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v3.20.4

SUBSEQUENT EVENTS (Details) $ in Millions | Jan. 20, 2021 USD ($)
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Community Capital Trust IV | Subsequent event |
Subsequent Event [Line Items] |
Authorized amount of debentures and associated preferred securities for redemption | $ 77.3

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- Definition Board of directors approved amount for redemption of debentures and associated preferred securities. + References No definition available. + Details
Name: | cbu_AuthorizedAmountOfDebenturesAndAssociatedPreferredSecuritiesForRedemption
Namespace Prefix: | cbu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | srt_OwnershipAxis=cbu_CommunityStatutoryTrust4Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |