Allowance for Credit Losses - Roll Forward Of the Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Allowance for loan losses:
Beginning balance	$ 145,387	$ 50,059	$ 52,162	$ 49,026	$ 49,026
Accounting standards update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201613Member
Charge-offs	(3,576)	(4,965)	$ (12,491)	(7,953)
Recoveries	3,187	4,020	9,593	5,641
Net (charge-offs) recoveries	(389)	(945)	(2,898)	(2,312)
Provision for credit losses on loans	23,100	1,700	76,350	4,100
Ending balance	168,098	50,814	168,098	50,814	$ 52,162
Individually evaluated	11,136	2,045	11,136	2,045
Collectively evaluated	156,962	46,221	156,962	46,221
Loans:
Individually evaluated	29,602		29,602		51,179
Collectively evaluated	11,055,136		11,055,136		9,466,195
Gross loans	11,084,738		11,084,738		9,689,638
Nonaccruing loans with no allowance for credit losses	8,380		8,380
Purchased with deteriorated credit quality
Allowance for loan losses:
Ending balance		2,548		2,548
Loans:
Gross loans					172,264
Commercial, financial, agricultural
Allowance for loan losses:
Beginning balance	30,685	9,534	10,658	8,269	8,269
Charge-offs	(420)	(757)	(1,969)	(1,709)
Recoveries	698	761	996	1,376
Net (charge-offs) recoveries	278	4	(973)	(333)
Provision for credit losses on loans	7,232	750	17,159	2,352
Ending balance	38,195	10,288	38,195	10,288	10,658
Individually evaluated	10,211	1,382	10,211	1,382
Collectively evaluated	27,984	8,778	27,984	8,778
Loans:
Individually evaluated	17,670		17,670		8,460
Collectively evaluated	2,630,392		2,630,392		1,329,974
Gross loans	2,648,062		2,648,062		1,367,972
Nonaccruing loans with no allowance for credit losses	589		589
Commercial, financial, agricultural | Purchased with deteriorated credit quality
Allowance for loan losses:
Ending balance		128		128
Loans:
Gross loans					29,538
Real estate – construction:
Allowance for loan losses:
Beginning balance	12,538	5,302	5,029	4,755	4,755
Charge-offs	(136)	0	(668)	0
Recoveries	31	0	31	7
Net (charge-offs) recoveries	(105)	0	(637)	7
Provision for credit losses on loans	1,386	(175)	5,922	365
Ending balance	13,819	5,127	13,819	5,127	5,029
Individually evaluated	0	24	0	24
Collectively evaluated	13,819	5,103	13,819	5,103
Loans:
Individually evaluated	0		0		12,416
Collectively evaluated	773,119		773,119		813,204
Gross loans	773,119		773,119		826,483
Nonaccruing loans with no allowance for credit losses	0		0
Real estate – construction: | Purchased with deteriorated credit quality
Allowance for loan losses:
Ending balance		0		0
Loans:
Gross loans					863
Real estate – 1-4 family mortgage
Allowance for loan losses:
Beginning balance	29,401	9,616	9,814	10,139	10,139
Charge-offs	(720)	(268)	(1,083)	(1,143)
Recoveries	152	219	288	531
Net (charge-offs) recoveries	(568)	(49)	(795)	(612)
Provision for credit losses on loans	3,872	282	9,372	322
Ending balance	32,705	9,849	32,705	9,849	9,814
Individually evaluated	275	186	275	186
Collectively evaluated	32,430	9,313	32,430	9,313
Loans:
Individually evaluated	4,718		4,718		20,262
Collectively evaluated	2,755,676		2,755,676		2,810,808
Gross loans	2,760,394		2,760,394		2,866,613
Nonaccruing loans with no allowance for credit losses	4,147		4,147
Real estate – 1-4 family mortgage | Purchased with deteriorated credit quality
Allowance for loan losses:
Ending balance		350		350
Loans:
Gross loans					35,543
Real estate – commercial mortgage:
Allowance for loan losses:
Beginning balance	60,061	24,302	24,990	24,492	24,492
Charge-offs	(553)	(677)	(2,600)	(1,406)
Recoveries	711	33	2,451	644
Net (charge-offs) recoveries	158	(644)	(149)	(762)
Provision for credit losses on loans	10,363	381	41,448	309
Ending balance	70,582	24,039	70,582	24,039	24,990
Individually evaluated	380	450	380	450
Collectively evaluated	70,202	21,521	70,202	21,521
Loans:
Individually evaluated	6,596		6,596		9,550
Collectively evaluated	4,570,413		4,570,413		4,131,582
Gross loans	4,577,009		4,577,009		4,244,265
Nonaccruing loans with no allowance for credit losses	3,644		3,644
Real estate – commercial mortgage: | Purchased with deteriorated credit quality
Allowance for loan losses:
Ending balance		2,068		2,068
Loans:
Gross loans					103,133
Lease financing
Allowance for loan losses:
Beginning balance	1,812		910
Charge-offs	(168)		(168)
Recoveries	1		11
Net (charge-offs) recoveries	(167)		(157)
Provision for credit losses on loans	187		558
Ending balance	1,832		1,832		910
Individually evaluated	0		0
Collectively evaluated	1,832		1,832
Loans:
Individually evaluated	0		0
Collectively evaluated	82,928		82,928
Gross loans	82,928		82,928
Nonaccruing loans with no allowance for credit losses	0		0
Installment and Other
Allowance for loan losses:
Beginning balance	10,890	1,305	761	1,371	1,371
Charge-offs	(1,579)	(3,263)	(6,003)	(3,695)
Recoveries	1,594	3,007	5,816	3,083
Net (charge-offs) recoveries	15	(256)	(187)	(612)
Provision for credit losses on loans	60	462	1,891	752
Ending balance	10,965	1,511	10,965	1,511	761
Individually evaluated	270	3	270	3
Collectively evaluated	10,695	1,506	10,695	1,506
Loans:
Individually evaluated	618		618		491
Collectively evaluated	242,608		242,608		380,627
Gross loans	243,226		243,226		384,305
Nonaccruing loans with no allowance for credit losses	$ 0		0
Installment and Other | Purchased with deteriorated credit quality
Allowance for loan losses:
Ending balance		$ 2		$ 2
Loans:
Gross loans					3,187
Cumulative Effect, Period Of Adoption, Adjustment
Allowance for loan losses:
Beginning balance			42,484
Ending balance					42,484
Cumulative Effect, Period Of Adoption, Adjustment | Commercial, financial, agricultural
Allowance for loan losses:
Beginning balance			11,351
Ending balance					11,351
Cumulative Effect, Period Of Adoption, Adjustment | Real estate – construction:
Allowance for loan losses:
Beginning balance			3,505
Ending balance					3,505
Cumulative Effect, Period Of Adoption, Adjustment | Real estate – 1-4 family mortgage
Allowance for loan losses:
Beginning balance			14,314
Ending balance					14,314
Cumulative Effect, Period Of Adoption, Adjustment | Real estate – commercial mortgage:
Allowance for loan losses:
Beginning balance			4,293
Ending balance					4,293
Cumulative Effect, Period Of Adoption, Adjustment | Lease financing
Allowance for loan losses:
Beginning balance			521
Ending balance					521
Cumulative Effect, Period Of Adoption, Adjustment | Installment and Other
Allowance for loan losses:
Beginning balance			$ 8,500
Ending balance					$ 8,500