Loans and Allowance for Credit Losses - Recorded Investment and Unpaid Principal Balance for Impaired Loans with Associated Allowance (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	$ 29,698	$ 28,735
Unpaid Principal Balance	39,122	38,787
Related Allowance	2,432	1,500
Average Recorded Investment	27,154	41,386	$ 38,171
Interest Income Recognized	684	1,098	1,451
Originated Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	1,848	8,735
Unpaid Principal Balance	6,997	16,442
Average Recorded Investment	2,411	18,480	10,282
Interest Income Recognized	66	602	394
Originated Loans [Member] | Real Estate Construction With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	0
Unpaid Principal Balance	0	0
Average Recorded Investment	0	0	0
Interest Income Recognized	0	0	0
Originated Loans [Member] | Residential real estate with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	10,372	10,726
Unpaid Principal Balance	12,437	12,571
Average Recorded Investment	10,819	10,651	11,366
Interest Income Recognized	365	271	355
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	3,015	3,599
Unpaid Principal Balance	3,210	3,812
Average Recorded Investment	7,455	7,919	6,469
Interest Income Recognized	156	177	583
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	406	281
Unpaid Principal Balance	640	408
Average Recorded Investment	371	310	353
Interest Income Recognized	17	11	19
Originated Loans [Member] | Subtotal with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	15,641	23,341
Unpaid Principal Balance	23,284	33,233
Average Recorded Investment	21,056	37,360	28,470
Interest Income Recognized	604	1,061	1,351
Originated Loans [Member] | Commercial, financial, agricultural and other With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	8,290	3,042
Unpaid Principal Balance	10,032	3,181
Related Allowance	1,580	797
Average Recorded Investment	4,110	2,531	9,391
Interest Income Recognized	77	20	96
Originated Loans [Member] | Real estate construction with an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	0
Unpaid Principal Balance	0	0
Related Allowance	0	0
Average Recorded Investment	0	0	0
Interest Income Recognized	0	0	0
Originated Loans [Member] | Residential real estate With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	474	486
Unpaid Principal Balance	498	495
Related Allowance	1	107
Average Recorded Investment	241	504	167
Interest Income Recognized	0	13	0
Originated Loans [Member] | Commercial real estate With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	5,293	1,866
Unpaid Principal Balance	5,308	1,878
Related Allowance	851	596
Average Recorded Investment	1,747	991	143
Interest Income Recognized	3	4	4
Originated Loans [Member] | Loans to individuals With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	0
Unpaid Principal Balance	0	0
Related Allowance	0	0
Average Recorded Investment	0	0	0
Interest Income Recognized	0	0	0
Originated Loans [Member] | Subtotal With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	14,057	5,394
Unpaid Principal Balance	15,838	5,554
Related Allowance	2,432	1,500
Average Recorded Investment	6,098	4,026	9,701
Interest Income Recognized	80	37	100
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	2,519	3,292
Unpaid Principal Balance	2,993	4,877
Related Allowance	0	131
Average Recorded Investment	5,225	3,712	3,406
Interest Income Recognized	26	15	0
Acquired Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	73	73
Unpaid Principal Balance	73	73
Average Recorded Investment	2,479	214	476
Interest Income Recognized	0	10	0
Acquired Loans [Member] | Real Estate Construction With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	0
Unpaid Principal Balance	0	0
Average Recorded Investment	0	0	25
Interest Income Recognized	0	0	0
Acquired Loans [Member] | Residential real estate with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	2,136	2,031
Unpaid Principal Balance	2,585	2,604
Average Recorded Investment	1,986	1,906	535
Interest Income Recognized	8	5	0
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	298	1,042
Unpaid Principal Balance	320	2,052
Average Recorded Investment	747	1,565	2,135
Interest Income Recognized	18	0	0
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	12	15
Unpaid Principal Balance	15	17
Average Recorded Investment	13	16	6
Interest Income Recognized	0	0	0
Acquired Loans [Member] | Subtotal with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	2,519	3,161
Unpaid Principal Balance	2,993	4,746
Average Recorded Investment	5,225	3,701	3,177
Interest Income Recognized	26	15	0
Acquired Loans [Member] | Commercial, financial, agricultural and other With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	131
Unpaid Principal Balance	0	131
Related Allowance	0	131
Average Recorded Investment	0	11	0
Interest Income Recognized	0	0	0
Acquired Loans [Member] | Real estate construction with an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	0
Unpaid Principal Balance	0	0
Related Allowance	0	0
Average Recorded Investment	0	0	0
Interest Income Recognized	0	0	0
Acquired Loans [Member] | Residential real estate With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	0
Unpaid Principal Balance	0	0
Related Allowance	0	0
Average Recorded Investment	0	0	74
Interest Income Recognized	0	0	0
Acquired Loans [Member] | Commercial real estate With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	0
Unpaid Principal Balance	0	0
Related Allowance	0	0
Average Recorded Investment	0	0	155
Interest Income Recognized	0	0	0
Acquired Loans [Member] | Loans to individuals With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	0
Unpaid Principal Balance	0	0
Related Allowance	0	0
Average Recorded Investment	0	0	0
Interest Income Recognized	0	0	0
Acquired Loans [Member] | Subtotal With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	0	131
Unpaid Principal Balance	0	131
Related Allowance	0	131
Average Recorded Investment	0	11	229
Interest Income Recognized	$ 0	$ 0	$ 0