Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses	$ 30,967	$ 4,095
Ending Balance			$ 79,075		$ 49,653
Ending balance: individually evaluated for impaired			9,799		2,045
Ending balance: collectively evaluated for impaired			69,276		47,608
Total			6,313,944	$ 6,189,148	5,871,070
Ending balance: individually evaluated for impaired			45,563		20,676
Ending balance: collectively evaluated for impaired			6,268,381		5,850,394
Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance			7,724		4,185
Total			1,691,140		1,565,349
Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance			32,503		19,218
Ending balance: individually evaluated for impaired			7,072		914
Ending balance: collectively evaluated for impaired			25,431		18,304
Total			2,190,098		2,138,376
Ending balance: individually evaluated for impaired			36,513		11,111
Ending balance: collectively evaluated for impaired			2,153,585		2,127,265
Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance			27,728		19,833
Ending balance: individually evaluated for impaired			2,727		1,012
Ending balance: collectively evaluated for impaired			25,001		18,821
Total			1,272,240		1,180,320
Ending balance: individually evaluated for impaired			7,544		5,627
Ending balance: collectively evaluated for impaired			1,264,696		1,174,693
Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance			2,852		2,254
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			2,852		2,254
Total			413,458		389,387
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			413,458		389,387
Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance: individually evaluated for impaired			0		119
Ending balance: collectively evaluated for impaired			7,724		4,066
Ending balance: individually evaluated for impaired			1,506		3,938
Ending balance: collectively evaluated for impaired			1,689,634		1,561,411
Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance			8,268		4,163
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			8,268		4,163
Total			747,008		597,638
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			747,008		597,638
Acquired Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	52	178
Charge-offs	(162)	(576)
Recoveries	33	44
Provision for credit losses	1,665	407
Ending Balance	52	178	1,588	52	53
Total			447,548	474,775
Acquired Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	2	35
Charge-offs	(25)	(45)
Recoveries	13	24
Provision for credit losses	10	21
Ending Balance	2	35	0	2	35
Total			251,834	265,554
Acquired Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	37	4
Charge-offs	(1)	0
Recoveries	0	0
Provision for credit losses	1,202	(4)
Ending Balance	37	4	1,238	37	0
Total			148,759	159,173
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	13	139
Charge-offs	0	(526)
Recoveries	13	11
Provision for credit losses	324	394
Ending Balance	13	139	350	13	18
Total			29,331	29,827
Acquired Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	0	0
Charge-offs	0	0
Recoveries	0	0
Provision for credit losses	0	0
Ending Balance	0	0	0	0	0
Total			5,224	6,262
Acquired Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	0	0
Charge-offs	(136)	(5)
Recoveries	7	9
Provision for credit losses	129	(4)
Ending Balance	0	0	0	0	0
Total			12,400	13,959
Originated Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	51,585	47,586
Charge-offs	(3,786)	(2,028)
Recoveries	386	354
Provision for credit losses	29,302	3,688
Ending Balance	51,585	47,586	77,487	51,585	49,600
Total			5,866,396	5,714,373
Originated Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	4,091	3,934
Charge-offs	(552)	(136)
Recoveries	62	81
Provision for credit losses	4,123	271
Ending Balance	4,091	3,934	7,724	4,091	4,150
Total			1,439,306	1,415,808
Originated Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	19,731	18,382
Charge-offs	(265)	(299)
Recoveries	44	41
Provision for credit losses	11,755	1,094
Ending Balance	19,731	18,382	31,265	19,731	19,218
Total			2,041,339	1,958,346
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	20,221	19,235
Charge-offs	(486)	(483)
Recoveries	68	76
Provision for credit losses	7,575	987
Ending Balance	20,221	19,235	27,378	20,221	19,815
Total			1,242,909	1,212,026
Originated Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	2,558	2,002
Charge-offs	0	0
Recoveries	0	42
Provision for credit losses	294	210
Ending Balance	2,558	2,002	2,852	2,558	2,254
Total			408,234	442,777
Originated Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	4,984	4,033
Charge-offs	(2,483)	(1,110)
Recoveries	212	114
Provision for credit losses	5,555	1,126
Ending Balance	$ 4,984	$ 4,033	8,268	4,984	$ 4,163
Total			$ 734,608	$ 685,416