Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses	$ (11,212)	$ (2,708)	$ (49,038)	$ (9,638)
Ending Balance					$ 88,307		$ 50,035
Ending balance: individually evaluated for impaired					7,418		1,546
Ending balance: collectively evaluated for impaired					80,889		48,489
Total					6,949,716	$ 6,189,148	6,099,561
Ending balance: individually evaluated for impaired					36,969		25,344
Ending balance: collectively evaluated for impaired					6,912,747		6,074,217
Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					8,252		3,917
Ending balance: individually evaluated for impaired							4
Ending balance: collectively evaluated for impaired							3,913
Total					1,744,020		1,666,220
Ending balance: individually evaluated for impaired							4,102
Ending balance: collectively evaluated for impaired							1,662,118
Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					42,244		19,048
Ending balance: individually evaluated for impaired					5,921		488
Ending balance: collectively evaluated for impaired					36,323		18,560
Total					2,215,311		2,124,240
Ending balance: individually evaluated for impaired					30,387		10,825
Ending balance: collectively evaluated for impaired					2,184,924		2,113,415
Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					25,488		19,947
Ending balance: individually evaluated for impaired					1,497		1,054
Ending balance: collectively evaluated for impaired					23,991		18,893
Total					1,736,736		1,210,936
Ending balance: individually evaluated for impaired					5,048		10,417
Ending balance: collectively evaluated for impaired					1,731,688		1,200,519
Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					3,508		2,410
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					3,508		2,410
Total					452,989		420,281
Ending balance: individually evaluated for impaired					308		0
Ending balance: collectively evaluated for impaired					452,681		420,281
Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance: individually evaluated for impaired					0
Ending balance: collectively evaluated for impaired					8,252
Ending balance: individually evaluated for impaired					1,226
Ending balance: collectively evaluated for impaired					1,742,794
Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					8,815		4,713
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					8,815		4,713
Total					800,660		677,884
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					800,660		677,884
Acquired Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	1,859	65	52	178
Charge-offs	(202)	(1,428)	(502)	(2,032)
Recoveries	28	32	76	113
Provision for credit losses	(293)	(1,360)	(2,352)	(1,770)
Ending Balance	1,859	65	52	178	1,978	52	29
Total					373,876	474,775
Acquired Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	0	25	2	35
Charge-offs	(122)	0	(213)	(46)
Recoveries	13	11	38	46
Provision for credit losses	(109)	35	(173)	34
Ending Balance	0	25	2	35	0	2	1
Total					209,071	265,554
Acquired Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	1,356	25	37	4
Charge-offs	0	(1,376)	(2)	(1,376)
Recoveries	0	0	0	0
Provision for credit losses	(286)	(1,372)	(1,607)	(1,393)
Ending Balance	1,356	25	37	4	1,642	37	21
Total					127,094	159,173
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	332	15	13	139
Charge-offs	0	(49)	0	(601)
Recoveries	13	21	28	53
Provision for credit losses	9	(20)	(295)	(416)
Ending Balance	332	15	13	139	336	13	7
Total					25,122	29,827
Acquired Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	171	0	0	0
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision for credit losses	171	0	0	0
Ending Balance	171	0	0	0	0	0	0
Total					2,400	6,262
Acquired Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	0	0	0	0
Charge-offs	(80)	(3)	(287)	(9)
Recoveries	2	0	10	14
Provision for credit losses	(78)	(3)	(277)	5
Ending Balance	0	0	0	0	0	0	0
Total					10,189	13,959
Originated Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	79,582	50,996	51,585	47,586
Charge-offs	(4,705)	(2,702)	(13,259)	(6,555)
Recoveries	533	364	1,317	1,107
Provision for credit losses	(10,919)	(1,348)	(46,686)	(7,868)
Ending Balance	79,582	50,996	51,585	47,586	86,329	51,585	50,006
Total					6,575,840	5,714,373
Originated Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	9,239	4,133	4,091	3,934
Charge-offs	(161)	(383)	(720)	(617)
Recoveries	153	6	274	190
Provision for credit losses	979	(160)	(4,607)	(409)
Ending Balance	9,239	4,133	4,091	3,934	8,252	4,091	3,916
Total					1,534,949	1,415,808
Originated Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	33,130	19,287	19,731	18,382
Charge-offs	0	(6)	(2,415)	(305)
Recoveries	110	81	154	160
Provision for credit losses	(7,362)	335	(23,132)	(790)
Ending Balance	33,130	19,287	19,731	18,382	40,602	19,731	19,027
Total					2,088,217	1,958,346
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	24,812	20,678	20,221	19,235
Charge-offs	(3,395)	(742)	(5,166)	(1,584)
Recoveries	44	41	161	180
Provision for credit losses	(3,691)	37	(9,936)	(2,109)
Ending Balance	24,812	20,678	20,221	19,235	25,152	20,221	19,940
Total					1,711,614	1,212,026
Originated Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	3,067	2,491	2,558	2,002
Charge-offs	0	0	0	0
Recoveries	0	74	26	158
Provision for credit losses	(441)	155	(924)	(250)
Ending Balance	3,067	2,491	2,558	2,002	3,508	2,558	2,410
Total					450,589	442,777
Originated Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	9,334	4,407	4,984	4,033
Charge-offs	(1,149)	(1,571)	(4,958)	(4,049)
Recoveries	226	162	702	419
Provision for credit losses	(404)	(1,715)	(8,087)	(4,310)
Ending Balance	$ 9,334	$ 4,407	$ 4,984	$ 4,033	8,815	4,984	$ 4,713
Total					$ 790,471	$ 685,416