Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) Contract	Sep. 30, 2016 USD ($) Contract	Sep. 30, 2017 USD ($) Contract	Sep. 30, 2016 USD ($) Contract
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	7	17	33	57
Total Pre-Modification Outstanding Recorded Investment	$ 203	$ 4,463	$ 9,814	$ 12,186
Post- Modification Outstanding Recorded Investment	$ 189	$ 4,424	$ 7,426	$ 11,262
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	1612000	671000	1680000
Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	1	1	6	5
Total Pre-Modification Outstanding Recorded Investment	$ 47	$ 3,853	$ 8,601	$ 7,955
Post- Modification Outstanding Recorded Investment	$ 47	$ 3,853	$ 6,307	$ 7,281
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	1612	669	1612
Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	4	11	15	35
Total Pre-Modification Outstanding Recorded Investment	$ 117	$ 473	$ 846	$ 2,762
Post- Modification Outstanding Recorded Investment	$ 106	$ 441	$ 777	$ 2,620
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	0	2	0
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	1	1	4	7
Total Pre-Modification Outstanding Recorded Investment	$ 27	$ 85	$ 290	$ 1,373
Post- Modification Outstanding Recorded Investment	$ 25	$ 85	$ 280	$ 1,285
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	0	0	68
Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	1	4	8	10
Total Pre-Modification Outstanding Recorded Investment	$ 12	$ 52	$ 77	$ 96
Post- Modification Outstanding Recorded Investment	$ 11	$ 45	$ 62	$ 76
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	$ 85	$ 7,076	$ 1,441
Extended Maturity [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	6,768	93
Extended Maturity [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	129	0
Extended Maturity [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	85	179	1,348
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	17	142	2,007	4,294
Contractual Interest Rate Reduction [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	1,786	4,009
Contractual Interest Rate Reduction [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	17	100	204	214
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	0	42	17	71
Contractual Payment Modification [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	186	4,236	731	6,451
Contractual Payment Modification [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	47	3,853	47	3,853
Contractual Payment Modification [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	100	373	513	2,548
Contractual Payment Modification [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	27	0	111	25
Contractual Payment Modification [Member] | Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 12	$ 10	$ 60	$ 25