Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) Contract	Sep. 30, 2018 USD ($) Contract	Sep. 30, 2019 USD ($) Contract	Sep. 30, 2018 USD ($) Contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	6	14	26	42
Total Pre-Modification Outstanding Recorded Investment	$ 217	$ 482	$ 7,381	$ 10,600
Post- Modification Outstanding Recorded Investment	215	452	6,917	9,547
Troubled Debt Restructuring [Policy Text Block]	$ 0	$ 0	$ 398	$ 2,811
Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	1	1	2	3
Total Pre-Modification Outstanding Recorded Investment	$ 95	$ 74	$ 156	$ 8,324
Post- Modification Outstanding Recorded Investment	96	74	157	7,393
Troubled Debt Restructuring [Policy Text Block]	$ 0	$ 0	$ 0	$ 2,811
Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	3	7	14	24
Total Pre-Modification Outstanding Recorded Investment	$ 85	$ 365	$ 1,008	$ 1,189
Post- Modification Outstanding Recorded Investment	85	338	933	1,108
Troubled Debt Restructuring [Policy Text Block]	$ 0	$ 0	$ 1	$ 0
Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract			3	2
Total Pre-Modification Outstanding Recorded Investment			$ 6,119	$ 966
Post- Modification Outstanding Recorded Investment			5,740	943
Troubled Debt Restructuring [Policy Text Block]			$ 397	$ 0
Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	2	6	7	13
Total Pre-Modification Outstanding Recorded Investment	$ 37	$ 43	$ 98	$ 121
Post- Modification Outstanding Recorded Investment	34	40	87	103
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	139	17	159
Extended Maturity [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	74	0	74
Extended Maturity [Member] | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	65	17	85
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	32	96	149	222
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	0	0
Contractual Interest Rate Reduction [Member] | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	32	70	149	145
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	0	26	0	77
Contractual Payment Modification [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	185	247	7,215	10,219
Contractual Payment Modification [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	95	0	156	8,250
Contractual Payment Modification [Member] | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	53	230	842	959
Contractual Payment Modification [Member] | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment			6,119	966
Contractual Payment Modification [Member] | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 37	$ 17	$ 98	$ 44