Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 101,309
Charge-offs	(3,765)
Recoveries	495
Provision for credit losses	(1,276)
Ending Balance	101,309		$ 96,763	$ 101,309	$ 79,075
Ending balance: individually evaluated for impaired			3,107		9,799
Ending balance: collectively evaluated for impaired			93,656		69,276
Total			6,736,894	6,761,183	6,313,944
Ending balance: individually evaluated for impaired			38,459		45,563
Ending balance: collectively evaluated for impaired			6,698,435		6,268,381
Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	17,187
Charge-offs	(569)
Recoveries	90
Provision for credit losses	5,093
Ending Balance	17,187		21,801	17,187	27,728
Ending balance: individually evaluated for impaired			2,506		2,727
Ending balance: collectively evaluated for impaired			19,295		25,001
Total			1,555,671	1,555,986	1,272,240
Ending balance: individually evaluated for impaired			9,991		7,544
Ending balance: collectively evaluated for impaired			1,545,680		1,264,696
Commercial, financial, agricultural and other [Member] | Time and Demand Loans
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	16,838
Charge-offs	(460)
Recoveries	89
Provision for credit losses	4,960
Ending Balance	16,838		21,427	16,838
Ending balance: individually evaluated for impaired			2,506
Ending balance: collectively evaluated for impaired			18,921
Total			1,541,280	1,541,382
Ending balance: individually evaluated for impaired			9,991
Ending balance: collectively evaluated for impaired			1,531,289
Commercial, financial, agricultural and other [Member] | Commercial Credit Cards
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	349
Charge-offs	(109)
Recoveries	1
Provision for credit losses	133
Ending Balance	349		374	349
Ending balance: individually evaluated for impaired			0
Ending balance: collectively evaluated for impaired			374
Total			14,391	14,604
Ending balance: individually evaluated for impaired			0
Ending balance: collectively evaluated for impaired			14,391
Construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	7,966
Charge-offs	0
Recoveries	0
Provision for credit losses	(3,945)
Ending Balance	7,966		4,021	7,966
Ending balance: individually evaluated for impaired			0
Ending balance: collectively evaluated for impaired			4,021
Total			404,580
Ending balance: individually evaluated for impaired			0
Ending balance: collectively evaluated for impaired			404,580
Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	14,358
Charge-offs	(105)
Recoveries	37
Provision for credit losses	(1,461)
Ending Balance	14,358		12,829	14,358	7,724
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			12,829		7,724
Total			1,756,615	1,750,592	1,691,140
Ending balance: individually evaluated for impaired			1,055		1,506
Ending balance: collectively evaluated for impaired			1,755,560		1,689,634
Residential real estate [Member] | Senior Lien
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	7,919
Charge-offs	(23)
Recoveries	23
Provision for credit losses	(692)
Ending Balance	7,919		7,227	7,919
Ending balance: individually evaluated for impaired			0
Ending balance: collectively evaluated for impaired			7,227
Total			1,152,314	1,144,323
Ending balance: individually evaluated for impaired			512
Ending balance: collectively evaluated for impaired			1,151,802
Residential real estate [Member] | Junior Lien
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	6,439
Charge-offs	(82)
Recoveries	14
Provision for credit losses	(769)
Ending Balance	6,439		5,602	6,439
Ending balance: individually evaluated for impaired			0
Ending balance: collectively evaluated for impaired			5,602
Total			604,301	606,269
Ending balance: individually evaluated for impaired			543
Ending balance: collectively evaluated for impaired			603,758
Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	41,953
Charge-offs	(1,550)
Recoveries	39
Provision for credit losses	(2,774)
Ending Balance	41,953		37,668	41,953	32,503
Ending balance: individually evaluated for impaired			601		7,072
Ending balance: collectively evaluated for impaired			37,067		25,431
Total			2,167,506	2,211,569	2,190,098
Ending balance: individually evaluated for impaired			27,413		36,513
Ending balance: collectively evaluated for impaired			2,140,093		2,153,585
Commercial Real Estate [Member] | Multifamily
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	6,240
Charge-offs	(1)
Recoveries	0
Provision for credit losses	(1,988)
Ending Balance	6,240		4,251	6,240
Ending balance: individually evaluated for impaired			115
Ending balance: collectively evaluated for impaired			4,136
Total			363,604	371,239
Ending balance: individually evaluated for impaired			459
Ending balance: collectively evaluated for impaired			363,145
Commercial Real Estate [Member] | Nonowner Occupied Commercial Real Estate
