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FAIR VALUE, Valuation Techniques (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Other Real Estate Owned [Member] | Fair Value of Collateral [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair value of assets classified as Level 3 1,855us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= cbu_OtherRealEstateOwnedMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_MarketApproachValuationTechniqueMember		 5,060us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= cbu_OtherRealEstateOwnedMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_MarketApproachValuationTechniqueMember
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%cbu_FairValueInputsEstimatedCostOfDisposalOrMarketAdjustment
/ us-gaap_FairValueByAssetClassAxis
= cbu_OtherRealEstateOwnedMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_MarketApproachValuationTechniqueMember		 11.00%cbu_FairValueInputsEstimatedCostOfDisposalOrMarketAdjustment
/ us-gaap_FairValueByAssetClassAxis
= cbu_OtherRealEstateOwnedMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_MarketApproachValuationTechniqueMember
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) 77.50%cbu_FairValueInputsEstimatedCostOfDisposalOrMarketAdjustment
/ us-gaap_FairValueByAssetClassAxis
= cbu_OtherRealEstateOwnedMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_MarketApproachValuationTechniqueMember		 54.40%cbu_FairValueInputsEstimatedCostOfDisposalOrMarketAdjustment
/ us-gaap_FairValueByAssetClassAxis
= cbu_OtherRealEstateOwnedMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_MarketApproachValuationTechniqueMember
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) 30.60%cbu_FairValueInputsEstimatedCostOfDisposalOrMarketAdjustment
/ us-gaap_FairValueByAssetClassAxis
= cbu_OtherRealEstateOwnedMember
/ us-gaap_RangeAxis
= us-gaap_WeightedAverageMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_MarketApproachValuationTechniqueMember		 28.10%cbu_FairValueInputsEstimatedCostOfDisposalOrMarketAdjustment
/ us-gaap_FairValueByAssetClassAxis
= cbu_OtherRealEstateOwnedMember
/ us-gaap_RangeAxis
= us-gaap_WeightedAverageMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_MarketApproachValuationTechniqueMember
Commitments To Originate Real Estate Loans For Sale [Member] | Discounted Cash Flow [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair value of assets classified as Level 3 185us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= cbu_CommitmentsToOriginateRealEstateLoansForSaleMember
/ us-gaap_ValuationTechniqueAxis
= cbu_DiscountedCashFlowMember		 44us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= cbu_CommitmentsToOriginateRealEstateLoansForSaleMember
/ us-gaap_ValuationTechniqueAxis
= cbu_DiscountedCashFlowMember
Embedded servicing value (in hundredths) 1.00%cbu_FairValueInputsEmbeddedServicingValue
/ us-gaap_FairValueByAssetClassAxis
= cbu_CommitmentsToOriginateRealEstateLoansForSaleMember
/ us-gaap_ValuationTechniqueAxis
= cbu_DiscountedCashFlowMember		 1.00%cbu_FairValueInputsEmbeddedServicingValue
/ us-gaap_FairValueByAssetClassAxis
= cbu_CommitmentsToOriginateRealEstateLoansForSaleMember
/ us-gaap_ValuationTechniqueAxis
= cbu_DiscountedCashFlowMember