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MANDATORILY REDEEMABLE PREFERRED SECURITIES (Details) $ in Millions	3 Months Ended
	Mar. 31, 2022 USD ($)
Community Capital Trust IV [Member]
Mandatorily Redeemable Preferred Securities [Abstract]
Redemption of debentures and associated preferred securities	$ 77.3

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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

Name:	us-gaap_LongTermDebtByComponentsCurrentAndNoncurrentAlternativeAbstract
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: