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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2020
Allowance for loan losses:
Beginning balance	$ 172,354	$ 145,387	$ 176,144	$ 52,162
Accounting standards update [Extensible List]					Accounting Standards Update 2016-13 [Member]
Charge-offs	(2,979)	(3,576)	(11,203)	(12,491)
Recoveries	1,863	3,187	6,297	9,593
Net (charge-offs) recoveries	(1,116)	(389)	(4,906)	(2,898)
(Recovery of) provision for credit losses on loans	(1,200)	23,100	(1,200)	76,350
Ending balance	170,038	168,098	170,038	168,098	$ 52,162
Individually evaluated	16,498	11,136	16,498	11,136
Collectively evaluated	153,540	156,962	153,540	156,962
Loans:
Individually evaluated	40,241	29,602	40,241	29,602
Collectively evaluated	9,976,583	11,055,136	9,976,583	11,055,136
Gross loans	10,016,824	11,084,738	10,016,824	11,084,738		$ 10,933,647
Nonaccruing loans with no allowance for credit losses	8,572	8,380	8,572	8,380
Commercial, financial, agricultural
Allowance for loan losses:
Beginning balance	36,994	30,685	39,031	10,658
Charge-offs	(1,225)	(420)	(5,907)	(1,969)
Recoveries	418	698	940	996
Net (charge-offs) recoveries	(807)	278	(4,967)	(973)
(Recovery of) provision for credit losses on loans	(1,210)	7,232	913	17,159
Ending balance	34,977	38,195	34,977	38,195	10,658
Individually evaluated	9,717	10,211	9,717	10,211
Collectively evaluated	25,260	27,984	25,260	27,984
Loans:
Individually evaluated	15,193	17,670	15,193	17,670
Collectively evaluated	1,420,826	2,630,392	1,420,826	2,630,392
Gross loans	1,436,019	2,648,062	1,436,019	2,648,062
Nonaccruing loans with no allowance for credit losses	2,658	589	2,658	589
Real estate – construction:
Allowance for loan losses:
Beginning balance	15,729	12,538	16,047	5,029
Charge-offs	0	(136)	(52)	(668)
Recoveries	0	31	13	31
Net (charge-offs) recoveries	0	(105)	(39)	(637)
(Recovery of) provision for credit losses on loans	440	1,386	161	5,922
Ending balance	16,169	13,819	16,169	13,819	5,029
Individually evaluated	0	0	0	0
Collectively evaluated	16,169	13,819	16,169	13,819
Loans:
Individually evaluated	0	0	0	0
Collectively evaluated	1,091,296	773,119	1,091,296	773,119
Gross loans	1,091,296	773,119	1,091,296	773,119
Nonaccruing loans with no allowance for credit losses	0	0	0	0
Real estate – 1-4 family mortgage:
Allowance for loan losses:
Beginning balance	31,303	29,401	32,165	9,814
Charge-offs	(276)	(720)	(529)	(1,083)
Recoveries	193	152	855	288
Net (charge-offs) recoveries	(83)	(568)	326	(795)
(Recovery of) provision for credit losses on loans	961	3,872	(310)	9,372
Ending balance	32,181	32,705	32,181	32,705	9,814
Individually evaluated	206	275	206	275
Collectively evaluated	31,975	32,430	31,975	32,430
Loans:
Individually evaluated	5,311	4,718	5,311	4,718
Collectively evaluated	2,719,432	2,755,676	2,719,432	2,755,676
Gross loans	2,724,743	2,760,394	2,724,743	2,760,394
Nonaccruing loans with no allowance for credit losses	3,039	4,147	3,039	4,147
Real estate – commercial mortgage:
Allowance for loan losses:
Beginning balance	74,893	60,061	76,127	24,990
Charge-offs	(184)	(553)	(416)	(2,600)
Recoveries	190	711	504	2,451
Net (charge-offs) recoveries	6	158	88	(149)
(Recovery of) provision for credit losses on loans	(1,004)	10,363	(2,320)	41,448
Ending balance	73,895	70,582	73,895	70,582	24,990
Individually evaluated	5,968	380	5,968	380
Collectively evaluated	67,927	70,202	67,927	70,202
Loans:
Individually evaluated	19,120	6,596	19,120	6,596
Collectively evaluated	4,516,610	4,570,413	4,516,610	4,570,413
Gross loans	4,535,730	4,577,009	4,535,730	4,577,009
Nonaccruing loans with no allowance for credit losses	2,865	3,644	2,865	3,644
Lease financing
Allowance for loan losses:
Beginning balance	1,511	1,812	1,624	910
Charge-offs	(13)	(168)	(13)	(168)
Recoveries	11	1	36	11
Net (charge-offs) recoveries	(2)	(167)	23	(157)
(Recovery of) provision for credit losses on loans	61	187	(77)	558
Ending balance	1,570	1,832	1,570	1,832	910
Individually evaluated	0	0	0	0
Collectively evaluated	1,570	1,832	1,570	1,832
Loans:
Individually evaluated	0	0	0	0
Collectively evaluated	79,215	82,928	79,215	82,928
Gross loans	79,215	82,928	79,215	82,928
Nonaccruing loans with no allowance for credit losses	0	0	0	0
Installment Loans to Individuals
Allowance for loan losses:
Beginning balance	11,924	10,890	11,150	761
Charge-offs	(1,281)	(1,579)	(4,286)	(6,003)
Recoveries	1,051	1,594	3,949	5,816
Net (charge-offs) recoveries	(230)	15	(337)	(187)
(Recovery of) provision for credit losses on loans	(448)	60	433	1,891
Ending balance	11,246	10,965	11,246	10,965	761
Individually evaluated	607	270	607	270
Collectively evaluated	10,639	10,695	10,639	10,695
Loans:
Individually evaluated	617	618	617	618
Collectively evaluated	149,204	242,608	149,204	242,608
Gross loans	149,821	243,226	149,821	243,226
Nonaccruing loans with no allowance for credit losses	$ 10	$ 0	$ 10	0
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 Loans to Individuals
Allowance for loan losses:
Beginning balance				$ 8,500
Ending balance					$ 8,500