 489	$ 0	$ 489	$ 398
Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	1	1	1	2
Total Pre-Modification Outstanding Recorded Investment	$ 629	$ 95	$ 629	$ 156
Post- Modification Outstanding Recorded Investment	625	96	625	157
Troubled Debt Restructuring [Policy Text Block]	$ 489	$ 0	$ 489	$ 0
Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	12	3	16	14
Total Pre-Modification Outstanding Recorded Investment	$ 613	$ 85	$ 877	$ 1,008
Post- Modification Outstanding Recorded Investment	477	85	729	933
Troubled Debt Restructuring [Policy Text Block]	$ 0	$ 0	$ 0	$ 1
Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract			2	3
Total Pre-Modification Outstanding Recorded Investment			$ 12	$ 6,119
Post- Modification Outstanding Recorded Investment			8	5,740
Troubled Debt Restructuring [Policy Text Block]			$ 0	$ 397
Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	4	2	14	7
Total Pre-Modification Outstanding Recorded Investment	$ 67	$ 37	$ 263	$ 98
Post- Modification Outstanding Recorded Investment	63	34	245	87
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	0	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	17
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] | 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	705	32	776	149
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	0	629	0
Contractual Interest Rate Reduction [Member] | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	33	32	33	149
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] | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	43	0	114	0
Contractual Payment Modification [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	604	185	1,005	7,215
Contractual Payment Modification [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	95	0	156
Contractual Payment Modification [Member] | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	580	53	844	842
Contractual Payment Modification [Member] | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment			12	6,119
Contractual Payment Modification [Member] | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 24	$ 37	$ 149	$ 98