Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) Contract	Jun. 30, 2017 USD ($) Contract	Jun. 30, 2018 USD ($) Contract	Jun. 30, 2017 USD ($) Contract
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	19	13	35	26
Total Pre-Modification Outstanding Recorded Investment	$ 13,066	$ 8,794	$ 21,274	$ 9,611
Post- Modification Outstanding Recorded Investment	$ 12,967	$ 6,659	$ 16,690	$ 7,419
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	3714000	964000	4301000	964000
Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	7	3	8	4
Total Pre-Modification Outstanding Recorded Investment	$ 11,679	$ 8,513	$ 16,389	$ 8,554
Post- Modification Outstanding Recorded Investment	$ 11,591	$ 6,385	$ 11,972	$ 6,422
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	3714	960	4095	960
Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	6	5	17	12
Total Pre-Modification Outstanding Recorded Investment	$ 383	$ 193	$ 824	$ 729
Post- Modification Outstanding Recorded Investment	$ 381	$ 189	$ 779	$ 683
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	4	0	4
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	2	1	3	3
Total Pre-Modification Outstanding Recorded Investment	$ 966	$ 68	$ 3,983	$ 263
Post- Modification Outstanding Recorded Investment	$ 960	$ 68	$ 3,870	$ 258
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	0	206	0
Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	4	4	7	7
Total Pre-Modification Outstanding Recorded Investment	$ 38	$ 20	$ 78	$ 65
Post- Modification Outstanding Recorded Investment	$ 35	$ 17	$ 69	$ 56
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	$ 0	$ 6,768	$ 7,747	$ 7,076
Extended Maturity [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	6,768	4,710	6,768
Extended Maturity [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	20	129
Extended Maturity [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	3,017	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	24	1,833	127	1,990
Contractual Interest Rate Reduction [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	1,745	0	1,786
Contractual Interest Rate Reduction [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	85	75	187
Contractual Interest Rate Reduction [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	0	0
Contractual Interest Rate Reduction [Member] | Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	24	3	52	17
Contractual Payment Modification [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	13,042	193	13,400	545
Contractual Payment Modification [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	11,679	0	11,679	0
Contractual Payment Modification [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	383	108	729	413
Contractual Payment Modification [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	966	68	966	84
Contractual Payment Modification [Member] | Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 14	$ 17	$ 26	$ 48