STOCK-BASED COMPENSATION PLANS - Performance Awards - Additional information (Details) $ in Thousands	12 Months Ended
Dec. 31, 2022 USD ($) item shares	Dec. 31, 2021 USD ($) shares	Dec. 31, 2020 USD ($) shares
STOCK-BASED COMPENSATION PLANS
Shares issuable \| shares	31,999	47,820	48,976
Performance Awards
STOCK-BASED COMPENSATION PLANS
Number of sets of performance criteria \| item	2
Cumulative period of Relative Total Shareholder Pattern	3 years
Relative TSR employment weightage (as percent)	50.00%
Core ROATCE period	3 years
Relative Total Performance Goal, weightage (as percent)	50.00%
Vesting period	3 years
Estimated payout percent	90.00%
Shares issuable \| shares	31,999
Unrecognized stock-based compensation expense \| $	$ 700
Weighted-average period over which unrecognized expense	2 years
Total fair value of restricted stock vested \| $	$ 20	$ 10	$ 100

Represents the Core ROATCE period.

No definition available.

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Namespace Prefix:	cbu_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the estimated payout on compensation cost under the terms of the plan for the different performance sets.

+ References

No definition available.

+ Details

Name:	cbu_PerformanceMeasurementPeriodEstimatedPayoutPercent
Namespace Prefix:	cbu_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of sets of performance criteria based on the individual award.

+ References

No definition available.

+ Details

Name:	cbu_PerformanceMeasurementPeriodNumberOfPerformanceCriteriaSetsBasedOnIndividualAward
Namespace Prefix:	cbu_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the percent of Relative Total Performance Goal weightage.

+ References

No definition available.

+ Details

Name:	cbu_RelativeTotalPerformanceGoalSecondPerformanceCriteriaWeightagePercent
Namespace Prefix:	cbu_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percent of Relative TSR employment weightage.

+ References

No definition available.

+ Details

Name:	cbu_TotalShareholderReturnEmploymentWeightageFirstPerformanceCriteriaPercent
Namespace Prefix:	cbu_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the cumulative period of Relative Total Shareholder Return.

+ References

No definition available.

+ Details

Name:	cbu_TotalShareholderReturnMeasurementPeriodFirstPerformanceCriteria
Namespace Prefix:	cbu_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(ii)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardTypeAxis=cbu_PerformanceAwardRestrictedStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: