FAIR VALUE, Significant Unobservable Inputs (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Other Real Estate Owned [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	1,767	1,855
Other Real Estate Owned [Member] \| Fair Value of Collateral [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant unobservable inputs	Estimated cost of disposal/market adjustment	Estimated cost of disposal/market adjustment
Other Real Estate Owned [Member] \| Fair Value of Collateral [Member] \| Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated cost of disposal/market adjustment (in hundredths)	10.00%	10.00%
Other Real Estate Owned [Member] \| Fair Value of Collateral [Member] \| Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated cost of disposal/market adjustment (in hundredths)	49.20%	77.50%
Other Real Estate Owned [Member] \| Fair Value of Collateral [Member] \| Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated cost of disposal/market adjustment (in hundredths)	23.30%	30.60%
Commitments To Originate Real Estate Loans For Sale [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	152	185
Commitments To Originate Real Estate Loans For Sale [Member] \| Discounted Cash Flow [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant unobservable inputs	Embedded servicing value	Embedded servicing value
Embedded servicing value (in hundredths)	1.00%	1.00%

Embedded servicing value, used as an input to measure fair value.

No definition available.

Name:	cbu_FairValueInputsEmbeddedServicingValue
Namespace Prefix:	cbu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Adjustment to the appraisal used to measure fair value for estimated cost of disposal or market adjustment.

+ References

No definition available.

+ Details

Name:	cbu_FairValueInputsEstimatedCostOfDisposalOrMarketAdjustment
Namespace Prefix:	cbu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

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

- Details

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

- Definition

Description of the inputs and valuation technique(s) used to measure fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bbb)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

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

- Details

Name:	us-gaap_FairValueByAssetClassAxis=cbu_CommitmentsToOriginateRealEstateLoansForSaleMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=cbu_OtherRealEstateOwnedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: