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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ in Millions | 12 Months Ended
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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 |  |

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