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MANDATORILY REDEEMABLE PREFERRED SECURITIES (Details) - USD ($) $ in Millions | Mar. 15, 2021 | Sep. 15, 2020
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Community Capital Trust IV [Member] |  |
Mandatorily Redeemable Preferred Securities [Abstract] |  |
Redemption of debentures and associated preferred securities | $ 77.3 |
Steuben Statutory Trust |  |
Mandatorily Redeemable Preferred Securities [Abstract] |  |
Redemption of debentures and associated preferred securities |  | $ 2.1

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- Definition The cash inflow from the exercise of the rights to redeem debentures and trust preferred securities, net of the cash outflow for repurchasing trust preferred securities, by a business trust or other special purpose entity, mainly established by a bank holding entity, to third party investors. The trust's assets are deeply subordinated debentures of the bank holding entity. Most trust preferred securities are subject to a mandatory redemption upon the repayment of the debentures. + References No definition available. + Details
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Namespace Prefix: | cbu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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