Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) Contract	Sep. 30, 2017 USD ($) Contract	Sep. 30, 2018 USD ($) Contract	Sep. 30, 2017 USD ($) Contract
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	14	7	42	33
Total Pre-Modification Outstanding Recorded Investment	$ 482	$ 203	$ 10,600	$ 9,814
Post- Modification Outstanding Recorded Investment	$ 452	$ 189	$ 9,547	$ 7,426
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	0	2811000	671000
Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	1	1	3	6
Total Pre-Modification Outstanding Recorded Investment	$ 74	$ 47	$ 8,324	$ 8,601
Post- Modification Outstanding Recorded Investment	$ 74	$ 47	$ 7,393	$ 6,307
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	0	2811	669
Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	7	4	24	15
Total Pre-Modification Outstanding Recorded Investment	$ 365	$ 117	$ 1,189	$ 846
Post- Modification Outstanding Recorded Investment	$ 338	$ 106	$ 1,108	$ 777
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	0	0	2
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract		1	2	4
Total Pre-Modification Outstanding Recorded Investment		$ 27	$ 966	$ 290
Post- Modification Outstanding Recorded Investment		$ 25	$ 943	$ 280
Financing Receivable, Modifications, Determination of Allowance for Credit Losses		0	0	0
Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	6	1	13	8
Total Pre-Modification Outstanding Recorded Investment	$ 43	$ 12	$ 121	$ 77
Post- Modification Outstanding Recorded Investment	$ 40	$ 11	$ 103	$ 62
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	0	0	0
Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 139	$ 0	$ 159	$ 7,076
Extended Maturity [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	74	0	74	6,768
Extended Maturity [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	65	0	85	129
Extended Maturity [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment		0	0	179
Extended Maturity [Member] | Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	0	0
Contractual Interest Rate Reduction [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	96	17	222	2,007
Contractual Interest Rate Reduction [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	0	1,786
Contractual Interest Rate Reduction [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	70	17	145	204
Contractual Interest Rate Reduction [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment		0	0	0
Contractual Interest Rate Reduction [Member] | Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	26	0	77	17
Contractual Payment Modification [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	247	186	10,219	731
Contractual Payment Modification [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	47	8,250	47
Contractual Payment Modification [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	230	100	959	513
Contractual Payment Modification [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment		27	966	111
Contractual Payment Modification [Member] | Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 17	$ 12	$ 44	$ 60