LOANS AND ALLOWANCE FOR CREDIT LOSSES - Loans by Class Modified as Troubled Debt Restructurings (Details) - Loans Excluding Purchased Loan $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) contract branch	Sep. 30, 2020 USD ($) branch contract	Sep. 30, 2021 USD ($) contract branch	Sep. 30, 2020 USD ($) branch contract	Dec. 31, 2020 USD ($) contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract	9	565	34	649
Troubled debt restructurings principal balances	$ 1,818	$ 92,562	$ 20,066	$ 103,409
Financing Receivables, 30 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract	10	14	33	27
Troubled debt restructurings principal balances	$ 514	$ 1,410	$ 1,189	$ 2,758
Financing Receivables, 30 Days Past Due | Accruing Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract			563		752
Troubled debt restructurings principal balances			$ 77,292		$ 78,843
Financing Receivables, 30 Days Past Due | Non-Accruing Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract			118		128
Troubled debt restructurings principal balances			$ 10,120		$ 6,145
Commercial, financial and agricultural
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract	0	1	6	2
Troubled debt restructurings principal balances	$ 0	$ 9	$ 532	$ 725
Commercial, financial and agricultural | Financing Receivables, 30 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract	1	0	1	1
Troubled debt restructurings principal balances	$ 1	$ 0	$ 1	$ 198
Commercial, financial and agricultural | Financing Receivables, 30 Days Past Due | Accruing Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract			14		9
Troubled debt restructurings principal balances			$ 1,683		$ 521
Commercial, financial and agricultural | Financing Receivables, 30 Days Past Due | Non-Accruing Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract			7		11
Troubled debt restructurings principal balances			$ 112		$ 849
Consumer installment
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract	0	0	2	4
Troubled debt restructurings principal balances	$ 0	$ 0	$ 7	$ 15
Consumer installment | Financing Receivables, 30 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract	1	0	1	4
Troubled debt restructurings principal balances	$ 1	$ 0	$ 1	$ 3
Consumer installment | Financing Receivables, 30 Days Past Due | Accruing Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract			8		10
Troubled debt restructurings principal balances			$ 22		$ 32
Consumer installment | Financing Receivables, 30 Days Past Due | Non-Accruing Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract			19		20
Troubled debt restructurings principal balances			$ 38		$ 56
Indirect automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | branch	0	530	0	530
Troubled debt restructurings principal balances	$ 0	$ 3,171	$ 0	$ 3,170
Indirect automobile | Financing Receivables, 30 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | branch	4	0	16	0
Troubled debt restructurings principal balances	$ 23	$ 0	$ 76	$ 0
Indirect automobile | Financing Receivables, 30 Days Past Due | Accruing Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract			282		437
Troubled debt restructurings principal balances			$ 1,284		$ 2,277
Indirect automobile | Financing Receivables, 30 Days Past Due | Non-Accruing Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract			52		51
Troubled debt restructurings principal balances			$ 297		$ 461
Real estate – construction and development
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract	0	0	0	1
Troubled debt restructurings principal balances	$ 0	$ 0	$ 0	$ 19
Real estate – construction and development | Financing Receivables, 30 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract	0	1	0	3
Troubled debt restructurings principal balances	$ 0	$ 403	$ 0	$ 689
Real estate – construction and development | Financing Receivables, 30 Days Past Due | Accruing Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract			5		4
Troubled debt restructurings principal balances			$ 887		$ 506
Real estate – construction and development | Financing Receivables, 30 Days Past Due | Non-Accruing Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract			3		5
Troubled debt restructurings principal balances			$ 271		$ 707
Real estate – commercial and farmland
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract	0	20	5	21
Troubled debt restructurings principal balances	$ 0	$ 86,773	$ 16,257	$ 86,788
Real estate – commercial and farmland | Financing Receivables, 30 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract	0	1	0	3
Troubled debt restructurings principal balances	$ 0	$ 50	$ 0	$ 726
Real estate – commercial and farmland | Financing Receivables, 30 Days Past Due | Accruing Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract			27		28
Troubled debt restructurings principal balances			$ 43,895		$ 36,707
Real estate – commercial and farmland | Financing Receivables, 30 Days Past Due | Non-Accruing Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract			7		7
Troubled debt restructurings principal balances			$ 6,715		$ 1,401
Real estate – residential
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract	9	14	21	91
Troubled debt restructurings principal balances	$ 1,818	$ 2,609	$ 3,270	$ 12,692
Real estate – residential | Financing Receivables, 30 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract	4	12	15	16
Troubled debt restructurings principal balances	$ 489	$ 957	$ 1,111	$ 1,142
Real estate – residential | Financing Receivables, 30 Days Past Due | Accruing Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract			227		264
Troubled debt restructurings principal balances			$ 29,521		$ 38,800
Real estate – residential | Financing Receivables, 30 Days Past Due | Non-Accruing Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts | contract			30		34
Troubled debt restructurings principal balances			$ 2,687		$ 2,671