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

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