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DERIVATIVE INSTRUMENTS (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]  
Derivative, Gain (Loss), net $ (12)us-gaap_DerivativeGainLossOnDerivativeNet
Not Designated as Hedging Instrument [Member]  
Derivatives, Estimated Fair Value and Balance Sheet Location [Line Items]  
Fair Value, net 130us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Commitments to originate real estate loans for sale [Member] | Mortgage banking and other services [Member]  
Derivative Instruments, Gain (Loss) [Line Items]  
Derivative, Gain (Loss), net (33)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= cbu_CommitmentsToOriginateRealEstateLoansForSaleMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesMember
Forward sales commitments [Member] | Mortgage banking and other services [Member]  
Derivative Instruments, Gain (Loss) [Line Items]  
Derivative, Gain (Loss), net 21us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_OtherContractMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesMember
Other Assets [Member] | Commitments to originate real estate loans for sale [Member] | Not Designated as Hedging Instrument [Member]  
Derivatives, Estimated Fair Value and Balance Sheet Location [Line Items]  
Derivative, Notional Amount 5,035invest_DerivativeNotionalAmount
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= cbu_CommitmentsToOriginateRealEstateLoansForSaleMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Fair Value, net 152us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= cbu_CommitmentsToOriginateRealEstateLoansForSaleMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Other Liabilities [Member] | Forward sales commitments [Member] | Not Designated as Hedging Instrument [Member]  
Derivatives, Estimated Fair Value and Balance Sheet Location [Line Items]  
Derivative, Notional Amount 5,236invest_DerivativeNotionalAmount
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_OtherContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Fair Value, net $ (22)us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_OtherContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember