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LOANS, Allowance for Loan Losses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Allowance for loan losses [Roll Forward]
Beginning balance	$ 45,341us-gaap_LoansAndLeasesReceivableAllowance	$ 44,319us-gaap_LoansAndLeasesReceivableAllowance
Charge-offs	(2,414)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs	(2,348)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
Recoveries	1,455us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts	1,226us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
Provision	623us-gaap_ProvisionForLoanAndLeaseLosses	1,000us-gaap_ProvisionForLoanAndLeaseLosses
Ending balance	45,005us-gaap_LoansAndLeasesReceivableAllowance	44,197us-gaap_LoansAndLeasesReceivableAllowance
Consumer Mortgage [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	10,286us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember	8,994us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember
Charge-offs	(443)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember	(167)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember
Recoveries	21us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember	41us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember
Provision	369us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember	413us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember
Ending balance	10,233us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember	9,281us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember
Business Lending [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	15,787us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember	17,507us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember
Charge-offs	(133)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember	(120)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember
Recoveries	82us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember	171us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember
Provision	(331)us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember	(512)us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember
Ending balance	15,405us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember	17,046us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember
Consumer Indirect [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	11,544us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember	10,248us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember
Charge-offs	(1,427)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember	(1,427)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember
Recoveries	1,152us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember	796us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember
Provision	(23)us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember	969us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember
Ending balance	11,246us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember	10,586us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember
Consumer Direct [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	3,083us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember	3,181us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember
Charge-offs	(345)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember	(492)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember
Recoveries	193us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember	212us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember
Provision	(52)us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember	186us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember
Ending balance	2,879us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember	3,087us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember
Home Equity [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	2,701us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember	1,830us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember
Charge-offs	(66)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember	(129)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember
Recoveries	7us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember	6us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember
Provision	21us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember	111us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember
Ending balance	2,663us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember	1,818us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember
Unallocated [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	1,767us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember	2,029us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember
Charge-offs	0us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember	0us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember
Recoveries	0us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember	0us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember
Provision	616us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember	149us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember
Ending balance	2,383us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember	2,178us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember
Acquired Impaired [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	173us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_AcquiredImpairedMember	530us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_AcquiredImpairedMember
Charge-offs	0us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_AcquiredImpairedMember	(13)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_AcquiredImpairedMember
Recoveries	0us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_AcquiredImpairedMember	0us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_AcquiredImpairedMember
Provision	23us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_AcquiredImpairedMember	(316)us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_AcquiredImpairedMember
Ending balance	$ 196us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_AcquiredImpairedMember	$ 201us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_AcquiredImpairedMember

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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

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Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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

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-Name Staff Accounting Bulletin (SAB)

-Number Topic 6

-Section L

-Subsection 1

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Amount of expense related to estimated loss from loan and lease transactions.

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-Topic 230

-SubTopic 10

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