Loans and Allowance for Credit Losses - Recorded Investment and Unpaid Principal Balance for Impaired Loans with Associated Allowance (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) Contract	Jun. 30, 2019 USD ($) Contract	Jun. 30, 2020 USD ($) Contract	Jun. 30, 2019 USD ($) Contract	Dec. 31, 2019 USD ($)
Financing Receivable, Impaired [Line Items]
Number of Contracts | Contract	5	10	16	20
Total Pre-Modification Outstanding Recorded Investment	$ 200	$ 6,233	$ 471	$ 7,164
Post- Modification Outstanding Recorded Investment	197	5,964	451	6,869
Troubled Debt Restructuring [Policy Text Block]	0	969	0	1,006
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	53	68	71	117
Contractual Payment Modification [Member]
Financing Receivable, Impaired [Line Items]
Total Pre-Modification Outstanding Recorded Investment	$ 147	$ 6,165	$ 400	$ 7,030
Loans to individuals [Member]
Financing Receivable, Impaired [Line Items]
Number of Contracts | Contract	3	3	10	5
Total Pre-Modification Outstanding Recorded Investment	$ 54	$ 14	$ 195	$ 62
Post- Modification Outstanding Recorded Investment	55	14	186	56
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	53	0	71	0
Loans to individuals [Member] | Contractual Payment Modification [Member]
Financing Receivable, Impaired [Line Items]
Total Pre-Modification Outstanding Recorded Investment	1	14	124	62
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	4,015	4,197	3,518	3,806
Impaired Financing Receivable, Interest Income, Accrual Method	15	21	17	23
Related allowance	347		347		$ 0
Recorded investment	3,955		3,955		2,519
Unpaid principal balance	4,385		4,385		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	1,630	74	1,035
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	0	0	0	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Acquired Loans [Member] | Residential Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	1,753	1,969	1,922	1,975
Impaired Financing Receivable, Interest Income, Accrual Method	15	4	17	5
Recorded investment	1,499		1,499		2,136
Unpaid principal balance	1,875		1,875		2,585
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	246	423	238	620
Impaired Financing Receivable, Interest Income, Accrual Method	0	17	0	18
Recorded investment	245		245		298
Unpaid principal balance	260		260		320
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	11	14	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	2,084	4,036	2,245	3,644
Impaired Financing Receivable, Interest Income, Accrual Method	15	21	17	23
Recorded investment	1,829		1,829		2,519
Unpaid principal balance	2,223		2,223		2,993
Acquired Loans [Member] | Real Estate Construction With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	103	0	51	0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	171		171		0
Recorded investment	308		308		0
Unpaid principal balance	308		308		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	1,828	161	1,222	162
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	176		176		0
Recorded investment	1,818		1,818		0
Unpaid principal balance	1,854		1,854		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	1,931	161	1,273	162
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Related allowance	347		347		0
Recorded investment	2,126		2,126		0
Unpaid principal balance	2,162		2,162		0
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	53,897	27,815	45,580	26,294
Impaired Financing Receivable, Interest Income, Accrual Method	144	214	271	345
Related allowance	6,548		6,548		2,432
Recorded investment	52,068		52,068		29,698
Unpaid principal balance	62,808		62,808		39,122
Originated Loans [Member] | Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	2,028	3,957	1,881	3,784
Impaired Financing Receivable, Interest Income, Accrual Method	0	16	1	30
Related allowance	958		958		1,580
Recorded investment	2,019		2,019		8,290
Unpaid principal balance	7,071		7,071		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	6,758	2,268	6,894	2,216
Impaired Financing Receivable, Interest Income, Accrual Method	12	5	34	7
Recorded investment	7,620		7,620		1,848
Unpaid principal balance	10,557		10,557		6,997
Originated Loans [Member] | Real Estate Construction With No 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
Recorded investment	0		0		0
Unpaid principal balance	0		0		0
Originated Loans [Member] | Residential Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	9,836	10,242	10,182	10,577
Impaired Financing Receivable, Interest Income, Accrual Method	80	115	159	186
Recorded investment	9,778		9,778		10,372
Unpaid principal balance	11,770		11,770		12,437
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	17,411	3,034	14,367	3,256
Impaired Financing Receivable, Interest Income, Accrual Method	48	65	67	101
Recorded investment	14,786		14,786		3,015
Unpaid principal balance	15,053		15,053		3,210
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	468	357	457	340
Impaired Financing Receivable, Interest Income, Accrual Method	4	5	6	7
Recorded investment	475		475		406
Unpaid principal balance	845		845		640
Originated Loans [Member] | With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	34,473	15,901	31,900	16,389
Impaired Financing Receivable, Interest Income, Accrual Method	144	190	266	301
Recorded investment	32,659		32,659		15,641
Unpaid principal balance	38,225		38,225		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	254	653	290	571
Impaired Financing Receivable, Interest Income, Accrual Method	0	7	0	12
Related allowance	0		0		1
Recorded investment	254		254		474
Unpaid principal balance	358		358		498
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	17,142	7,304	11,509	5,550
Impaired Financing Receivable, Interest Income, Accrual Method	0	1	4	2
Related allowance	5,590		5,590		851
Recorded investment	17,136		17,136		5,293
Unpaid principal balance	17,154		17,154		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	19,424	11,914	13,680	9,905
Impaired Financing Receivable, Interest Income, Accrual Method	0	$ 24	5	$ 44
Related allowance	6,548		6,548		2,432
Recorded investment	19,409		19,409		14,057
Unpaid principal balance	$ 24,583		$ 24,583		$ 15,838