LOANS - Narrative (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	$ 14,599,805,000		$ 14,480,925,000
Deferred interest on loans, allowance for credit losses	556,000		718,000
Interest income on nonaccrual loans	0		0
Loans modified that are not troubled debt restructurings	118,000,000.0	$ 71,300,000
COVID-19 Deferrals	$ 235,421,000		332,779,000
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment modification term	9 months
Loan Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued interest receivable	$ 67,800,000		73,400,000
Commercial, financial and agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	1,611,029,000		1,627,477,000
COVID-19 Deferrals	4,460,000		12,471,000
Payment Protection Plan Loans | Commercial, financial and agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	792,000,000.0		827,400,000
Loans Excluding Purchased Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructuring	87,100,000		85,000,000.0
Troubled debt restructurings, previous charge-offs, excluding purchased loans	1,200,000		1,200,000
Allowance for loan losses allocated to troubled debt restructuring	14,200,000		$ 13,000,000.0
Troubled debt restructurings principal balances	8,693,000	976,000
Financing receivable, modifications, subsequent default, recorded investment	6,200,000	3,000,000.0
Loans Excluding Purchased Loan | Commercial, financial and agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings principal balances	$ 463,000	$ 0