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LOANS, Troubled Debt Restructuring (TDR) (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Loan
Dec. 31, 2013
Loan
Financing Receivable, Modifications [Line Items]    
Individual commercial loan evaluated for impairment, minimum $ 500,000us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment  
TDR included in general loan loss allocation and the qualitative review, maximum 500,000cbu_FinancingReceivableAllowanceForCreditLossesCollectivelyModifiedEvaluatedForImpairment  
Troubled debt restructurings, nonaccrual, number of loans 68cbu_TroubledDebtRestructuringsNonaccrual 47cbu_TroubledDebtRestructuringsNonaccrual
Troubled debt restructurings, nonaccrual, amount 2,752,000cbu_TroubledDebtRestructuringsNonaccrualAmount 2,046,000cbu_TroubledDebtRestructuringsNonaccrualAmount
Troubled debt restructurings, accruing, number of loans 157cbu_TroubledDebtRestructuringsAccruing 213cbu_TroubledDebtRestructuringsAccruing
Troubled debt restructurings, accruing, amount 3,200,000cbu_TroubledDebtRestructuringsAccruingCarryingBalance 3,389,000cbu_TroubledDebtRestructuringsAccruingCarryingBalance
Troubled debt restructurings, total, number of loans 225cbu_TroubledDebtRestructurings 260cbu_TroubledDebtRestructurings
Troubled debt restructurings, total, amount 5,952,000us-gaap_FinancingReceivableModificationsRecordedInvestment 5,435,000us-gaap_FinancingReceivableModificationsRecordedInvestment
Loans modified in TDR during the year, number of contracts 82us-gaap_FinancingReceivableModificationsNumberOfContracts2 106us-gaap_FinancingReceivableModificationsNumberOfContracts2
Loans modified in TDR during the year, amount 2,201,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 2,641,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Number of non-Chapter 7 bankruptcy TDRs 3cbu_FinancingReceivableModificationsNonChapter7BankruptcyTDRsNumberOfContracts 2cbu_FinancingReceivableModificationsNonChapter7BankruptcyTDRsNumberOfContracts
Consumer Mortgage [Member]    
Financing Receivable, Modifications [Line Items]    
Troubled debt restructurings, nonaccrual, number of loans 49cbu_TroubledDebtRestructuringsNonaccrual
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
31cbu_TroubledDebtRestructuringsNonaccrual
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Troubled debt restructurings, nonaccrual, amount 2,092,000cbu_TroubledDebtRestructuringsNonaccrualAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
1,682,000cbu_TroubledDebtRestructuringsNonaccrualAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Troubled debt restructurings, accruing, number of loans 37cbu_TroubledDebtRestructuringsAccruing
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
48cbu_TroubledDebtRestructuringsAccruing
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Troubled debt restructurings, accruing, amount 1,770,000cbu_TroubledDebtRestructuringsAccruingCarryingBalance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
2,171,000cbu_TroubledDebtRestructuringsAccruingCarryingBalance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Troubled debt restructurings, total, number of loans 86cbu_TroubledDebtRestructurings
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
79cbu_TroubledDebtRestructurings
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Troubled debt restructurings, total, amount 3,862,000us-gaap_FinancingReceivableModificationsRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
3,853,000us-gaap_FinancingReceivableModificationsRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Loans modified in TDR during the year, number of contracts 22us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
31us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Loans modified in TDR during the year, amount 949,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
1,758,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Business Lending [Member]    
Financing Receivable, Modifications [Line Items]    
Troubled debt restructurings, nonaccrual, number of loans 6cbu_TroubledDebtRestructuringsNonaccrual
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
4cbu_TroubledDebtRestructuringsNonaccrual
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Troubled debt restructurings, nonaccrual, amount 442,000cbu_TroubledDebtRestructuringsNonaccrualAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
162,000cbu_TroubledDebtRestructuringsNonaccrualAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Troubled debt restructurings, accruing, number of loans 3cbu_TroubledDebtRestructuringsAccruing
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
1cbu_TroubledDebtRestructuringsAccruing
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Troubled debt restructurings, accruing, amount 468,000cbu_TroubledDebtRestructuringsAccruingCarryingBalance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
47,000cbu_TroubledDebtRestructuringsAccruingCarryingBalance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Troubled debt restructurings, total, number of loans 9cbu_TroubledDebtRestructurings
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
5cbu_TroubledDebtRestructurings
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Troubled debt restructurings, total, amount 910,000us-gaap_FinancingReceivableModificationsRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
209,000us-gaap_FinancingReceivableModificationsRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Loans modified in TDR during the year, number of contracts 7us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
3us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Loans modified in TDR during the year, amount 769,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
183,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Number of loans restructured via granting a waiver of payments for a period of time 2cbu_FinancingReceivableModificationsGrantingAWaiverOfPaymentsForAPeriodOfTimeNumberOfContracts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
 
Number of loans restructured by extension of terms 1cbu_FinancingReceivableModificationsTermExtensionNumberOfContracts
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
 
Consumer Indirect [Member]    
Financing Receivable, Modifications [Line Items]    
Troubled debt restructurings, nonaccrual, number of loans 0cbu_TroubledDebtRestructuringsNonaccrual
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
0cbu_TroubledDebtRestructuringsNonaccrual
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
Troubled debt restructurings, nonaccrual, amount 0cbu_TroubledDebtRestructuringsNonaccrualAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
0cbu_TroubledDebtRestructuringsNonaccrualAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
Troubled debt restructurings, accruing, number of loans 79cbu_TroubledDebtRestructuringsAccruing
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
98cbu_TroubledDebtRestructuringsAccruing
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
Troubled debt restructurings, accruing, amount 615,000cbu_TroubledDebtRestructuringsAccruingCarryingBalance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
692,000cbu_TroubledDebtRestructuringsAccruingCarryingBalance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
Troubled debt restructurings, total, number of loans 79cbu_TroubledDebtRestructurings
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
98cbu_TroubledDebtRestructurings
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
Troubled debt restructurings, total, amount 615,000us-gaap_FinancingReceivableModificationsRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
692,000us-gaap_FinancingReceivableModificationsRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
Loans modified in TDR during the year, number of contracts 33us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
36us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
Loans modified in TDR during the year, amount 312,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
327,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentIndirectMember
Consumer Direct [Member]    
Financing Receivable, Modifications [Line Items]    
Troubled debt restructurings, nonaccrual, number of loans 0cbu_TroubledDebtRestructuringsNonaccrual
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
0cbu_TroubledDebtRestructuringsNonaccrual
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
Troubled debt restructurings, nonaccrual, amount 0cbu_TroubledDebtRestructuringsNonaccrualAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
0cbu_TroubledDebtRestructuringsNonaccrualAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
Troubled debt restructurings, accruing, number of loans 25cbu_TroubledDebtRestructuringsAccruing
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
46cbu_TroubledDebtRestructuringsAccruing
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
Troubled debt restructurings, accruing, amount 69,000cbu_TroubledDebtRestructuringsAccruingCarryingBalance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
116,000cbu_TroubledDebtRestructuringsAccruingCarryingBalance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
Troubled debt restructurings, total, number of loans 25cbu_TroubledDebtRestructurings
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
46cbu_TroubledDebtRestructurings
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
Troubled debt restructurings, total, amount 69,000us-gaap_FinancingReceivableModificationsRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
116,000us-gaap_FinancingReceivableModificationsRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
Loans modified in TDR during the year, number of contracts 14us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
22us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
Loans modified in TDR during the year, amount 26,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
75,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_ConsumerInstallmentDirectMember
Home Equity [Member]    
Financing Receivable, Modifications [Line Items]    
Troubled debt restructurings, nonaccrual, number of loans 13cbu_TroubledDebtRestructuringsNonaccrual
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember
12cbu_TroubledDebtRestructuringsNonaccrual
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember
Troubled debt restructurings, nonaccrual, amount 218,000cbu_TroubledDebtRestructuringsNonaccrualAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember
202,000cbu_TroubledDebtRestructuringsNonaccrualAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember
Troubled debt restructurings, accruing, number of loans 13cbu_TroubledDebtRestructuringsAccruing
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember
20cbu_TroubledDebtRestructuringsAccruing
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember
Troubled debt restructurings, accruing, amount 278,000cbu_TroubledDebtRestructuringsAccruingCarryingBalance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember
363,000cbu_TroubledDebtRestructuringsAccruingCarryingBalance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember
Troubled debt restructurings, total, number of loans 26cbu_TroubledDebtRestructurings
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember
32cbu_TroubledDebtRestructurings
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember
Troubled debt restructurings, total, amount 496,000us-gaap_FinancingReceivableModificationsRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember
565,000us-gaap_FinancingReceivableModificationsRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember
Loans modified in TDR during the year, number of contracts 6us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember
14us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember
Loans modified in TDR during the year, amount $ 145,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember
$ 298,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbu_HomeEquityReceivablesMember