Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) Contract	Mar. 31, 2017 USD ($) Contract
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	18	13
Total Pre-Modification Outstanding Recorded Investment	$ 8,387	$ 817
Post- Modification Outstanding Recorded Investment	$ 7,393	$ 778
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	775000	0
Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	2	1
Total Pre-Modification Outstanding Recorded Investment	$ 4,871	$ 42
Post- Modification Outstanding Recorded Investment	$ 3,942	$ 38
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	531	0
Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	11	7
Total Pre-Modification Outstanding Recorded Investment	$ 441	$ 536
Post- Modification Outstanding Recorded Investment	$ 404	$ 504
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	17	0
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	1	2
Total Pre-Modification Outstanding Recorded Investment	$ 3,017	$ 195
Post- Modification Outstanding Recorded Investment	$ 2,994	$ 193
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	227	0
Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	4	3
Total Pre-Modification Outstanding Recorded Investment	$ 58	$ 44
Post- Modification Outstanding Recorded Investment	$ 53	$ 43
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	0
Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 7,746	$ 308
Extended Maturity [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	4,709	0
Extended Maturity [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	20	129
Extended Maturity [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	3,017	179
Extended Maturity [Member] | Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0
Contractual Interest Rate Reduction [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	103	157
Contractual Interest Rate Reduction [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	42
Contractual Interest Rate Reduction [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	75	101
Contractual Interest Rate Reduction [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0
Contractual Interest Rate Reduction [Member] | Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	28	14
Contractual Payment Modification [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	538	352
Contractual Payment Modification [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	162	0
Contractual Payment Modification [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	346	306
Contractual Payment Modification [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	16
Contractual Payment Modification [Member] | Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 30	$ 30