Loans and Allowance for Credit Losses - Recorded Investment and Unpaid Principal Balance for Impaired Loans with Associated Allowance (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) Contract	Sep. 30, 2019 USD ($) Contract	Sep. 30, 2020 USD ($) Contract	Sep. 30, 2019 USD ($) Contract	Dec. 31, 2019 USD ($)
Financing Receivable, Impaired [Line Items]
Number of Contracts | Contract	17	6	33	26
Total Pre-Modification Outstanding Recorded Investment	$ 1,309	$ 217	$ 1,781	$ 7,381
Post- Modification Outstanding Recorded Investment	1,165	215	1,607	6,917
Troubled Debt Restructuring [Policy Text Block]	489	0	489	398
Extended Maturity [Member]
Financing Receivable, Impaired [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	0	17
Contractual Interest Rate Reduction [Member]
Financing Receivable, Impaired [Line Items]
Total Pre-Modification Outstanding Recorded Investment	705	32	776	149
Contractual Payment Modification [Member]
Financing Receivable, Impaired [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 604	$ 185	$ 1,005	$ 7,215
Loans to individuals [Member]
Financing Receivable, Impaired [Line Items]
Number of Contracts | Contract	4	2	14	7
Total Pre-Modification Outstanding Recorded Investment	$ 67	$ 37	$ 263	$ 98
Post- Modification Outstanding Recorded Investment	63	34	245	87
Troubled Debt Restructuring [Policy Text Block]	0	0	0	0
Loans to individuals [Member] | Extended Maturity [Member]
Financing Receivable, Impaired [Line Items]
Total Pre-Modification Outstanding Recorded Investment	0	0	0	0
Loans to individuals [Member] | Contractual Interest Rate Reduction [Member]
Financing Receivable, Impaired [Line Items]
Total Pre-Modification Outstanding Recorded Investment	43	0	114	0
Loans to individuals [Member] | Contractual Payment Modification [Member]
Financing Receivable, Impaired [Line Items]
Total Pre-Modification Outstanding Recorded Investment	24	37	149	98
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	3,306	7,481	3,447	5,031
Impaired Financing Receivable, Interest Income, Accrual Method	77	1	94	24
Related allowance	0		0		$ 0
Recorded investment	2,055		2,055		2,519
Unpaid principal balance	2,447		2,447		2,993
Acquired Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Acquired Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	74	4,697	74	2,255
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Recorded investment	74		74		73
Unpaid principal balance	74		74		73
Acquired Loans [Member] | Real Estate Construction With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	308	0	137	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Recorded investment	308		308		0
Unpaid principal balance	308		308		0
Acquired Loans [Member] | Residential Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	1,473	1,950	1,772	1,966
Impaired Financing Receivable, Interest Income, Accrual Method	1	1	18	6
Recorded investment	1,429		1,429		2,136
Unpaid principal balance	1,794		1,794		2,585
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	1,440	666	1,453	636
Impaired Financing Receivable, Interest Income, Accrual Method	76	0	76	18
Recorded investment	233		233		298
Unpaid principal balance	257		257		320
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	11	13	11	14
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Recorded investment	11		11		12
Unpaid principal balance	14		14		15
Acquired Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	3,306	7,326	3,447	4,871
Impaired Financing Receivable, Interest Income, Accrual Method	77	1	94	24
Recorded investment	2,055		2,055		2,519
Unpaid principal balance	2,447		2,447		2,993
Acquired Loans [Member] | Real Estate Construction With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Acquired Loans [Member] | Residential Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Acquired Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	155	0	160
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Acquired Loans [Member] | Loans To Individuals With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Acquired Loans [Member] | With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	155	0	160
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	50,770	27,987	47,310	26,858
Impaired Financing Receivable, Interest Income, Accrual Method	127	131	403	474
Related allowance	7,418		7,418		2,432
Recorded investment	47,673		47,673		29,698
Unpaid principal balance	58,556		58,556		39,122
Originated Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	6,423	4,677	4,569	4,064
Impaired Financing Receivable, Interest Income, Accrual Method	16	8	45	36
Related allowance	1,497		1,497		1,580
Recorded investment	4,750		4,750		8,290
Unpaid principal balance	12,778		12,778		10,032
Originated Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	1,918	1,902	4,062	2,129
Impaired Financing Receivable, Interest Income, Accrual Method	10	3	15	10
Recorded investment	795		795		1,848
Unpaid principal balance	987		987		6,997
Originated Loans [Member] | Real Estate Construction With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	18	0	6	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Recorded investment	54		54		0
Unpaid principal balance	53		53		0
Originated Loans [Member] | Residential Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	10,198	10,254	10,380	10,751
Impaired Financing Receivable, Interest Income, Accrual Method	72	86	235	280
Recorded investment	9,962		9,962		10,372
Unpaid principal balance	11,965		11,965		12,437
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	14,312	3,582	13,994	3,854
Impaired Financing Receivable, Interest Income, Accrual Method	23	28	90	129
Recorded investment	14,228		14,228		3,015
Unpaid principal balance	14,523		14,523		3,210
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	494	389	469	356
Impaired Financing Receivable, Interest Income, Accrual Method	3	4	9	11
Recorded investment	495		495		406
Unpaid principal balance	808		808		640
Originated Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	26,940	16,127	28,911	17,090
Impaired Financing Receivable, Interest Income, Accrual Method	108	121	349	430
Recorded investment	25,534		25,534		15,641
Unpaid principal balance	28,336		28,336		23,284
Originated Loans [Member] | Real Estate Construction With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Originated Loans [Member] | Residential Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	740	0	347
Impaired Financing Receivable, Interest Income, Accrual Method	0	1	0	6
Related allowance	0		0		1
Recorded investment	0		0		474
Unpaid principal balance	0		0		498
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	17,407	6,443	13,830	5,357
Impaired Financing Receivable, Interest Income, Accrual Method	3	1	9	2
Related allowance	5,921		5,921		851
Recorded investment	17,389		17,389		5,293
Unpaid principal balance	17,442		17,442		5,308
Originated Loans [Member] | Loans To Individuals With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	0	0	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	0		0		0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Originated Loans [Member] | With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	23,830	11,860	18,399	9,768
Impaired Financing Receivable, Interest Income, Accrual Method	19	$ 10	54	$ 44
Related allowance	7,418		7,418		2,432
Recorded investment	22,139		22,139		14,057
Unpaid principal balance	$ 30,220		$ 30,220		$ 15,838