Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses	$ (6,859)	$ (2,835)	$ (37,826)	$ (6,930)
Ending Balance					$ 81,441		$ 51,061
Ending balance: individually evaluated for impaired					6,895		2,530
Ending balance: collectively evaluated for impaired					74,546		48,531
Total					6,922,075	$ 6,189,148	6,003,059
Ending balance: individually evaluated for impaired					37,679		25,487
Ending balance: collectively evaluated for impaired					6,884,396		5,977,572
Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					9,239		4,158
Ending balance: individually evaluated for impaired							87
Ending balance: collectively evaluated for impaired							4,071
Total					1,723,288		1,579,441
Ending balance: individually evaluated for impaired							4,065
Ending balance: collectively evaluated for impaired							1,575,376
Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					34,486		19,312
Ending balance: individually evaluated for impaired					5,766		1,113
Ending balance: collectively evaluated for impaired					28,720		18,199
Total					2,224,710		2,118,582
Ending balance: individually evaluated for impaired					32,537		10,216
Ending balance: collectively evaluated for impaired					2,192,173		2,108,366
Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					25,144		20,693
Ending balance: individually evaluated for impaired					958		1,330
Ending balance: collectively evaluated for impaired					24,186		19,363
Total					1,773,099		1,236,424
Ending balance: individually evaluated for impaired					3,336		11,206
Ending balance: collectively evaluated for impaired					1,769,763		1,225,218
Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					3,238		2,491
Ending balance: individually evaluated for impaired					171		0
Ending balance: collectively evaluated for impaired					3,067		2,491
Total					416,329		441,854
Ending balance: individually evaluated for impaired					308		0
Ending balance: collectively evaluated for impaired					416,021		441,854
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					9,239
Ending balance: individually evaluated for impaired					1,498
Ending balance: collectively evaluated for impaired					1,721,790
Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					9,334		4,407
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					9,334		4,407
Total					784,649		626,758
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					784,649		626,758
Acquired Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	1,588	53	52	178
Charge-offs	(138)	(28)	(300)	(604)
Recoveries	15	37	48	81
Provision for credit losses	(394)	(3)	(2,059)	(410)
Ending Balance	1,588	53	52	178	1,859	52	65
Total					404,125	474,775
Acquired Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	0	35	2	35
Charge-offs	(66)	(1)	(91)	(46)
Recoveries	12	11	25	35
Provision for credit losses	(54)	20	(64)	(1)
Ending Balance	0	35	2	35	0	2	25
Total					228,363	265,554
Acquired Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	1,238	0	37	4
Charge-offs	(1)	0	(2)	0
Recoveries	0	0	0	0
Provision for credit losses	(119)	(25)	(1,321)	(21)
Ending Balance	1,238	0	37	4	1,356	37	25
Total					135,052	159,173
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	350	18	13	139
Charge-offs	0	(26)	0	(552)
Recoveries	2	21	15	32
Provision for credit losses	20	(2)	(304)	(396)
Ending Balance	350	18	13	139	332	13	15
Total					26,604	29,827
Acquired Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	0	0	0	0
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision for credit losses	(171)	0	(171)	0
Ending Balance	0	0	0	0	171	0	0
Total					2,924	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	(71)	(1)	(207)	(6)
Recoveries	1	5	8	14
Provision for credit losses	(70)	4	(199)	8
Ending Balance	0	0	0	0	0	0	0
Total					11,182	13,959
Originated Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	77,487	49,600	51,585	47,586
Charge-offs	(4,768)	(1,825)	(8,554)	(3,853)
Recoveries	398	389	784	743
Provision for credit losses	(6,465)	(2,832)	(35,767)	(6,520)
Ending Balance	77,487	49,600	51,585	47,586	79,582	51,585	50,996
Total					6,517,950	5,714,373
Originated Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	7,724	4,150	4,091	3,934
Charge-offs	(7)	(98)	(559)	(234)
Recoveries	59	103	121	184
Provision for credit losses	(1,463)	22	(5,586)	(249)
Ending Balance	7,724	4,150	4,091	3,934	9,239	4,091	4,133
Total					1,494,925	1,415,808
Originated Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	31,265	19,218	19,731	18,382
Charge-offs	(2,150)	0	(2,415)	(299)
Recoveries	0	38	44	79
Provision for credit losses	(4,015)	(31)	(15,770)	(1,125)
Ending Balance	31,265	19,218	19,731	18,382	33,130	19,731	19,287
Total					2,089,658	1,958,346
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	27,378	19,815	20,221	19,235
Charge-offs	(1,285)	(359)	(1,771)	(842)
Recoveries	49	63	117	139
Provision for credit losses	1,330	(1,159)	(6,245)	(2,146)
Ending Balance	27,378	19,815	20,221	19,235	24,812	20,221	20,678
Total					1,746,495	1,212,026
Originated Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	2,852	2,254	2,558	2,002
Charge-offs	0	0	0	0
Recoveries	26	42	26	84
Provision for credit losses	(189)	(195)	(483)	(405)
Ending Balance	2,852	2,254	2,558	2,002	3,067	2,558	2,491
Total					413,405	442,777
Originated Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	8,268	4,163	4,984	4,033
Charge-offs	(1,326)	(1,368)	(3,809)	(2,478)
Recoveries	264	143	476	257
Provision for credit losses	(2,128)	(1,469)	(7,683)	(2,595)
Ending Balance	$ 8,268	$ 4,163	$ 4,984	$ 4,033	9,334	4,984	$ 4,407
Total					$ 773,467	$ 685,416