LOANS - Troubled Debt Restructurings (TDRs) (Details) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) loan borrower	Sep. 30, 2020 USD ($) loan	Dec. 31, 2020 USD ($) loan borrower
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	233		247
TDRs, amount	$ 7,078		$ 6,984
Loans modified in TDR during the year, number | loan	20	25
Loans modified in TDR during the year, amount	$ 2,733	$ 835
Number of Borrowers Representing $75.6 Million in Outstanding Loan Balances | borrower	4
Outstanding Loan Balances of 47 Number of Borrowers	$ 3,900
Percentage of total loan outstanding	0.10%		0.90%
Number of Business Borrowers Representing $75.1 Million in Outstanding Loan Balances | borrower	3
Outstanding loan balances of 42 number of business borrowers	$ 3,800
Number of Consumer Borrowers Representing $0.5 Million in Outstanding Loan Balances | borrower	1
Outstanding Loan Balances of 5 Number of Consumer Borrowers	$ 100
Number of Borrowers Representing $66.5 Million in Outstanding Loan Balances | borrower			74
Outstanding Loan Balances of 74 Number of Borrowers			$ 66,500
Number of Borrowers Representing $65.7 Million in Outstanding Loan Balances | borrower			63
Outstanding Loan Balances of 63 Number of Borrowers			$ 65,700
Number of Borrowers Representing $0.8 Million in Outstanding Loan Balances | borrower			11
Outstanding Loan Balances of 11 Number of Borrowers			$ 800
Minimum [Member]
Financing Receivable, Modifications [Line Items]
Threshold balance of loan individually evaluated for impairment	500
Maximum [Member]
Financing Receivable, Modifications [Line Items]
Threshold balance of TDR loans collectively included in general loan loss allocation and qualitative review	$ 500
Nonaccrual [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	82		73
TDRs, amount	$ 3,530		$ 3,227
Accruing [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	151		174
TDRs, amount	$ 3,548		$ 3,757
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Loans modified in TDR during the year, number | loan	43	46
Loans modified in TDR during the year, amount	$ 3,348	$ 1,707
Commercial Portfolio Segment [Member] | Business Lending [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	15		10
TDRs, amount	$ 633		$ 720
Loans modified in TDR during the year, number | loan	5	0
Loans modified in TDR during the year, amount	$ 1,925	$ 0
Commercial Portfolio Segment [Member] | Business Lending [Member] | Nonaccrual [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	10		6
TDRs, amount	$ 458		$ 529
Commercial Portfolio Segment [Member] | Business Lending [Member] | Accruing [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	5		4
TDRs, amount	$ 175		$ 191
Commercial Portfolio Segment [Member] | Consumer Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Loans modified in TDR during the year, number | loan	19	17
Loans modified in TDR during the year, amount	$ 1,193	$ 1,378
Commercial Portfolio Segment [Member] | Consumer Indirect [Member]
Financing Receivable, Modifications [Line Items]
Loans modified in TDR during the year, number | loan	17	25
Loans modified in TDR during the year, amount	$ 222	$ 261
Commercial Portfolio Segment [Member] | Consumer Direct [Member]
Financing Receivable, Modifications [Line Items]
Loans modified in TDR during the year, number | loan	2	2
Loans modified in TDR during the year, amount	$ 8	$ 12
Commercial Portfolio Segment [Member] | Home Equity [Member]
Financing Receivable, Modifications [Line Items]
Loans modified in TDR during the year, number | loan	0	2
Loans modified in TDR during the year, amount	$ 0	$ 56
Residential Portfolio Segment [Member] | Consumer Mortgage [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	106		104
TDRs, amount	$ 5,135		$ 4,679
Loans modified in TDR during the year, number | loan	9	8
Loans modified in TDR during the year, amount	$ 728	$ 646
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Nonaccrual [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	62		56
TDRs, amount	$ 2,828		$ 2,413
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Accruing [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	44		48
TDRs, amount	$ 2,307		$ 2,266
Consumer Portfolio Segment [Member] | Consumer Indirect [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	73		86
TDRs, amount	$ 816		$ 951
Loans modified in TDR during the year, number | loan	5	14
Loans modified in TDR during the year, amount	$ 78	$ 131
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Nonaccrual [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	0		0
TDRs, amount	$ 0		$ 0
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Accruing [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	73		86
TDRs, amount	$ 816		$ 951
Consumer Portfolio Segment [Member] | Consumer Direct [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	17		23
TDRs, amount	$ 10		$ 85
Loans modified in TDR during the year, number | loan	1	1
Loans modified in TDR during the year, amount	$ 2	$ 2
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Nonaccrual [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	0		0
TDRs, amount	$ 0		$ 0
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Accruing [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	17		23
TDRs, amount	$ 10		$ 85
Consumer Portfolio Segment [Member] | Home Equity [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	22		24
TDRs, amount	$ 484		$ 549
Loans modified in TDR during the year, number | loan	0	2
Loans modified in TDR during the year, amount	$ 0	$ 56
Consumer Portfolio Segment [Member] | Home Equity [Member] | Nonaccrual [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	10		11
TDRs, amount	$ 244		$ 285
Consumer Portfolio Segment [Member] | Home Equity [Member] | Accruing [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number | loan	12		13
TDRs, amount	$ 240		$ 264