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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ in Millions	12 Months Ended
	Dec. 31, 2022 USD ($) segment component country subsidiary Office item	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Nature of Operations
Number of significant consolidated subsidiaries owned \| subsidiary	2
Number of bank branches \| Office	203
Number of operating location \| item	13
Risk and Uncertainties
Number of main components of economic risk \| component	3
Wealth Management Services
Number of types of performance obligations \| item	2
Contract Balances
Accounts receivable	$ 33.3	$ 31.6
Unbilled fee revenue	8.8	9.1
Unearned revenue	1.1	2.2
Cash and Cash Equivalents
Cash due from banks	10.0	11.0
Allowance for Credit Losses - Debt Securities
Accrued interest and fees receivable	$ 25.1	16.7
Debt Securities, Held-to-Maturity, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration]	Interest Receivable
Other Real Estate
Other real estate	$ 0.5	0.7
Investments in Real Estate Limited Partnerships
Impairment losses related to real estate partnerships	0.0
Advertising
Advertising costs	8.2	5.2	$ 6.1
Available-for-sale Securities
Allowance for Credit Losses - Debt Securities
Accrued interest and fees receivable	19.3	$ 15.8
Accrued interest receivable on held-to-maturity securities, included in accrued interest and fees receivable	$ 4.7
Land Improvements
Premises and Equipment
Estimated useful life	20 years
Minimum \| LITCH
Investments in Real Estate Limited Partnerships
Ownership interest	5.00%
Minimum \| Premises and Equipment
Premises and Equipment
Estimated useful life	2 years
Minimum \| Software
Premises and Equipment
Estimated useful life	3 years
Minimum \| Hardware
Premises and Equipment
Estimated useful life	3 years
Minimum \| Building and Building Improvements
Premises and Equipment
Estimated useful life	10 years
Minimum \| Core deposit intangibles
Intangible Assets
Intangible asset useful life (amortization period)	7 years
Minimum \| Customer relationships
Intangible Assets
Intangible asset useful life (amortization period)	7 years
Maximum \| LITCH
Investments in Real Estate Limited Partnerships
Ownership interest	99.99%
Maximum \| Premises and Equipment
Premises and Equipment
Estimated useful life	20 years
Maximum \| Software
Premises and Equipment
Estimated useful life	7 years
Maximum \| Hardware
Premises and Equipment
Estimated useful life	7 years
Maximum \| Building and Building Improvements
Premises and Equipment
Estimated useful life	40 years
Maximum \| Core deposit intangibles
Intangible Assets
Intangible asset useful life (amortization period)	20 years
Maximum \| Customer relationships
Intangible Assets
Intangible asset useful life (amortization period)	20 years
BPAS
Nature of Operations
Number of significant consolidated subsidiaries owned \| segment	5
Hand Benefits & Trust
Nature of Operations
Number of significant consolidated subsidiaries owned \| segment	1
NRS
Nature of Operations
Number of significant consolidated subsidiaries owned \| segment	1
Upstate New York
Nature of Operations
Number of counties where the bank has facilities \| country	42
Northeastern Pennsylvania
Nature of Operations
Number of counties where the bank has facilities \| country	6
Vermont
Nature of Operations
Number of counties where the bank has facilities \| country	12
Western Massachusetts
Nature of Operations
Number of counties where the bank has facilities \| country	1