Loans and Allowance for Credit Losses - Recorded Investment and Unpaid Principal Balance for Impaired Loans with Associated Allowance (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	$ 33,806		$ 34,086
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	44,211		40,124
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	39,444
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	98
Impaired Financing Receivable, with Related Allowance, Recorded Investment	16,575		19,967
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	16,731		25,207
Related allowance	3,107		4,906
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	12,357
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	18
Recorded investment	50,381		54,053
Unpaid principal balance	60,942		65,331
Impaired Financing Receivable, Average Recorded Investment	51,801
Impaired Financing Receivable, Interest Income, Cash Basis Method	116
Commercial, financial, agricultural and other [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	1,780		2,025
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	7,659		2,725
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	2,280
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	11
Impaired Financing Receivable, with Related Allowance, Recorded Investment	8,957		4,210
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	9,038		9,377
Related allowance	2,506		1,268
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	4,895
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	18
Construction
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	54		54
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	53		53
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	54
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0
Related allowance	0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0
Residential real estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	10,814		10,939
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	13,199		13,258
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	10,892
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	63
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0
Related allowance	0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	20,653		20,650
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	22,754		23,641
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	25,739
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	22
Impaired Financing Receivable, with Related Allowance, Recorded Investment	7,618		15,757
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	7,693		15,830
Related allowance	601		3,638
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	7,462
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0
Loans to individuals [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	505		418
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	546		447
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	479
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	2
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0
Related allowance	0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0
Time and Demand Loans | Commercial, financial, agricultural and other [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	1,780		2,025
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	7,659		2,725
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	2,280
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	11
Impaired Financing Receivable, with Related Allowance, Recorded Investment	8,957		4,210
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	9,038		9,377
Related allowance	2,506		1,268
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	4,895
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	18
Senior Lien | Residential real estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	5,856		6,062
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	7,402		7,575
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	5,982
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	45
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0
Related allowance	0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0
Junior Lien | Residential real estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	4,958		4,877
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	5,797		5,683
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	4,910
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	18
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0
Related allowance	0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0
Multifamily | Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	0		1
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	0		82
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	0
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	459		459
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	473		470
Related allowance	115		116
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	464
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0
Nonowner Occupied Commercial Real Estate | Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	18,125		16,786
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	19,933		19,459
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	22,593
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	7
Impaired Financing Receivable, with Related Allowance, Recorded Investment	6,917		15,060
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	6,979		15,122
Related allowance	459		3,508
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	6,917
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0
Owner Occupied Commercial Real Estate | Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	2,528		3,863
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	2,821		4,100
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	3,146
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	15
Impaired Financing Receivable, with Related Allowance, Recorded Investment	242		238
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	241		238
Related allowance	27		14
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	81
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0
Automobiles | Loans to individuals [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	429		405
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	465		430
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	437
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	2
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0
Related allowance	0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0
Other Consumer Loans | Loans to individuals [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	76		13
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	81		17
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	42
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0
Related allowance	0		$ 0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	$ 0
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		$ 15,985
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		107
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		21,277
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		19
Impaired Financing Receivable, Average Recorded Investment		37,262
Impaired Financing Receivable, Interest Income, Cash Basis Method		126
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		925
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		4
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		7,838
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		18
Originated Loans [Member] | Construction
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		0
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0
Originated Loans [Member] | Residential real estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		10,529
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		78
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		325
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0
Originated Loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		4,086
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		22
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		13,114
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		1
Originated Loans [Member] | Loans to individuals [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		445
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		3
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		2,405
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		2
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		616
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0
Impaired Financing Receivable, Average Recorded Investment		3,021
Impaired Financing Receivable, Interest Income, Cash Basis Method		2
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		74
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0
Acquired Loans [Member] | Construction
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		0
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0
Acquired Loans [Member] | Residential real estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		2,090
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		2
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0
Acquired Loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		229
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		616
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0
Acquired Loans [Member] | Loans to individuals [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		12
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		$ 0