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, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Impaired [Line Items]
Recorded Investment	$ 54,053
Unpaid Principal Balance	65,331
Related Allowance	4,906
Average Recorded Investment	50,728
Interest Income Recognized	639
Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	2,025
Unpaid Principal Balance	2,725
Average Recorded Investment	6,371
Interest Income Recognized	80
Commercial Financial Agricultural And Other With No Related Allowance [Member] | Time and demand loans
Financing Receivable, Impaired [Line Items]
Recorded Investment	2,025
Unpaid Principal Balance	2,725
Average Recorded Investment	6,371
Interest Income Recognized	80
Real Estate Construction With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	54
Unpaid Principal Balance	53
Average Recorded Investment	146
Interest Income Recognized	14
Residential real estate with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	10,939
Unpaid Principal Balance	13,258
Average Recorded Investment	11,913
Interest Income Recognized	335
Residential real estate with no related allowance recorded [Member] | Senior Lien
Financing Receivable, Impaired [Line Items]
Recorded Investment	6,062
Unpaid Principal Balance	7,575
Average Recorded Investment	6,605
Interest Income Recognized	241
Residential real estate with no related allowance recorded [Member] | Junior Lien
Financing Receivable, Impaired [Line Items]
Recorded Investment	4,877
Unpaid Principal Balance	5,683
Average Recorded Investment	5,308
Interest Income Recognized	94
Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	20,650
Unpaid Principal Balance	23,641
Average Recorded Investment	22,287
Interest Income Recognized	184
Commercial Real Estate With No Related Allowance [Member] | Multifamily
Financing Receivable, Impaired [Line Items]
Recorded Investment	1
Unpaid Principal Balance	82
Average Recorded Investment	1
Interest Income Recognized	0
Commercial Real Estate With No Related Allowance [Member] | Nonowner Occupied Commercial Real Estate
Financing Receivable, Impaired [Line Items]
Recorded Investment	16,786
Unpaid Principal Balance	19,459
Average Recorded Investment	18,536
Interest Income Recognized	82
Commercial Real Estate With No Related Allowance [Member] | Owner Occupied Commercial Real Estate
Financing Receivable, Impaired [Line Items]
Recorded Investment	3,863
Unpaid Principal Balance	4,100
Average Recorded Investment	3,750
Interest Income Recognized	102
Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	418
Unpaid Principal Balance	447
Average Recorded Investment	470
Interest Income Recognized	13
Loans To Individuals With No Related Allowance [Member] | Automobiles
Financing Receivable, Impaired [Line Items]
Recorded Investment	405
Unpaid Principal Balance	430
Average Recorded Investment	447
Interest Income Recognized	13
Loans To Individuals With No Related Allowance [Member] | Other consumer loans
Financing Receivable, Impaired [Line Items]
Recorded Investment	13
Unpaid Principal Balance	17
Average Recorded Investment	23
Interest Income Recognized	0
Subtotal with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	34,086
Unpaid Principal Balance	40,124
Average Recorded Investment	41,187
Interest Income Recognized	626
Commercial, financial, agricultural and other With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	4,210
Unpaid Principal Balance	9,377
Related Allowance	1,268
Average Recorded Investment	1,544
Interest Income Recognized	3
Commercial, financial, agricultural and other With an allowance recorded [Member] | Time and demand loans
Financing Receivable, Impaired [Line Items]
Recorded Investment	4,210
Unpaid Principal Balance	9,377
Related Allowance	1,268
Average Recorded Investment	1,544
Interest Income Recognized	3
Real estate construction with an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	0
Unpaid Principal Balance	0
Related Allowance	0
Average Recorded Investment	0
Interest Income Recognized	0
Residential real estate With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	0
Unpaid Principal Balance	0
Related Allowance	0
Average Recorded Investment	0
Interest Income Recognized	0
Residential real estate With an allowance recorded [Member] | Senior Lien
Financing Receivable, Impaired [Line Items]
Recorded Investment	0
Unpaid Principal Balance	0
Related Allowance	0
Average Recorded Investment	0
Interest Income Recognized	0
Residential real estate With an allowance recorded [Member] | Junior Lien
Financing Receivable, Impaired [Line Items]
Recorded Investment	0
Unpaid Principal Balance	0
Related Allowance	0
Average Recorded Investment	0
Interest Income Recognized	0
Commercial real estate With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	15,757
Unpaid Principal Balance	15,830
Related Allowance	3,638
Average Recorded Investment	7,997
Interest Income Recognized	10
Commercial real estate With an allowance recorded [Member] | Multifamily
Financing Receivable, Impaired [Line Items]
Recorded Investment	459
Unpaid Principal Balance	470
Related Allowance	116
Average Recorded Investment	395
Interest Income Recognized	0
Commercial real estate With an allowance recorded [Member] | Nonowner Occupied Commercial Real Estate
Financing Receivable, Impaired [Line Items]
Recorded Investment	15,060
Unpaid Principal Balance	15,122
Related Allowance	3,508
Average Recorded Investment	7,363
Interest Income Recognized	0
Commercial real estate With an allowance recorded [Member] | Owner Occupied Commercial Real Estate
Financing Receivable, Impaired [Line Items]
Recorded Investment	238
Unpaid Principal Balance	238
Related Allowance	14
Average Recorded Investment	239
Interest Income Recognized	10
Loans to individuals With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	0
Unpaid Principal Balance	0
Related Allowance	0
Average Recorded Investment	0
Interest Income Recognized	0
Loans to individuals With an allowance recorded [Member] | Automobiles
Financing Receivable, Impaired [Line Items]
Recorded Investment	0
Unpaid Principal Balance	0
