Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) Contract	Jun. 30, 2016 USD ($) Contract	Jun. 30, 2017 USD ($) Contract	Jun. 30, 2016 USD ($) Contract
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	13	27	26	42
Total Pre-Modification Outstanding Recorded Investment	$ 8,794	$ 2,982	$ 9,611	$ 7,960
Post- Modification Outstanding Recorded Investment	$ 6,659	$ 2,833	$ 7,419	$ 7,405
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	964000	114000	964000	114000
Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	3	4	4	5
Total Pre-Modification Outstanding Recorded Investment	$ 8,513	$ 332	$ 8,554	$ 4,101
Post- Modification Outstanding Recorded Investment	$ 6,385	$ 217	$ 6,422	$ 3,708
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	960	40	960	40
Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	5	17	12	26
Total Pre-Modification Outstanding Recorded Investment	$ 193	$ 1,435	$ 729	$ 2,530
Post- Modification Outstanding Recorded Investment	$ 189	$ 1,428	$ 683	$ 2,440
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	4	0	4	0
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	1	4	3	6
Total Pre-Modification Outstanding Recorded Investment	$ 68	$ 1,198	$ 263	$ 1,289
Post- Modification Outstanding Recorded Investment	$ 68	$ 1,173	$ 258	$ 1,227
Financing Receivable, Modifications, Determination of Allowance for Credit Losses	0	74	0	74
Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	4	2	7	5
Total Pre-Modification Outstanding Recorded Investment	$ 20	$ 17	$ 65	$ 40
Post- Modification Outstanding Recorded Investment	$ 17	$ 15	$ 56	$ 30
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	$ 6,768	$ 1,290	$ 7,076	$ 1,356
Extended Maturity [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	6,768	92	6,768	92
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	1,198	179	1,264
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	1,833	251	1,990	4,152
Contractual Interest Rate Reduction [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	1,745	240	1,786	4,009
Contractual Interest Rate Reduction [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	85	0	187	114
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	3	11	17	29
Contractual Payment Modification [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	193	1,441	545	2,452
Contractual Payment Modification [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	0	0
Contractual Payment Modification [Member] | Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	108	1,435	413	2,416
Contractual Payment Modification [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	68	0	84	25
Contractual Payment Modification [Member] | Loans to individuals [Member]
Financing Receivable, Modifications [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 17	$ 6	$ 48	$ 11