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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) country trust subsidiary segment component item Office	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Nature of Operations [Abstract]
Number of significant consolidated subsidiaries owned \| subsidiary	2
Number of unconsolidated subsidiary business trusts owned \| trust	1
Number of bank branches \| Office	232
Risks and Uncertainties [Abstract]
Number of main components of economic risk \| component	3
Wealth Management Services [Abstract]
Number of types of performance obligations \| item	2
Contract Balances [Abstract]
Accounts receivable	$ 30.3	$ 26.8
Unbilled fee revenue	7.7	7.5
Unearned revenue	1.4	1.8
Cash and Cash Equivalents [Abstract]
Cash due from banks	7.7	10.4
Allowance for Loan Losses [Abstract]
Accrued interest and fees receivable	22.2
Other Real Estate [Abstract]
Other real estate	0.9	1.3
Investments in Real Estate Limited Partnerships [Abstract]
Unfunded commitments	0.9
Impairment losses related to real estate partnerships	0.0
Advertising [Abstract]
Advertising costs	6.1	$ 7.1	$ 5.1
Available-for-sale Securities [Member]
Allowance for Loan Losses [Abstract]
Accrued interest and fees receivable	$ 13.3
Land Improvements [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	20 years
Minimum [Member] \| Investments in Real Estate Limited Partnerships [Member]
Investments in Real Estate Limited Partnerships [Abstract]
Ownership interest	5.00%
Minimum [Member] \| Equipment [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	2 years
Minimum [Member] \| Software [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	3 years
Minimum [Member] \| Hardware [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	3 years
Minimum [Member] \| Building and Building Improvements [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	10 years
Minimum [Member] \| Core Deposit Intangibles [Member]
Intangible Assets [Abstract]
Intangible asset useful life (amortization period)	7 years
Minimum [Member] \| Customer Relationships [Member]
Intangible Assets [Abstract]
Intangible asset useful life (amortization period)	7 years
Maximum [Member]
Impaired and Other Nonaccrual Loans [Abstract]
Threshold balance of loans individually evaluated for impairment	$ 0.5
Maximum [Member] \| Investments in Real Estate Limited Partnerships [Member]
Investments in Real Estate Limited Partnerships [Abstract]
Ownership interest	99.99%
Maximum [Member] \| Equipment [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	20 years
Maximum [Member] \| Software [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	7 years
Maximum [Member] \| Hardware [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	7 years
Maximum [Member] \| Building and Building Improvements [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	40 years
Maximum [Member] \| Core Deposit Intangibles [Member]
Intangible Assets [Abstract]
Intangible asset useful life (amortization period)	20 years
Maximum [Member] \| Customer Relationships [Member]
Intangible Assets [Abstract]
Intangible asset useful life (amortization period)	20 years
BPAS [Member]
Nature of Operations [Abstract]
Number of significant consolidated subsidiaries owned \| segment	5
Hand Benefits & Trust [Member]
Nature of Operations [Abstract]
Number of significant consolidated subsidiaries owned \| segment	1
NRS [Member]
Nature of Operations [Abstract]
Number of significant consolidated subsidiaries owned \| segment	1
Upstate New York [Member]
Nature of Operations [Abstract]
Number of counties where the bank has facilities \| country	42
PENNSYLVANIA
Nature of Operations [Abstract]
Number of counties where the bank has facilities \| country	6
Vermont [Member]
Nature of Operations [Abstract]
Number of counties where the bank has facilities \| country	12
Massachusetts [Member]
Nature of Operations [Abstract]
Number of counties where the bank has facilities \| country	1