SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) Calculation Type Subsidiary County Branch Factor Component Trust Segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 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 | Branch	231
Risks and Uncertainties [Abstract]
Number of main components of economic risk | Component	3
Wealth Management Services [Abstract]
Number of types of performance obligations | Type	2
Contract Balances [Abstract]
Accounts receivable	$ 26.8	$ 26.4
Unbilled fee revenue	7.5	7.8
Unearned revenue	1.8	2.2
Cash and Cash Equivalents [Abstract]
Cash due from banks	$ 10.4	15.0
Impaired and Other Nonaccrual Loans [Abstract]
Number of days past due for loans to be placed on nonaccrual status	90 days
Period for which a nonaccrual TDR loan remains on nonaccrual status	12 months
Allowance for Loan Losses [Abstract]
Number of broad components used in allowance methodology | Component	2
Number of calculations used for general loan loss allocations | Calculation	2
Number of main loan segments | Segment	5
Period of historical net charge-off data for each loan class used in determining allowance level	36 months
Number of qualitative environmental factors used in qualitative calculation for general loan loss allocations | Factor	8
Other Real Estate [Abstract]
Other real estate	$ 1.3	1.3
Advertising [Abstract]
Advertising costs	7.1	$ 5.1	$ 5.7
ASU 2016-07 [Member]
Investments in Real Estate Limited Partnerships [Abstract]
Unfunded commitments	0.4
Impairment losses related to real estate partnerships	$ 0.0
Investments in Real Estate Limited Partnerships [Member] | Minimum [Member]
Investments in Real Estate Limited Partnerships [Abstract]
Ownership interest	5.00%
Investments in Real Estate Limited Partnerships [Member] | Maximum [Member]
Investments in Real Estate Limited Partnerships [Abstract]
Ownership interest	99.99%
Upstate New York [Member]
Nature of Operations [Abstract]
Number of counties where the bank has facilities | County	40
Northeastern Pennsylvania [Member]
Nature of Operations [Abstract]
Number of counties where the bank has facilities | County	6
Vermont [Member]
Nature of Operations [Abstract]
Number of counties where the bank has facilities | County	12
Western Massachusetts [Member]
Nature of Operations [Abstract]
Number of counties where the bank has facilities | County	1
Consumer Installment Loans [Member]
Impaired and Other Nonaccrual Loans [Abstract]
Number of days past due after which loans are charged off to the extent outstanding principal balance exceeds fair value of collateral/property	90 days
Consumer Mortgage Loans [Member]
Impaired and Other Nonaccrual Loans [Abstract]
Number of days past due after which loans are charged off to the extent outstanding principal balance exceeds fair value of collateral/property	180 days
Home Equity Loans [Member]
Impaired and Other Nonaccrual Loans [Abstract]
Number of days past due after which loans are charged off to the extent outstanding principal balance exceeds fair value of collateral/property	180 days
Equipment [Member] | Minimum [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	2 years
Equipment [Member] | Maximum [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	20 years
Software [Member] | Minimum [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	3 years
Software [Member] | Maximum [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	7 years
Hardware [Member] | Minimum [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	3 years
Hardware [Member] | Maximum [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	7 years
Building and Building Improvements [Member] | Minimum [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	10 years
Building and Building Improvements [Member] | Maximum [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	40 years
Land Improvements [Member] | Maximum [Member]
Property, Plant and Equipment, Gross [Abstract]
Estimated useful life	20 years
BPAS [Member]
Nature of Operations [Abstract]
Number of significant consolidated subsidiaries owned | Subsidiary	5
Hand Benefits & Trust [Member]
Nature of Operations [Abstract]
Number of significant consolidated subsidiaries owned | Subsidiary	1
NRS [Member]
Nature of Operations [Abstract]
Number of significant consolidated subsidiaries owned | Subsidiary	1
Core Deposits [Member] | Minimum [Member]
Intangible Assets [Abstract]
Intangible asset useful life (amortization period)	7 years
Core Deposits [Member] | Maximum [Member]
Intangible Assets [Abstract]
Intangible asset useful life (amortization period)	20 years
Customer Relationships [Member] | Minimum [Member]
Intangible Assets [Abstract]
Intangible asset useful life (amortization period)	7 years
Customer Relationships [Member] | Maximum [Member]
Intangible Assets [Abstract]
Intangible asset useful life (amortization period)	20 years
Commercial Portfolio Segment [Member] | Minimum [Member]
Impaired and Other Nonaccrual Loans [Abstract]
Threshold balance of loans individually evaluated for impairment	$ 0.5