LOANS AND ALLOWANCE FOR CREDIT LOSSES - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred interest on loans, allowance for credit losses	$ 224		$ 718
Interest income on nonaccrual loans	0	$ 0
Loans modified that are not troubled debt restructurings	322,800	313,600
COVID-19 deferrals	$ 76,525		332,779
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	$ 57,200		73,400
Commercial, financial and agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
COVID-19 deferrals	755		12,471
Payment Protection Plan Loans | Commercial, financial and agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	280,400		827,400
Loans Excluding Purchased Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructuring	87,400		85,000
Troubled debt restructurings, previous charge-offs, excluding purchased loans	654	$ 1,200
Allowance for loan losses allocated to troubled debt restructuring	$ 14,300		$ 13,000