Loans and Allowance for Credit Losses - Allowance for Credit Losses (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
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 96,763		$ 101,309
Charge-offs	(4,839)		(8,604)
Recoveries	912		1,407
Provision for credit losses	4,202		2,926
Ending Balance	97,038	$ 81,441	97,038	$ 81,441	$ 101,309
Ending balance: individually evaluated for impaired	2,946	6,895	2,946	6,895
Ending balance: collectively evaluated for impaired	94,092	74,546	94,092	74,546
Total	6,740,535	6,922,075	6,740,535	6,922,075	6,761,183
Ending balance: individually evaluated for impaired	41,328	37,679	41,328	37,679
Ending balance: collectively evaluated for impaired	6,699,207	6,884,396	6,699,207	6,884,396
Provision for credit losses	5,413	6,859	1,023	37,826
Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance		25,144		25,144
Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance		3,238		3,238
Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance		9,239		9,239
Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance		34,486		34,486
Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance		9,334		9,334
Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	21,801		17,187
Charge-offs	(3,887)		(4,456)
Recoveries	103		193
Provision for credit losses	3,449		8,542
Ending Balance	21,466	25,144	21,466	25,144	17,187
Ending balance: individually evaluated for impaired	2,358	958	2,358	958
Ending balance: collectively evaluated for impaired	19,108	24,186	19,108	24,186
Total	1,374,177	1,773,099	1,374,177	1,773,099	1,555,986
Ending balance: individually evaluated for impaired	13,688	3,336	13,688	3,336
Ending balance: collectively evaluated for impaired	1,360,489	1,769,763	1,360,489	1,769,763
Commercial, financial, agricultural and other [Member] | Time and Demand Loans
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	21,427		16,838
Charge-offs	(3,878)		(4,338)
Recoveries	99		188
Provision for credit losses	3,450		8,410
Ending Balance	21,098		21,098		16,838
Ending balance: individually evaluated for impaired	2,358		2,358
Ending balance: collectively evaluated for impaired	18,740		18,740
Total	1,360,065		1,360,065		1,541,382
Ending balance: individually evaluated for impaired	13,688		13,688
Ending balance: collectively evaluated for impaired	1,346,377		1,346,377
Commercial, financial, agricultural and other [Member] | Commercial Credit Cards
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	374		349
Charge-offs	(9)		(118)
Recoveries	4		5
Provision for credit losses	(1)		132
Ending Balance	368		368		349
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	368		368
Total	14,112		14,112		14,604
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	14,112		14,112
Construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	4,021		7,966
Charge-offs	0		0
Recoveries	135		135
Provision for credit losses	129		(3,816)
Ending Balance	4,285		4,285		7,966
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	4,285		4,285
Total	414,816		414,816
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	414,816		414,816
Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	12,829		14,358
Charge-offs	(14)		(119)
Recoveries	174		211
Provision for credit losses	(56)		(1,517)
Ending Balance	12,933	9,239	12,933	9,239	14,358
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	12,933	9,239	12,933	9,239
Total	1,828,783	1,723,288	1,828,783	1,723,288	1,750,592
Ending balance: individually evaluated for impaired	541	1,498	541	1,498
Ending balance: collectively evaluated for impaired	1,828,242	1,721,790	1,828,242	1,721,790
Residential real estate [Member] | Senior Lien
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	7,227		7,919
Charge-offs	(13)		(36)
Recoveries	159		182
Provision for credit losses	21		(671)
Ending Balance	7,394		7,394		7,919
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	7,394		7,394
Total	1,218,300		1,218,300		1,144,323
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	1,218,300		1,218,300
Residential real estate [Member] | Junior Lien
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	5,602		6,439
Charge-offs	(1)		(83)
Recoveries	15		29
Provision for credit losses	(77)		(846)
Ending Balance	5,539		5,539		6,439
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	5,539		5,539
Total	610,483		610,483		606,269
Ending balance: individually evaluated for impaired	541		541
Ending balance: collectively evaluated for impaired	609,942		609,942
Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	37,668		41,953
Charge-offs	(7)		(1,557)
Recoveries	1		40
Provision for credit losses	(1,867)		(4,641)
Ending Balance	35,795	34,486	35,795	34,486	41,953
Ending balance: individually evaluated for impaired	588	5,766	588	5,766
Ending balance: collectively evaluated for impaired	35,207	28,720	35,207	28,720
Total	2,205,758	2,224,710	2,205,758	2,224,710	2,211,569
Ending balance: individually evaluated for impaired	27,099	32,537	27,099	32,537
Ending balance: collectively evaluated for impaired	2,178,659	2,192,173	2,178,659	2,192,173
Commercial Real Estate [Member] | Multifamily
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	4,251		6,240
Charge-offs	0		(1)
Recoveries	0		0
Provision for credit losses	128		(1,860)
Ending Balance	4,379		4,379		6,240
Ending balance: individually evaluated for impaired	113		113
Ending balance: collectively evaluated for impaired	4,266		4,266
Total	385,905		385,905		371,239
Ending balance: individually evaluated for impaired	445		445
Ending balance: collectively evaluated for impaired	385,460		385,460
Commercial Real Estate [Member] | Nonowner Occupied Commercial Real Estate
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	27,889		28,414
Charge-offs	(7)		(1,556)
Recoveries	1		40
Provision for credit losses	(972)		13
Ending Balance	26,911		26,911		28,414
Ending balance: individually evaluated for impaired	475		475
Ending balance: collectively evaluated for impaired	26,436		26,436
Total	1,429,192		1,429,192		1,421,151
Ending balance: individually evaluated for impaired	24,624		24,624
