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		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	$ 29,732		$ 29,732		$ 34,086
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	41,391		41,391		40,124
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	30,369		31,102
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	201		300
Impaired Financing Receivable, with Related Allowance, Recorded Investment	23,061		23,061		19,967
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	24,739		24,739		25,207
Related allowance	2,946		2,946		4,906
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	23,120		21,543
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	19		36
Recorded investment	52,793		52,793		54,053
Unpaid principal balance	66,130		66,130		65,331
Impaired Financing Receivable, Average Recorded Investment	53,489		52,645
Impaired Financing Receivable, Interest Income, Cash Basis Method	220		336
Commercial, financial, agricultural and other [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	5,598		5,598		2,025
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	14,230		14,230		2,725
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	5,591		3,966
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	31		43
Impaired Financing Receivable, with Related Allowance, Recorded Investment	8,737		8,737		4,210
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	8,889		8,889		9,377
Related allowance	2,358		2,358		1,268
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	8,791		6,813
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	19		36
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	10,029		10,029		10,939
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	12,207		12,207		13,258
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	10,560		10,726
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	159		221
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	13,575		13,575		20,650
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	14,388		14,388		23,641
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	13,689		15,879
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	7		30
Impaired Financing Receivable, with Related Allowance, Recorded Investment	14,324		14,324		15,757
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	15,850		15,850		15,830
Related allowance	588		588		3,638
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	14,329		14,730
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	476		476		418
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	513		513		447
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	475		477
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	4		6
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	5,598		5,598		2,025
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	14,230		14,230		2,725
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	5,591		3,966
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	31		43
Impaired Financing Receivable, with Related Allowance, Recorded Investment	8,737		8,737		4,210
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	8,889		8,889		9,377
Related allowance	2,358		2,358		1,268
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	8,791		6,813
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	19		36
Senior Lien | Residential real estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	5,364		5,364		6,062
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	6,746		6,746		7,575
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	5,661		5,822
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	132		176
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	4,665		4,665		4,877
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	5,461		5,461		5,683
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	4,899		4,904
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	27		45
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	445		445		459
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	463		463		470
Related allowance	113		113		116
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	450		457
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	11,134		11,134		16,786
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	11,621		11,621		19,459
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	11,227		13,079
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	2		10
Impaired Financing Receivable, with Related Allowance, Recorded Investment	13,614		13,614		15,060
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	15,122		15,122		15,122
Related allowance	475		475		3,508
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	13,614		14,096
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	2,441		2,441		3,863
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	2,767		2,767		4,100
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	2,462		2,800
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	5		20
Impaired Financing Receivable, with Related Allowance, Recorded Investment	265		265		238
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	265		265		238
Related allowance	0		0		14
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	265		177
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	419		419		405
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	452		452		430
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	415		426
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	4		6
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	57		57		13
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	61		61		17
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	60		51
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		$ 34,473		$ 31,900
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		144		266
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		19,424		13,680
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0		5
Impaired Financing Receivable, Average Recorded Investment		53,897		45,580
Impaired Financing Receivable, Interest Income, Cash Basis Method		144		271
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		6,758		6,894
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		12		34
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		2,028		1,881
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0		1
Originated Loans [Member] | Construction
Financing Receivable, Impaired [Line Items]
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, 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		9,836		10,182
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		80		159
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		254		290
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		17,411		14,367
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		48		67
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		17,142		11,509
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0		4
Originated Loans [Member] | Loans to individuals [Member]
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		468		457
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		4		6
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		2,084		2,245
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		15		17
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		1,931		1,273
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0		0
Impaired Financing Receivable, Average Recorded Investment		4,015		3,518
Impaired Financing Receivable, Interest Income, Cash Basis Method		15		17
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		0		0
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		103		51
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		1,753		1,922
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		15		17
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		246		238
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method		0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		1,828		1,222
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