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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	$ 28,474		$ 28,474		$ 34,086
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	41,980		41,980		40,124
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	36,443		41,123
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	131		438
Impaired Financing Receivable, with Related Allowance, Recorded Investment	9,612		9,612		19,967
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	9,692		9,692		25,207
Related allowance	980		980		4,906
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	9,318		9,228
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	19		50
Recorded investment	38,086		38,086		54,053
Unpaid principal balance	51,672		51,672		65,331
Impaired Financing Receivable, Average Recorded Investment	45,761		50,351
Impaired Financing Receivable, Interest Income, Cash Basis Method	150		488
Commercial, financial, agricultural and other [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	4,836		4,836		2,025
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	15,308		15,308		2,725
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	9,858		9,266
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	3		50
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,899		1,899		4,210
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	1,898		1,898		9,377
Related allowance	479		479		1,268
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	1,842		1,820
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	19		50
Construction
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	54		54		54
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	53		53		53
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	54		54
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0		0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0		0
Related allowance	0		0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0		0
Residential real estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	9,910		9,910		10,939
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	12,101		12,101		13,258
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	10,060		10,504
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	54		275
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0		0
Related allowance	0		0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0		0
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	13,176		13,176		20,650
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	13,981		13,981		23,641
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	15,967		20,813
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	68		101
Impaired Financing Receivable, with Related Allowance, Recorded Investment	7,713		7,713		15,757
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	7,794		7,794		15,830
Related allowance	501		501		3,638
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	7,476		7,408
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0		0
Loans to individuals [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	498		498		418
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	537		537		447
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	504		486
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	6		12
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0		0
Related allowance	0		0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0		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	4,836		4,836		2,025
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	15,308		15,308		2,725
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	9,858		9,266
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	3		50
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,899		1,899		4,210
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	1,898		1,898		9,377
Related allowance	479		479		1,268
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	1,842		1,820
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	19		50
Senior Lien | Residential real estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	5,599		5,599		6,062
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	7,003		7,003		7,575
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	5,558		5,734
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	34		210
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0		0
Related allowance	0		0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0		0
Junior Lien | Residential real estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	4,311		4,311		4,877
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	5,098		5,098		5,683
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	4,502		4,770
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	20		65
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0		0
Related allowance	0		0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0		0
Multifamily | Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	0		0		1
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	0		0		82
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	0		0
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0		0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	435		435		459
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	456		456		470
Related allowance	103		103		116
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	439		451
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0		0
Nonowner Occupied Commercial Real Estate | Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	11,531		11,531		16,786
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	12,147		12,147		19,459
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	13,633		18,050
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	4		15
Impaired Financing Receivable, with Related Allowance, Recorded Investment	6,917		6,917		15,060
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	6,979		6,979		15,122
Related allowance	338		338		3,508
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	6,917		6,917
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0		0
Owner Occupied Commercial Real Estate | Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	1,645		1,645		3,863
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	1,834		1,834		4,100
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	2,334		2,763
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	64		86
Impaired Financing Receivable, with Related Allowance, Recorded Investment	361		361		238
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	359		359		238
Related allowance	60		60		14
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	120		40
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0		0
Automobiles | Loans to individuals [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	442		442		405
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	476		476		430
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	448		433
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	6		12
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0		0
Related allowance	0		0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0		0
Other Consumer Loans | Loans to individuals [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	56		56		13
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	61		61		17
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	56		53
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0		0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0		0
Related allowance	0		0		$ 0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	$ 0		$ 0
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		$ 26,940		$ 28,911
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		108		349
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		23,830		18,399
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		19		54
Impaired Financing Receivable, Average Recorded Investment		50,770		47,310
Impaired Financing Receivable, Interest Income, Cash Basis Method		127		403
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		1,918		4,062
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		10		15
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		6,423		4,569
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		16		45
Originated Loans [Member] | Construction
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		18		6
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0		0
Originated Loans [Member] | Residential real estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		10,198		10,380
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		72		235
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0		0
Originated Loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		14,312		13,994
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		23		90
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		17,407		13,830
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		3		9
Originated Loans [Member] | Loans to individuals [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		494		469
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		3		9
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0		0
Acquired Loans [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		3,306		3,447
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		77		94
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0		0
Impaired Financing Receivable, Average Recorded Investment		3,306		3,447
Impaired Financing Receivable, Interest Income, Cash Basis Method		77		94
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		74
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0		0
Acquired Loans [Member] | Construction
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		308		137
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0		0
Acquired Loans [Member] | Residential real estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		1,473		1,772
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		1		18
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0		0
Acquired Loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		1,440		1,453
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		76		76
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0		0
Acquired Loans [Member] | Loans to individuals [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		11		11
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0		0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		$ 0		$ 0