Ending balance: collectively evaluated for impaired	1,404,568		1,404,568
Commercial Real Estate [Member] | Owner Occupied Commercial Real Estate
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	5,528		7,299
Charge-offs	0		0
Recoveries	0		0
Provision for credit losses	(1,023)		(2,794)
Ending Balance	4,505		4,505		7,299
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	4,505		4,505
Total	390,661		390,661		419,179
Ending balance: individually evaluated for impaired	2,030		2,030
Ending balance: collectively evaluated for impaired	388,631		388,631
Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	20,444		19,845
Charge-offs	(931)		(2,472)
Recoveries	499		828
Provision for credit losses	2,547		4,358
Ending Balance	22,559	9,334	22,559	9,334	19,845
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	22,559	9,334	22,559	9,334
Total	917,001	784,649	917,001	784,649	815,815
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	917,001	784,649	917,001	784,649
Loans to individuals [Member] | Automobiles
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	16,888		16,133
Charge-offs	(388)		(1,068)
Recoveries	394		575
Provision for credit losses	2,404		3,658
Ending Balance	19,298		19,298		16,133
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	19,298		19,298
Total	829,150		829,150		712,800
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	829,150		829,150
Loans to individuals [Member] | Consumer Credit Cards
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	689		635
Charge-offs	(79)		(247)
Recoveries	25		42
Provision for credit losses	(107)		98
Ending Balance	528		528		635
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	528		528
Total	10,834		10,834		12,360
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	10,834		10,834
Loans to individuals [Member] | Other Consumer Loans
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	2,867		3,077
Charge-offs	(464)		(1,157)
Recoveries	80		211
Provision for credit losses	250		602
Ending Balance	2,733		2,733		3,077
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	2,733		2,733
Total	77,017		77,017		90,655
Ending balance: individually evaluated for impaired	0		0
Ending balance: collectively evaluated for impaired	77,017		77,017
Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance		3,238		3,238
Ending balance: individually evaluated for impaired		171		171
Ending balance: collectively evaluated for impaired		3,067		3,067
Total	$ 414,816	416,329	$ 414,816	416,329	$ 427,221
Ending balance: individually evaluated for impaired		308		308
Ending balance: collectively evaluated for impaired		416,021		416,021
Acquired Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		1,588		52
Charge-offs		(138)		(300)
Recoveries		15		48
Provision for credit losses				2,059
Ending Balance		1,859		1,859
Provision for credit losses		394
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		350
Charge-offs		0
Recoveries		2
Ending Balance		332		332
Provision for credit losses		(20)
Acquired Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		0
Charge-offs		0
Recoveries		0
Ending Balance		171		171
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		(66)
Recoveries		12
Ending Balance		0		0
Provision for credit losses		54
Acquired Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		1,238
Charge-offs		(1)
Recoveries		0
Ending Balance		1,356		1,356
Provision for credit losses		119
Acquired Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		0
Charge-offs		(71)
Recoveries		1
Ending Balance		0		0
Provision for credit losses		70
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				13
Charge-offs				0
Recoveries				15
Provision for credit losses				304
Ending Balance		332		332
Acquired Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				2
Charge-offs				(91)
Recoveries				25
Provision for credit losses				64
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				1,321
Ending Balance		1,356		1,356
Acquired Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				0
Charge-offs				(207)
Recoveries				8
Provision for credit losses				199
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				171
Ending Balance		171		171
Originated Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		77,487		51,585
Charge-offs		(4,768)		(8,554)
Recoveries		398		784
Provision for credit losses				35,767
Ending Balance		79,582		79,582
Provision for credit losses		6,465
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		27,378
Charge-offs		(1,285)
Recoveries		49
Ending Balance		24,812		24,812
Provision for credit losses		(1,330)
Originated Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		2,852
Charge-offs		0
Recoveries		26
Ending Balance		3,067		3,067
Provision for credit losses		189
Originated Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		7,724
Charge-offs		(7)
Recoveries		59
Ending Balance		9,239		9,239
Provision for credit losses		1,463
Originated Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		31,265
Charge-offs		(2,150)
Recoveries		0
Ending Balance		33,130		33,130
Provision for credit losses		4,015
Originated Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		8,268
Charge-offs		(1,326)
Recoveries		264
Ending Balance		9,334		9,334
Provision for credit losses		2,128
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				20,221
Charge-offs				(1,771)
Recoveries				117
Provision for credit losses				6,245
Ending Balance		24,812		24,812
Originated Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				4,091
Charge-offs				(559)
Recoveries				121
Provision for credit losses				5,586
Ending Balance		9,239		9,239
Originated Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				19,731
Charge-offs				(2,415)
Recoveries				44
Provision for credit losses				15,770
Ending Balance		33,130		33,130
Originated Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				4,984
Charge-offs				(3,809)
Recoveries				476
Provision for credit losses				7,683
Ending Balance		9,334		9,334
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				483
Ending Balance		$ 3,067		$ 3,067