Related Allowance	0
Average Recorded Investment	0
Interest Income Recognized	0
Loans to individuals With an allowance recorded [Member] | Other consumer loans
Financing Receivable, Impaired [Line Items]
Recorded Investment	0
Unpaid Principal Balance	0
Related Allowance	0
Average Recorded Investment	0
Interest Income Recognized	0
Subtotal With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	19,967
Unpaid Principal Balance	25,207
Related Allowance	4,906
Average Recorded Investment	9,541
Interest Income Recognized	$ 13
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		$ 29,698
Unpaid Principal Balance		39,122
Related Allowance		2,432
Average Recorded Investment		27,154	$ 41,386
Interest Income Recognized		684	1,098
Originated Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		1,848
Unpaid Principal Balance		6,997
Average Recorded Investment		2,411	18,480
Interest Income Recognized		66	602
Originated Loans [Member] | Real Estate Construction With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		0
Unpaid Principal Balance		0
Average Recorded Investment		0	0
Interest Income Recognized		0	0
Originated Loans [Member] | Residential real estate with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		10,372
Unpaid Principal Balance		12,437
Average Recorded Investment		10,819	10,651
Interest Income Recognized		365	271
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		3,015
Unpaid Principal Balance		3,210
Average Recorded Investment		7,455	7,919
Interest Income Recognized		156	177
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		406
Unpaid Principal Balance		640
Average Recorded Investment		371	310
Interest Income Recognized		17	11
Originated Loans [Member] | Subtotal with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		15,641
Unpaid Principal Balance		23,284
Average Recorded Investment		21,056	37,360
Interest Income Recognized		604	1,061
Originated Loans [Member] | Commercial, financial, agricultural and other With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		8,290
Unpaid Principal Balance		10,032
Related Allowance		1,580
Average Recorded Investment		4,110	2,531
Interest Income Recognized		77	20
Originated Loans [Member] | Real estate construction with an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		0
Unpaid Principal Balance		0
Related Allowance		0
Average Recorded Investment		0	0
Interest Income Recognized		0	0
Originated Loans [Member] | Residential real estate With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		474
Unpaid Principal Balance		498
Related Allowance		1
Average Recorded Investment		241	504
Interest Income Recognized		0	13
Originated Loans [Member] | Commercial real estate With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		5,293
Unpaid Principal Balance		5,308
Related Allowance		851
Average Recorded Investment		1,747	991
Interest Income Recognized		3	4
Originated Loans [Member] | Loans to individuals With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		0
Unpaid Principal Balance		0
Related Allowance		0
Average Recorded Investment		0	0
Interest Income Recognized		0	0
Originated Loans [Member] | Subtotal With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		14,057
Unpaid Principal Balance		15,838
Related Allowance		2,432
Average Recorded Investment		6,098	4,026
Interest Income Recognized		80	37
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		2,519
Unpaid Principal Balance		2,993
Related Allowance		0
Average Recorded Investment		5,225	3,712
Interest Income Recognized		26	15
Acquired Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		73
Unpaid Principal Balance		73
Average Recorded Investment		2,479	214
Interest Income Recognized		0	10
Acquired Loans [Member] | Real Estate Construction With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		0
Unpaid Principal Balance		0
Average Recorded Investment		0	0
Interest Income Recognized		0	0
Acquired Loans [Member] | Residential real estate with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		2,136
Unpaid Principal Balance		2,585
Average Recorded Investment		1,986	1,906
Interest Income Recognized		8	5
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		298
Unpaid Principal Balance		320
Average Recorded Investment		747	1,565
Interest Income Recognized		18	0
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		12
Unpaid Principal Balance		15
Average Recorded Investment		13	16
Interest Income Recognized		0	0
Acquired Loans [Member] | Subtotal with no related allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		2,519
Unpaid Principal Balance		2,993
Average Recorded Investment		5,225	3,701
Interest Income Recognized		26	15
Acquired Loans [Member] | Commercial, financial, agricultural and other With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		0
Unpaid Principal Balance		0
Related Allowance		0
Average Recorded Investment		0	11
Interest Income Recognized		0	0
Acquired Loans [Member] | Real estate construction with an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		0
Unpaid Principal Balance		0
Related Allowance		0
Average Recorded Investment		0	0
Interest Income Recognized		0	0
Acquired Loans [Member] | Residential real estate With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		0
Unpaid Principal Balance		0
Related Allowance		0
Average Recorded Investment		0	0
Interest Income Recognized		0	0
Acquired Loans [Member] | Commercial real estate With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		0
Unpaid Principal Balance		0
Related Allowance		0
Average Recorded Investment		0	0
Interest Income Recognized		0	0
Acquired Loans [Member] | Loans to individuals With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		0
Unpaid Principal Balance		0
Related Allowance		0
Average Recorded Investment		0	0
Interest Income Recognized		0	0
Acquired Loans [Member] | Subtotal With an allowance recorded [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment		0
Unpaid Principal Balance		0
Related Allowance		0
Average Recorded Investment		0	11
Interest Income Recognized		$ 0	$ 0