Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) Contract	Jun. 30, 2018 USD ($) Contract	Jun. 30, 2019 USD ($) Contract	Jun. 30, 2018 USD ($) Contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	10	19	20	35
Total Pre-Modification Outstanding Recorded Investment	$ 6,233	$ 13,066	$ 7,164	$ 21,274
Post- Modification Outstanding Recorded Investment	5,964	12,967	6,869	16,690
Troubled Debt Restructuring [Policy Text Block]	$ 969	$ 3,714	$ 1,006	$ 4,301
Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	0	7	1	8
Total Pre-Modification Outstanding Recorded Investment	$ 0	$ 11,679	$ 61	$ 16,389
Post- Modification Outstanding Recorded Investment	0	11,591	62	11,972
Troubled Debt Restructuring [Policy Text Block]	$ 0	$ 3,714	$ 0	$ 4,095
Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	5	6	11	17
Total Pre-Modification Outstanding Recorded Investment	$ 341	$ 383	$ 922	$ 824
Post- Modification Outstanding Recorded Investment	337	381	897	779
Troubled Debt Restructuring [Policy Text Block]	$ 0	$ 0	$ 37	$ 0
Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	2	2	3	3
Total Pre-Modification Outstanding Recorded Investment	$ 5,878	$ 966	$ 6,119	$ 3,983
Post- Modification Outstanding Recorded Investment	5,613	960	5,854	3,870
Troubled Debt Restructuring [Policy Text Block]	$ 969	$ 0	$ 969	$ 206
Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	3	4	5	7
Total Pre-Modification Outstanding Recorded Investment	$ 14	$ 38	$ 62	$ 78
Post- Modification Outstanding Recorded Investment	14	35	56	69
Troubled Debt Restructuring [Policy Text Block]	$ 0	0	$ 0	0
Real estate construction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | Contract	0		0
Total Pre-Modification Outstanding Recorded Investment	$ 0		$ 0
Post- Modification Outstanding Recorded Investment	0		0
Troubled Debt Restructuring [Policy Text Block]	0		0
Extended Maturity [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	17	7,747
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	4,710
Extended Maturity [Member] | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	17	20
Extended Maturity [Member] | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	0	3,017
Extended Maturity [Member] | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	0	0
Extended Maturity [Member] | Real estate construction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0		0
Contractual Interest Rate Reduction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	68	24	117	127
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	68	0	117	75
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	0
Contractual Interest Rate Reduction [Member] | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	24	0	52
Contractual Interest Rate Reduction [Member] | Real estate construction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0		0
Contractual Payment Modification [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	6,165	13,042	7,030	13,400
Contractual Payment Modification [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	11,679	61	11,679
Contractual Payment Modification [Member] | Residential real estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	273	383	788	729
Contractual Payment Modification [Member] | Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	5,878	966	6,119	966
Contractual Payment Modification [Member] | Loans to individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	14	$ 14	62	$ 26
Contractual Payment Modification [Member] | Real estate construction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 0		$ 0