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ACQUISITIONS - Intangible asset, goodwill and acquisition related expenses (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022 | May 13, 2022
Identifiable Intangible Assets |  |  |  |  |
Merger and acquisition integration related (recoveries) expenses | $ 0 | $ 409 | $ 56 | $ 4,668 |
Other intangibles |  |  |  |  |
Identifiable Intangible Assets |  |  |  |  |
Intangible asset useful life (amortization period) | 10 years |  | 10 years |  |
Elmira Savings Bank ("Elmira") |  |  |  |  |
Identifiable Intangible Assets |  |  |  |  |
Federal Home Loan Bank of New York borrowings |  |  |  |  | $ 17,600
Weighted-average rate |  |  |  |  | 2.48%
Axiom | Core deposit intangibles |  |  |  |  |
Identifiable Intangible Assets |  |  |  |  |
Intangible asset useful life (amortization period) | 8 years |  | 8 years |  |

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- Definition This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 23 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479405/805-10-25-23 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Weighted average interest rate of debt outstanding. + References No definition available. + Details
Name: | us-gaap_DebtWeightedAverageInterestRate
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Amount of Federal Home Loan Bank (FHLBank) advances to member financial institutions. + References No definition available. + Details
Name: | us-gaap_FederalHomeLoanBankAdvances
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | instant

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_BusinessAcquisitionAxis=cbu_ElmiraSavingsBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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