Allowance for Credit Losses - Roll Forward Of the Allowance for Loan Losses (Details) - USD ($)	3 Months Ended		6 Months Ended	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Jun. 30, 2020	Dec. 31, 2019	Dec. 31, 2020
Allowance for loan losses:
Beginning balance	$ 176,144,000	$ 52,162,000	$ 52,162,000
Accounting standards update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member
Charge-offs	(5,370,000)	(5,349,000)
Recoveries	2,332,000	4,538,000
Net (charge-offs) recoveries	(3,038,000)	(811,000)
Provision for credit losses on loans	0	26,350,000
Ending balance	173,106,000	120,185,000		$ 52,162,000
Individually evaluated	15,593,000	5,149,000
Collectively evaluated	157,513,000	115,036,000
Loans:
Individually evaluated	40,875,000	27,186,000
Collectively evaluated	10,647,533,000	9,742,191,000
Gross loans	10,688,408,000	9,769,377,000			$ 10,933,647,000
Nonaccruing loans with no allowance for credit losses	8,656,000	12,855,000
Commercial, financial, agricultural
Allowance for loan losses:
Beginning balance	39,031,000	10,658,000	$ 10,658,000
Charge-offs	(3,498,000)	(393,000)
Recoveries	289,000	190,000
Net (charge-offs) recoveries	(3,209,000)	(203,000)
Provision for credit losses on loans	1,770,000	4,131,000
Ending balance	37,592,000	25,937,000		10,658,000
Individually evaluated	9,908,000	3,653,000
Collectively evaluated	27,684,000	22,284,000
Loans:
Individually evaluated	15,435,000	10,460,000
Collectively evaluated	2,233,852,000	1,414,116,000
Gross loans	2,249,287,000	1,424,576,000
Nonaccruing loans with no allowance for credit losses	1,848,000	4,224,000
Real estate – construction:
Allowance for loan losses:
Beginning balance	16,047,000	5,029,000	5,029,000
Charge-offs	(52,000)	0
Recoveries	13,000	0
Net (charge-offs) recoveries	(39,000)	0
Provision for credit losses on loans	(1,031,000)	2,390,000
Ending balance	14,977,000	10,924,000		5,029,000
Individually evaluated	0	0
Collectively evaluated	14,977,000	10,924,000
Loans:
Individually evaluated	0	2,728,000
Collectively evaluated	955,918,000	785,167,000
Gross loans	955,918,000	787,895,000
Nonaccruing loans with no allowance for credit losses	0	2,728,000
Real estate – 1-4 family mortgage
Allowance for loan losses:
Beginning balance	32,165,000	9,814,000	9,814,000
Charge-offs	(101,000)	(221,000)
Recoveries	261,000	88,000
Net (charge-offs) recoveries	160,000	(133,000)
Provision for credit losses on loans	(631,000)	3,325,000
Ending balance	31,694,000	27,320,000		9,814,000
Individually evaluated	232,000	370,000
Collectively evaluated	31,462,000	26,950,000
Loans:
Individually evaluated	6,311,000	5,865,000
Collectively evaluated	2,679,750,000	2,840,436,000
Gross loans	2,686,061,000	2,846,301,000
Nonaccruing loans with no allowance for credit losses	4,695,000	3,309,000
Real estate – commercial mortgage:
Allowance for loan losses:
Beginning balance	76,127,000	24,990,000	24,990,000
Charge-offs	(61,000)	(2,047,000)
Recoveries	171,000	1,699,000
Net (charge-offs) recoveries	110,000	(348,000)
Provision for credit losses on loans	(12,000)	15,302,000
Ending balance	76,225,000	44,237,000		24,990,000
Individually evaluated	4,846,000	856,000
Collectively evaluated	71,379,000	43,381,000
Loans:
Individually evaluated	18,508,000	7,508,000
Collectively evaluated	4,530,519,000	4,301,200,000
Gross loans	4,549,027,000	4,308,708,000
Nonaccruing loans with no allowance for credit losses	2,113,000	2,594,000
Lease financing
Allowance for loan losses:
Beginning balance	1,624,000
Charge-offs	0	0
Recoveries	11,000	5,000
Net (charge-offs) recoveries	11,000	5,000
Provision for credit losses on loans	(89,000)	152,000
Ending balance	1,546,000	1,588,000
Individually evaluated	0	0
Collectively evaluated	1,546,000	1,588,000
Loans:
Individually evaluated	0	0
Collectively evaluated	75,256,000	84,679,000
Gross loans	75,256,000	84,679,000
Nonaccruing loans with no allowance for credit losses	0	0
Installment and Other
Allowance for loan losses:
Beginning balance	11,150,000	761,000	$ 761,000
Charge-offs	(1,658,000)	(2,688,000)
Recoveries	1,587,000	2,556,000
Net (charge-offs) recoveries	(71,000)	(132,000)
Provision for credit losses on loans	(7,000)	1,050,000
Ending balance	11,072,000	10,179,000		$ 761,000
Individually evaluated	607,000	270,000
Collectively evaluated	10,465,000	9,909,000
Loans:
Individually evaluated	621,000	625,000
Collectively evaluated	172,238,000	316,593,000
Gross loans	172,859,000	317,218,000
Nonaccruing loans with no allowance for credit losses	0	$ 0
Cumulative Effect, Period Of Adoption, Adjustment
Allowance for loan losses:
Beginning balance	42,484,000
Cumulative Effect, Period Of Adoption, Adjustment | Commercial, financial, agricultural
Allowance for loan losses:
Beginning balance	11,351,000
Cumulative Effect, Period Of Adoption, Adjustment | Real estate – construction:
Allowance for loan losses:
Beginning balance	3,505,000
Cumulative Effect, Period Of Adoption, Adjustment | Real estate – 1-4 family mortgage
Allowance for loan losses:
Beginning balance	14,314,000
Cumulative Effect, Period Of Adoption, Adjustment | Real estate – commercial mortgage:
Allowance for loan losses:
Beginning balance	4,293,000
Cumulative Effect, Period Of Adoption, Adjustment | Lease financing
Allowance for loan losses:
Beginning balance	521,000
Cumulative Effect, Period Of Adoption, Adjustment | Installment and Other
Allowance for loan losses:
Beginning balance	$ 8,500,000