LOANS AND ALLOWANCE FOR CREDIT LOSSES - Performance of Loan Modifications (Details) $ in Thousands	3 Months Ended
Mar. 31, 2024 USD ($)
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	$ 2,990
Total Class of Financing Receivable (in percent)	0.01%
Financial Asset, Not Past Due
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	$ 2,711
Past Due 30 - 89 Days
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
90+ Days Past Due and Still Accruing
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
Non-Accrual
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	279
Term Extension
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	685
Commercial Portfolio Segment \| CRE - owner occupied
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	$ 1,686
Total Class of Financing Receivable (in percent)	0.05%
Commercial Portfolio Segment \| CRE - owner occupied \| Financial Asset, Not Past Due
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	$ 1,686
Commercial Portfolio Segment \| CRE - owner occupied \| Past Due 30 - 89 Days
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
Commercial Portfolio Segment \| CRE - owner occupied \| 90+ Days Past Due and Still Accruing
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
Commercial Portfolio Segment \| CRE - owner occupied \| Non-Accrual
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
Commercial Portfolio Segment \| CRE - owner occupied \| Term Extension
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	353
Commercial Portfolio Segment \| CRE - non-owner occupied
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	$ 886
Total Class of Financing Receivable (in percent)	0.01%
Commercial Portfolio Segment \| CRE - non-owner occupied \| Financial Asset, Not Past Due
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	$ 886
Commercial Portfolio Segment \| CRE - non-owner occupied \| Past Due 30 - 89 Days
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
Commercial Portfolio Segment \| CRE - non-owner occupied \| 90+ Days Past Due and Still Accruing
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
Commercial Portfolio Segment \| CRE - non-owner occupied \| Non-Accrual
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
Commercial Portfolio Segment \| CRE - non-owner occupied \| Term Extension
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	224
Commercial Portfolio Segment \| Commercial & industrial and other business loans
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	$ 108
Total Class of Financing Receivable (in percent)	0.00%
Commercial Portfolio Segment \| Commercial & industrial and other business loans \| Financial Asset, Not Past Due
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	$ 108
Commercial Portfolio Segment \| Commercial & industrial and other business loans \| Past Due 30 - 89 Days
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
Commercial Portfolio Segment \| Commercial & industrial and other business loans \| 90+ Days Past Due and Still Accruing
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
Commercial Portfolio Segment \| Commercial & industrial and other business loans \| Non-Accrual
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
Commercial Portfolio Segment \| Commercial & industrial and other business loans \| Term Extension
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	108
Consumer Portfolio Segment \| Consumer mortgage
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	279
Consumer Portfolio Segment \| Consumer mortgage \| Financial Asset, Not Past Due
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
Consumer Portfolio Segment \| Consumer mortgage \| Past Due 30 - 89 Days
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
Consumer Portfolio Segment \| Consumer mortgage \| 90+ Days Past Due and Still Accruing
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
Consumer Portfolio Segment \| Consumer mortgage \| Non-Accrual
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	279
Residential Portfolio Segment \| Home equity
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	31
Residential Portfolio Segment \| Home equity \| Financial Asset, Not Past Due
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	31
Residential Portfolio Segment \| Home equity \| Past Due 30 - 89 Days
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
Residential Portfolio Segment \| Home equity \| 90+ Days Past Due and Still Accruing
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	0
Residential Portfolio Segment \| Home equity \| Non-Accrual
LOANS AND ALLOWANCE FOR CREDIT LOSSES
Loan modifications amount	$ 0

Amortized cost excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty.

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

- Definition

Percentage, excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty to total financing receivables.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-42

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-42

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481933/310-10-55-12A

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-33

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-34

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancialAssetNotPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=cbu_FinancingReceivables30To89DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=cbu_FinancingReceivables90PlusDaysPastDueAndStillAccruingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=cbu_FinancingReceivablesNonAccrualMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cbu_CommercialRealEstateOwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cbu_CommercialRealEstateNonOwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cbu_CommercialIndustrialAndOtherBusinessLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cbu_HomeEquityReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: