Loans and Allowance for Credit Losses - Allowance for Credit Losses (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
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 97,038		$ 101,309
Charge-offs	(2,906)		(11,510)
Recoveries	629		2,036
Provision for credit losses	(576)		2,350
Ending Balance	94,185	$ 88,307	94,185	$ 88,307	$ 101,309
Ending balance: individually evaluated for impaired	980	7,418	980	7,418
Ending balance: collectively evaluated for impaired	93,205	80,889	93,205	80,889
Loans and Leases Receivable, Net of Deferred Income	(6,732,580)	(6,949,716)	(6,732,580)	(6,949,716)	(6,761,183)
Ending balance: individually evaluated for impaired	26,148	36,969	26,148	36,969
Ending balance: collectively evaluated for impaired	6,706,432	6,912,747	6,706,432	6,912,747
Provision for credit losses	330	11,212	1,353	49,038
Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance		25,488		25,488
Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance		3,508		3,508
Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance		8,252		8,252
Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance		42,244		42,244
Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance		8,815		8,815
Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	21,466		17,187
Charge-offs	(2,066)		(6,522)
Recoveries	106		299
Provision for credit losses	(1,644)		6,898
Ending Balance	17,862	25,488	17,862	25,488	17,187
Ending balance: individually evaluated for impaired	479	1,497	479	1,497
Ending balance: collectively evaluated for impaired	17,383	23,991	17,383	23,991
Loans and Leases Receivable, Net of Deferred Income	(1,224,591)	(1,736,736)	(1,224,591)	(1,736,736)	(1,555,986)
Ending balance: individually evaluated for impaired	5,561	5,048	5,561	5,048
Ending balance: collectively evaluated for impaired	1,219,030	1,731,688	1,219,030	1,731,688
Commercial, financial, agricultural and other [Member] | Time and Demand Loans
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	21,098		16,838
Charge-offs	(2,059)		(6,397)
Recoveries	104		292
Provision for credit losses	(1,561)		6,849
Ending Balance	17,582		17,582		16,838
Ending balance: individually evaluated for impaired	479		479
Ending balance: collectively evaluated for impaired	17,103		17,103
Loans and Leases Receivable, Net of Deferred Income	(1,210,694)		(1,210,694)		(1,541,382)
Ending balance: individually evaluated for impaired	5,561		5,561
Ending balance: collectively evaluated for impaired	1,205,133		1,205,133
Commercial, financial, agricultural and other [Member] | Commercial Credit Cards
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	368		349
Charge-offs	(7)		(125)
Recoveries	2		7
Provision for credit losses	(83)		49
Ending Balance	280		280		349
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	280		280
Loans and Leases Receivable, Net of Deferred Income	(13,897)		(13,897)		(14,604)
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	13,897		13,897
Construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	4,285		7,966
Charge-offs	0		0
Recoveries	0		135
Provision for credit losses	48		(3,768)
Ending Balance	4,333		4,333		7,966
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	4,333		4,333
Loans and Leases Receivable, Net of Deferred Income	(424,016)		(424,016)
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	424,016		424,016
Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	12,933		14,358
Charge-offs	(11)		(130)
Recoveries	98		309
Provision for credit losses	(215)		(1,732)
Ending Balance	12,805	8,252	12,805	8,252	14,358
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	12,805	8,252	12,805	8,252
Loans and Leases Receivable, Net of Deferred Income	(1,875,375)	(1,744,020)	(1,875,375)	(1,744,020)	(1,750,592)
Ending balance: individually evaluated for impaired	254	1,226	254	1,226
Ending balance: collectively evaluated for impaired	1,875,121	1,742,794	1,875,121	1,742,794
Residential real estate [Member] | Senior Lien
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	7,394		7,919
Charge-offs	0		(36)
Recoveries	24		206
Provision for credit losses	39		(632)
Ending Balance	7,457		7,457		7,919
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	7,457		7,457
Loans and Leases Receivable, Net of Deferred Income	(1,254,142)		(1,254,142)		(1,144,323)
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	1,254,142		1,254,142
Residential real estate [Member] | Junior Lien
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	5,539		6,439
Charge-offs	(11)		(94)
Recoveries	74		103
Provision for credit losses	(254)		(1,100)
Ending Balance	5,348		5,348		6,439
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	5,348		5,348
Loans and Leases Receivable, Net of Deferred Income	(621,233)		(621,233)		(606,269)
Ending balance: individually evaluated for impaired	254		254
Ending balance: collectively evaluated for impaired	620,979		620,979
Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	35,795		41,953
Charge-offs	(102)		(1,659)
Recoveries	90		130
Provision for credit losses	(1,143)		(5,784)
Ending Balance	34,640	42,244	34,640	42,244	41,953
Ending balance: individually evaluated for impaired	501	5,921	501	5,921
Ending balance: collectively evaluated for impaired	34,139	36,323	34,139	36,323
Loans and Leases Receivable, Net of Deferred Income	(2,231,890)	(2,215,311)	(2,231,890)	(2,215,311)	(2,211,569)
Ending balance: individually evaluated for impaired	20,333	30,387	20,333	30,387
Ending balance: collectively evaluated for impaired	2,211,557	2,184,924	2,211,557	2,184,924
Commercial Real Estate [Member] | Multifamily
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	4,379		6,240
Charge-offs	0		(1)
Recoveries	0		0
Provision for credit losses	(548)		(2,408)
Ending Balance	3,831		3,831		6,240
Ending balance: individually evaluated for impaired	103		103
Ending balance: collectively evaluated for impaired	3,728		3,728
Loans and Leases Receivable, Net of Deferred Income	(405,720)		(405,720)		(371,239)
Ending balance: individually evaluated for impaired	435		435
Ending balance: collectively evaluated for impaired	405,285		405,285
Commercial Real Estate [Member] | Nonowner Occupied Commercial Real Estate
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	26,911		28,414
Charge-offs	0		(1,556)
Recoveries	80		120
Provision for credit losses	(893)		(880)
Ending Balance	26,098		26,098		28,414
Ending balance: individually evaluated for impaired	338		338
Ending balance: collectively evaluated for impaired	25,760		25,760
Loans and Leases Receivable, Net of Deferred Income	(1,442,845)		(1,442,845)		(1,421,151)
Ending balance: individually evaluated for impaired	18,314		18,314
Ending balance: collectively evaluated for impaired	1,424,531		1,424,531
Commercial Real Estate [Member] | Owner Occupied Commercial Real Estate
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	4,505		7,299
Charge-offs	(102)		(102)
Recoveries	10		10
Provision for credit losses	298		(2,496)
Ending Balance	4,711		4,711		7,299
Ending balance: individually evaluated for impaired	60		60
Ending balance: collectively evaluated for impaired	4,651		4,651
Loans and Leases Receivable, Net of Deferred Income	(383,325)		(383,325)		(419,179)
Ending balance: individually evaluated for impaired	1,584		1,584
Ending balance: collectively evaluated for impaired	381,741		381,741
Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	22,559		19,845
Charge-offs	(727)		(3,199)
Recoveries	335		1,163
Provision for credit losses	2,378		6,736
Ending Balance	24,545	8,815	24,545	8,815	19,845
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	24,545	8,815	24,545	8,815
Loans and Leases Receivable, Net of Deferred Income	(976,708)	(800,660)	(976,708)	(800,660)	(815,815)
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	976,708	800,660	976,708	800,660
Loans to individuals [Member] | Automobiles
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	19,298		16,133
Charge-offs	(331)		(1,399)
Recoveries	251		826
Provision for credit losses	1,984		5,642
Ending Balance	21,202		21,202		16,133
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	21,202		21,202
Loans and Leases Receivable, Net of Deferred Income	(886,367)		(886,367)		(712,800)
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	886,367		886,367
Loans to individuals [Member] | Consumer Credit Cards
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	528		635
Charge-offs	(125)		(372)
Recoveries	9		51
Provision for credit losses	84		182
Ending Balance	496		496		635
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	496		496
Loans and Leases Receivable, Net of Deferred Income	(10,617)		(10,617)		(12,360)
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	10,617		10,617
Loans to individuals [Member] | Other Consumer Loans
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	2,733		3,077
Charge-offs	(271)		(1,428)
Recoveries	75		286
Provision for credit losses	310		912
Ending Balance	2,847		2,847		3,077
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	2,847		2,847
Loans and Leases Receivable, Net of Deferred Income	(79,724)		(79,724)		(90,655)
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	79,724		79,724
Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance		3,508		3,508
Ending balance: individually evaluated for impaired		0		0
Ending balance: collectively evaluated for impaired		3,508		3,508
Loans and Leases Receivable, Net of Deferred Income	$ (424,016)	(452,989)	$ (424,016)	(452,989)	$ (427,221)
Ending balance: individually evaluated for impaired		308		308
Ending balance: collectively evaluated for impaired		452,681		452,681
Acquired Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		1,859		52
Charge-offs		(202)		(502)
Recoveries		28		76
Provision for credit losses				2,352
Ending Balance		1,978		1,978
Provision for credit losses		293
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		332
Charge-offs		0
Recoveries		13
Ending Balance		336		336
Provision for credit losses		(9)
Acquired Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		171
Charge-offs		0
Recoveries		0
Ending Balance		0		0
Provision for credit losses		(171)
Acquired Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		0
Charge-offs		(122)
Recoveries		13
Ending Balance		0		0
Provision for credit losses		109
Acquired Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		1,356
Charge-offs		0
Recoveries		0
Ending Balance		1,642		1,642
Provision for credit losses		286
Acquired Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		0
Charge-offs		(80)
Recoveries		2
Ending Balance		0		0
Provision for credit losses		78
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				13
Charge-offs				0
Recoveries				28
Provision for credit losses				295
Ending Balance		336		336
Acquired Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				2
Charge-offs				(213)
Recoveries				38
Provision for credit losses				173
Ending Balance		0		0
Acquired Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				37
Charge-offs				(2)
Recoveries				0
Provision for credit losses				1,607
Ending Balance		1,642		1,642
Acquired Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				0
Charge-offs				(287)
Recoveries				10
Provision for credit losses				277
Ending Balance		0		0
Acquired Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				0
Charge-offs				0
Recoveries				0
Provision for credit losses				0
Ending Balance		0		0
Originated Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		79,582		51,585
Charge-offs		(4,705)		(13,259)
Recoveries		533		1,317
Provision for credit losses				46,686
Ending Balance		86,329		86,329
Provision for credit losses		10,919
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		24,812
Charge-offs		(3,395)
Recoveries		44
Ending Balance		25,152		25,152
Provision for credit losses		3,691
Originated Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		3,067
Charge-offs		0
Recoveries		0
Ending Balance		3,508		3,508
Provision for credit losses		441
Originated Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		9,239
Charge-offs		(161)
Recoveries		153
Ending Balance		8,252		8,252
Provision for credit losses		(979)
Originated Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		33,130
Charge-offs		0
Recoveries		110
Ending Balance		40,602		40,602
Provision for credit losses		7,362
Originated Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		9,334
Charge-offs		(1,149)
Recoveries		226
Ending Balance		8,815		8,815
Provision for credit losses		404
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				20,221
Charge-offs				(5,166)
Recoveries				161
Provision for credit losses				9,936
Ending Balance		25,152		25,152
Originated Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				4,091
Charge-offs				(720)
Recoveries				274
Provision for credit losses				4,607
Ending Balance		8,252		8,252
Originated Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				19,731
Charge-offs				(2,415)
Recoveries				154
Provision for credit losses				23,132
Ending Balance		40,602		40,602
Originated Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				4,984
Charge-offs				(4,958)
Recoveries				702
Provision for credit losses				8,087
Ending Balance		8,815		8,815
Originated Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				2,558
Charge-offs				0
Recoveries				26
Provision for credit losses				924
Ending Balance		$ 3,508		$ 3,508