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	28,414
Charge-offs	(1,549)
Recoveries	39
Provision for credit losses	985
Ending Balance	28,414		27,889	28,414
Ending balance: individually evaluated for impaired			459
Ending balance: collectively evaluated for impaired			27,430
Total			1,393,968	1,421,151
Ending balance: individually evaluated for impaired			24,883
Ending balance: collectively evaluated for impaired			1,369,085
Commercial Real Estate [Member] | Owner Occupied Commercial Real Estate
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	7,299
Charge-offs	0
Recoveries	0
Provision for credit losses	(1,771)
Ending Balance	7,299		5,528	7,299
Ending balance: individually evaluated for impaired			27
Ending balance: collectively evaluated for impaired			5,501
Total			409,934	419,179
Ending balance: individually evaluated for impaired			2,071
Ending balance: collectively evaluated for impaired			407,863
Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	19,845
Charge-offs	(1,541)
Recoveries	329
Provision for credit losses	1,811
Ending Balance	19,845		20,444	19,845	8,268
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			20,444		8,268
Total			852,522	815,815	747,008
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			852,522		747,008
Loans to individuals [Member] | Automobiles
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	16,133
Charge-offs	(680)
Recoveries	181
Provision for credit losses	1,254
Ending Balance	16,133		16,888	16,133
Ending balance: individually evaluated for impaired			0
Ending balance: collectively evaluated for impaired			16,888
Total			759,061	712,800
Ending balance: individually evaluated for impaired			0
Ending balance: collectively evaluated for impaired			759,061
Loans to individuals [Member] | Consumer Credit Cards
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	635
Charge-offs	(168)
Recoveries	17
Provision for credit losses	205
Ending Balance	635		689	635
Ending balance: individually evaluated for impaired			0
Ending balance: collectively evaluated for impaired			689
Total			10,901	12,360
Ending balance: individually evaluated for impaired			0
Ending balance: collectively evaluated for impaired			10,901
Loans to individuals [Member] | Other Consumer Loans
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	3,077
Charge-offs	(693)
Recoveries	131
Provision for credit losses	352
Ending Balance	$ 3,077		2,867	3,077
Ending balance: individually evaluated for impaired			0
Ending balance: collectively evaluated for impaired			2,867
Total			82,560	90,655
Ending balance: individually evaluated for impaired			0
Ending balance: collectively evaluated for impaired			82,560
Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					2,852
Ending balance: individually evaluated for impaired					0
Ending balance: collectively evaluated for impaired					2,852
Total			$ 404,580	$ 427,221	413,458
Ending balance: individually evaluated for impaired					0
Ending balance: collectively evaluated for impaired					413,458
Acquired Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		$ 52
Charge-offs		(162)
Recoveries		33
Provision for credit losses		1,665
Ending Balance		52			1,588
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		13
Charge-offs		0
Recoveries		13
Provision for credit losses		324
Ending Balance		13			350
Acquired Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		2
Charge-offs		(25)
Recoveries		13
Provision for credit losses		10
Ending Balance		2			0
Acquired Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		37
Charge-offs		(1)
Recoveries		0
Provision for credit losses		1,202
Ending Balance		37			1,238
Acquired Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		0
Charge-offs		(136)
Recoveries		7
Provision for credit losses		129
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		51,585
Charge-offs		(3,786)
Recoveries		386
Provision for credit losses		29,302
Ending Balance		51,585			77,487
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		20,221
Charge-offs		(486)
Recoveries		68
Provision for credit losses		7,575
Ending Balance		20,221			27,378
Originated Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		4,091
Charge-offs		(552)
Recoveries		62
Provision for credit losses		4,123
Ending Balance		4,091			7,724
Originated Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		19,731
Charge-offs		(265)
Recoveries		44
Provision for credit losses		11,755
Ending Balance		19,731			31,265
Originated Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		4,984
Charge-offs		(2,483)
Recoveries		212
Provision for credit losses		5,555
Ending Balance		4,984			8,268
Originated Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		2,558
Charge-offs		0
Recoveries		0
Provision for credit losses		294
Ending Balance		$ 2,558			$ 2,852