ACCOUNTING POLICIES (Details) $ in Millions	9 Months Ended
Sep. 30, 2016 USD ($) Component Calculation Segment Factor
Financing Receivable, Allowance for Credit Losses [Line Items]
Number of broad components used in allowance methodology \| Component	2
Number of calculations used for general loan loss allocation \| Calculation	2
Number of main loan segments \| Segment	5
Period of historical net charge-off data for each loan class used in determining allowance level	36 months
Number of qualitative environmental factors used in qualitative calculation for loan loss allocation \| Factor	8
Core Deposits [Member] \| Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible asset useful life (amortization period)	7 years
Core Deposits [Member] \| Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible asset useful life (amortization period)	20 years
Customer Relationships [Member] \| Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible asset useful life (amortization period)	7 years
Customer Relationships [Member] \| Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible asset useful life (amortization period)	20 years
Commercial Portfolio Segment [Member] \| Minimum [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Threshold balance of loan individually evaluated for impairment \| $	$ 0.5

The threshold balance of financing receivable that is individually evaluated for impairment.

No definition available.

Name:	cbu_FinancingReceivableIndividuallyEvaluatedForImpairmentThresholdBalance
Namespace Prefix:	cbu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Refers to the number of broad components used in the entity's allowance methodology.

+ References

No definition available.

+ Details

Name:	cbu_NumberOfBroadComponentsUsedInAllowanceMethodology
Namespace Prefix:	cbu_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Refers to the number of calculations used for general loan loss allocation.

+ References

No definition available.

+ Details

Name:	cbu_NumberOfCalculationsUsedForGeneralLoanLossAllocation
Namespace Prefix:	cbu_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Refers to the number of main loan segments of the entity.

+ References

No definition available.

+ Details

Name:	cbu_NumberOfMainLoanSegments
Namespace Prefix:	cbu_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Refers to the number of qualitative environmental factors used in qualitative calculation for loan loss allocation.

+ References

No definition available.

+ Details

Name:	cbu_NumberOfQualitativeEnvironmentalFactorsUsedInQualitativeCalculationForLoanLossAllocation
Namespace Prefix:	cbu_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Refers to the period of historical net charge-off data for each loan class used in determining allowance level, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	cbu_PeriodOfHistoricalNetChargeOffDataForEachLoanClassUsedInDeterminingAllowanceLevel
Namespace Prefix:	cbu_
Data Type:	xbrli:durationItemType
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

No definition available.

+ Details

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

- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
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

No definition available.

+ Details

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

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CoreDepositsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CustomerRelationshipsMember
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: