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BENEFIT PLANS, Plan Assets (Details) (USD $)
 12 Months Ended 3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Sep. 30, 2013
Plan Assets [Abstract]        
Maximum percentage of alternative investments authorized to be purchased (in hundredths) 15.00%cbu_DefinedBenefitPlanTargetPlanAssetAllocationsAlternativeInvestments      
Maximum percentage of portfolio invested in company stock (in hundredths) 10.00%cbu_MaximumPercentageOfTotalPortfolioInvestedInCompanyStock      
Fair value of plan assets $ 177,551,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets [1] $ 173,081,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets [1]    
Total dividends and interest receivable excluded from plan assets 314,000cbu_TotalDividendsAndInterestReceivableExcludedFromPlanAssets 335,000cbu_TotalDividendsAndInterestReceivableExcludedFromPlanAssets    
Assumed Health Care Cost Trend Rates [Abstract]        
Assumed health care cost trend rate for medical costs for participants younger than 65 (in hundredths) 8.00%cbu_AssumedHealthCareCostTrendRateForParticipantsYoungerThan65      
Assumed health care cost trend rate for medical cost for participants older than 65 (in hundredths) 6.00%cbu_AssumedHealthCareCostTrendRateForParticipantsOlderThan65      
Assumed health care cost trend rate for prescription drugs (in hundredths) 9.00%cbu_AssumedHealthCareCostTrendRateForPrescriptionDrugs      
Ultimate health care cost trend rate (in hundredths) 5.00%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate      
Year that rate reaches ultimate trend rate 2024      
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract]        
Effect of one percentage point increase on service and interest cost components 100us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnServiceAndInterestCostComponents      
Effect of one percentage point increase on benefit obligation 2,000us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnAccumulatedPostretirementBenefitObligation      
Effect of one percentage point decrease on service and interest cost components 100us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnServiceAndInterestCostComponents1      
Effect of one percentage point decrease on benefit obligation 2,000us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnAccumulatedPostretirementBenefitObligation1      
Other Deferred Compensation Arrangement [Member]        
Nonqualified deferred compensation arrangements [Abstract]        
(Income)/expense recognized under plan 330,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= cbu_OtherDeferredCompensationArrangementMember		 (52,000)us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= cbu_OtherDeferredCompensationArrangementMember		 439,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= cbu_OtherDeferredCompensationArrangementMember		  
Liability recorded for compensation plan 3,895,000us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= cbu_OtherDeferredCompensationArrangementMember		 4,238,000us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= cbu_OtherDeferredCompensationArrangementMember		    
Employee Stock Ownership Plan 401K Plan [Member]        
401(k) Employee Stock Ownership Plan [Abstract]        
Contribution from eligible compensation, minimum (in hundredths) 1.00%cbu_ContributionFromEligibleCompensationMinimum
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= cbu_EmployeeStockOwnershipPlan401KPlanMember		      
Contribution from eligible compensation, maximum (in hundredths) 90.00%cbu_ContributionFromEligibleCompensationMaximum
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= cbu_EmployeeStockOwnershipPlan401KPlanMember		      
Percentage of first eligible compensation fully matched by employer (in hundredths) 3.00%cbu_PercentageOfFirstEligibleCompensationMatchedFullyByEmployer
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= cbu_EmployeeStockOwnershipPlan401KPlanMember		      
Percentage of matching contribution in the form of common stock (in hundredths) 100.00%cbu_PercentageOfMatchingContributionInFormOfCommonStock
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= cbu_EmployeeStockOwnershipPlan401KPlanMember		      
Percentage of the next eligible compensation matched at 50% by employer (in hundredths) 3.00%cbu_PercentageOfNextEligibleCompensationMatchedAt50ByEmployer
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= cbu_EmployeeStockOwnershipPlan401KPlanMember		      
Percentage of matching contributions for the next eligible compensation in the form of company stock (in hundredths) 50.00%cbu_PercentageOfMatchingContributionsForNextEligibleCompensation
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= cbu_EmployeeStockOwnershipPlan401KPlanMember		      
Interest credit contribution expense recognized for 401(k) plan 858,000cbu_DefinedContributionPlanInterestCreditContributionExpensedRecognized
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= cbu_EmployeeStockOwnershipPlan401KPlanMember		 650,000cbu_DefinedContributionPlanInterestCreditContributionExpensedRecognized
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= cbu_EmployeeStockOwnershipPlan401KPlanMember		 419,000cbu_DefinedContributionPlanInterestCreditContributionExpensedRecognized
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= cbu_EmployeeStockOwnershipPlan401KPlanMember		  
Nonqualified deferred compensation arrangements [Abstract]        
(Income)/expense recognized under plan 3,405,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= cbu_EmployeeStockOwnershipPlan401KPlanMember		 3,215,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= cbu_EmployeeStockOwnershipPlan401KPlanMember		 2,956,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= cbu_EmployeeStockOwnershipPlan401KPlanMember		  
Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 158,365,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 [1] 155,959,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 [1]    
Significant Observable Inputs, Level 2 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 19,186,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 [1] 17,122,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 [1]    
Significant Unobservable Inputs, Level 3 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 [1] 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 [1]    
Money Market Accounts [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 12,901,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashAndCashEquivalentsMember		 10,940,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashAndCashEquivalentsMember		    
Money Market Accounts [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 431,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashAndCashEquivalentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 709,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashAndCashEquivalentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		    
Money Market Accounts [Member] | Significant Observable Inputs, Level 2 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 12,470,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashAndCashEquivalentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 10,231,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashAndCashEquivalentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		    
Money Market Accounts [Member] | Significant Unobservable Inputs, Level 3 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashAndCashEquivalentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashAndCashEquivalentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		    
Equity Securities [Member]        
Plan Assets [Abstract]        
Target allocation percentage of assets (in hundredths) 60.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember		      
Fair value of plan assets 109,468,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember		 113,219,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember		    
Equity Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 109,468,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 113,219,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		    
Equity Securities [Member] | Significant Observable Inputs, Level 2 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_EquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		    
Equity Securities [Member] | Significant Unobservable Inputs, Level 3 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_EquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		    
U.S. Large-Cap [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 51,197,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USLargeCapMember		 50,513,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USLargeCapMember		    
U.S. Large-Cap [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 51,197,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USLargeCapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 50,513,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USLargeCapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		    
U.S. Large-Cap [Member] | Significant Observable Inputs, Level 2 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USLargeCapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USLargeCapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		    
U.S. Large-Cap [Member] | Significant Unobservable Inputs, Level 3 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USLargeCapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USLargeCapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		    
U.S Mid/Small Cap [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 14,026,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USMidSmallCapMember		 19,308,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USMidSmallCapMember		    
U.S Mid/Small Cap [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 14,026,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USMidSmallCapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 19,308,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USMidSmallCapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		    
U.S Mid/Small Cap [Member] | Significant Observable Inputs, Level 2 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USMidSmallCapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USMidSmallCapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		    
U.S Mid/Small Cap [Member] | Significant Unobservable Inputs, Level 3 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USMidSmallCapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_USMidSmallCapMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		    
CBSI Stock [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 15,354,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_CbsiStockMember		 9,913,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_CbsiStockMember		    
CBSI Stock [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 15,354,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_CbsiStockMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 9,913,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_CbsiStockMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		    
CBSI Stock [Member] | Significant Observable Inputs, Level 2 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_CbsiStockMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= cbu_CbsiStockMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		    
CBSI Stock [Member] | Significant Unobservable Inputs, Level 3 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_CbsiStockMember
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= us-gaap_FairValueInputsLevel3Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= cbu_CbsiStockMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		    
International [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 28,891,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InternationalMember		 33,485,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InternationalMember		    
International [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 28,891,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InternationalMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 33,485,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InternationalMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		    
International [Member] | Significant Observable Inputs, Level 2 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InternationalMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InternationalMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		    
International [Member] | Significant Unobservable Inputs, Level 3 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InternationalMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= cbu_InternationalMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		    
Fixed Income Securities [Member]        
Plan Assets [Abstract]        
Target allocation percentage of assets (in hundredths) 40.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_FixedIncomeMember		      
Fair value of plan assets 46,958,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_FixedIncomeMember		 48,845,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_FixedIncomeMember		    
Fixed Income Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 40,312,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_FixedIncomeMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 42,031,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_FixedIncomeMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		    
Fixed Income Securities [Member] | Significant Observable Inputs, Level 2 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 6,646,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_FixedIncomeMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 6,814,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_FixedIncomeMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		    
Fixed Income Securities [Member] | Significant Unobservable Inputs, Level 3 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_FixedIncomeMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_FixedIncomeMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		    
Government Securities [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 13,137,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_USTreasuryAndGovernmentMember		 14,490,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_USTreasuryAndGovernmentMember		    
Government Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 6,491,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 7,676,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		    
Government Securities [Member] | Significant Observable Inputs, Level 2 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 6,646,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 6,814,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		    
Government Securities [Member] | Significant Unobservable Inputs, Level 3 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		    
Investment Grade Bonds [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 21,539,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentGradeBondsMember		 17,400,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentGradeBondsMember		    
Investment Grade Bonds [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 21,539,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentGradeBondsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 17,400,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentGradeBondsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		    
Investment Grade Bonds [Member] | Significant Observable Inputs, Level 2 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentGradeBondsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentGradeBondsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		    
Investment Grade Bonds [Member] | Significant Unobservable Inputs, Level 3 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentGradeBondsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentGradeBondsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		    
International bonds [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 5,085,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentBondsMember		 2,712,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentBondsMember		    
International bonds [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 5,085,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentBondsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 2,712,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentBondsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		    
International bonds [Member] | Significant Observable Inputs, Level 2 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentBondsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentBondsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		    
International bonds [Member] | Significant Unobservable Inputs, Level 3 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentBondsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_InvestmentBondsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		    
High Yield [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 7,197,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_HighYieldMember		 [2] 14,243,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_HighYieldMember		 [2]    
High Yield [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 7,197,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_HighYieldMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 [2] 14,243,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_HighYieldMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 [2]    
High Yield [Member] | Significant Observable Inputs, Level 2 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_HighYieldMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 [2] 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_HighYieldMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 [2]    
High Yield [Member] | Significant Unobservable Inputs, Level 3 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_HighYieldMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 [2] 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= cbu_HighYieldMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 [2]    
Other Investments [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 8,224,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_OtherInvestmentsMember		 [3] 77,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_OtherInvestmentsMember		 [3]    
Other Investments [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 8,154,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 [3] 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 [3]    
Other Investments [Member] | Significant Observable Inputs, Level 2 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 70,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 [3] 77,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 [3]    
Other Investments [Member] | Significant Unobservable Inputs, Level 3 [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 [3] 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 [3]    
Director [Member] | Deferred Compensation Plan [Member]        
Nonqualified deferred compensation arrangements [Abstract]        
(Income)/expense recognized under plan 168,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= cbu_DeferredCompensationPlanMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_DirectorMember		 162,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= cbu_DeferredCompensationPlanMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_DirectorMember		 148,000us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= cbu_DeferredCompensationPlanMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_DirectorMember		  
Liability recorded for compensation plan 3,525,000us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= cbu_DeferredCompensationPlanMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_DirectorMember		 3,424,000us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= cbu_DeferredCompensationPlanMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_DirectorMember		    
Deferred compensation arrangement with individual, shares credited (in shares) 147,934us-gaap_DeferredCompensationArrangementWithIndividualSharesIssued
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= cbu_DeferredCompensationPlanMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_DirectorMember		 153,396us-gaap_DeferredCompensationArrangementWithIndividualSharesIssued
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= cbu_DeferredCompensationPlanMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_DirectorMember		    
Pension Benefits [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 177,865,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 173,416,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 143,661,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Employer contributions 616,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 10,622,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		   10,000,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
Projected benefit obligation 127,513,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 111,174,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 123,739,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Post-retirement Benefits [Member]        
Plan Assets [Abstract]        
Fair value of plan assets 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		  
Employer contributions 241,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		 498,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		    
Projected benefit obligation 2,256,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		 2,340,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		 3,051,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		  
Pentegra DB Plan [Member] | Tupper Lake National Bank [Member]        
Plan Assets [Abstract]        
Fair value of plan assets   3,000,000,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cbu_PentegraDbPlanMember
/ dei_LegalEntityAxis
= cbu_TupperLakeNationalBankMember		    
Projected benefit obligation   3,000,000,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cbu_PentegraDbPlanMember
/ dei_LegalEntityAxis
= cbu_TupperLakeNationalBankMember		    
Defined benefit plan, minimum percentage plan funded (in hundredths)   80.00%cbu_DefinedBenefitPlanPercentagePlanFunded
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cbu_PentegraDbPlanMember
/ dei_LegalEntityAxis
= cbu_TupperLakeNationalBankMember		    
Multi-employer plan, contributions 59,000us-gaap_MultiemployerPlanPeriodContributions
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cbu_PentegraDbPlanMember
/ dei_LegalEntityAxis
= cbu_TupperLakeNationalBankMember		 22,000us-gaap_MultiemployerPlanPeriodContributions
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cbu_PentegraDbPlanMember
/ dei_LegalEntityAxis
= cbu_TupperLakeNationalBankMember		 53,000us-gaap_MultiemployerPlanPeriodContributions
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cbu_PentegraDbPlanMember
/ dei_LegalEntityAxis
= cbu_TupperLakeNationalBankMember		  
Unfunded Supplementary Pension Plans [Member]        
Plan Assets [Abstract]        
Projected benefit obligation 11,000,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cbu_UnfundedSupplementaryPensionPlansMember		 11,400,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cbu_UnfundedSupplementaryPensionPlansMember		    
Unfunded Stock Balance [Member] | Nonemployee Directors [Member]        
Plan Assets [Abstract]        
Projected benefit obligation $ 100,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cbu_UnfundedStockBalanceMember
/ us-gaap_TitleOfIndividualAxis
= cbu_NonemployeeDirectorsMember		 $ 100,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cbu_UnfundedStockBalanceMember
/ us-gaap_TitleOfIndividualAxis
= cbu_NonemployeeDirectorsMember		    
[1] (c) Excludes dividends and interest receivable totaling $314,000 and $335,000 at December 31, 2014 and 2013, respectively.
[2] This category is exchange-traded funds representing a diversified index of high yield corporate bonds.
[3] This category is comprised of non-traditional investment classes including private equity funds and alternative exchange funds.