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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 | $ 177,551,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets | [1] | $ 173,081,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets | [1] |  |
Total dividends and interest receivable excluded from plan assets | 314,000 cbu_TotalDividendsAndInterestReceivableExcludedFromPlanAssets |  | 335,000 cbu_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 | 100 us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnServiceAndInterestCostComponents |  |  |  |  |
Effect of one percentage point increase on benefit obligation | 2,000 us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnAccumulatedPostretirementBenefitObligation |  |  |  |  |
Effect of one percentage point decrease on service and interest cost components | 100 us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnServiceAndInterestCostComponents1 |  |  |  |  |
Effect of one percentage point decrease on benefit obligation | 2,000 us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnAccumulatedPostretirementBenefitObligation1 |  |  |  |  |
Other Deferred Compensation Arrangement [Member] |  |  |  |  |  |
Nonqualified deferred compensation arrangements [Abstract] |  |  |  |  |  |
(Income)/expense recognized under plan | 330,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = cbu_OtherDeferredCompensationArrangementMember |  | (52,000)us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = cbu_OtherDeferredCompensationArrangementMember |  | 439,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = cbu_OtherDeferredCompensationArrangementMember |
Liability recorded for compensation plan | 3,895,000 us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = cbu_OtherDeferredCompensationArrangementMember |  | 4,238,000 us-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 | 858,000 cbu_DefinedContributionPlanInterestCreditContributionExpensedRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = cbu_EmployeeStockOwnershipPlan401KPlanMember |  | 650,000 cbu_DefinedContributionPlanInterestCreditContributionExpensedRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = cbu_EmployeeStockOwnershipPlan401KPlanMember |  | 419,000 cbu_DefinedContributionPlanInterestCreditContributionExpensedRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = cbu_EmployeeStockOwnershipPlan401KPlanMember |
Nonqualified deferred compensation arrangements [Abstract] |  |  |  |  |  |
(Income)/expense recognized under plan | 3,405,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = cbu_EmployeeStockOwnershipPlan401KPlanMember |  | 3,215,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = cbu_EmployeeStockOwnershipPlan401KPlanMember |  | 2,956,000 us-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 | 158,365,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | [1] | 155,959,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | [1] |  |
Significant Observable Inputs, Level 2 [Member] |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |
Fair value of plan assets | 19,186,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | [1] | 17,122,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | [1] |  |
Significant Unobservable Inputs, Level 3 [Member] |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | [1] | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | [1] |  |
Money Market Accounts [Member] |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |
Fair value of plan assets | 12,901,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember |  | 10,940,000 us-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 | 431,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 709,000 us-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 | 12,470,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 10,231,000 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 0 us-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 | 109,468,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember |  | 113,219,000 us-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 | 109,468,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 113,219,000 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = 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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  |  |
U.S. Large-Cap [Member] |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |
Fair value of plan assets | 51,197,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_USLargeCapMember |  | 50,513,000 us-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 | 51,197,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_USLargeCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 50,513,000 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_USLargeCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 0 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_USLargeCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 0 us-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 | 14,026,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_USMidSmallCapMember |  | 19,308,000 us-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 | 14,026,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_USMidSmallCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 19,308,000 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_USMidSmallCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 0 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_USMidSmallCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_USMidSmallCapMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  |  |
CBSI Stock [Member] |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |
Fair value of plan assets | 15,354,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_CbsiStockMember |  | 9,913,000 us-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 | 15,354,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_CbsiStockMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 9,913,000 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_CbsiStockMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = 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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_CbsiStockMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_CbsiStockMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  |  |
International [Member] |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |
Fair value of plan assets | 28,891,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_InternationalMember |  | 33,485,000 us-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 | 28,891,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_InternationalMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 33,485,000 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_InternationalMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 0 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_InternationalMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = 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 | 46,958,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_FixedIncomeMember |  | 48,845,000 us-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 | 40,312,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_FixedIncomeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 42,031,000 us-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 | 6,646,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_FixedIncomeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 6,814,000 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_FixedIncomeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_FixedIncomeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  |  |
Government Securities [Member] |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |
Fair value of plan assets | 13,137,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 14,490,000 us-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 | 6,491,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 7,676,000 us-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 | 6,646,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 6,814,000 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 0 us-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 | 21,539,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_InvestmentGradeBondsMember |  | 17,400,000 us-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 | 21,539,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_InvestmentGradeBondsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 17,400,000 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_InvestmentGradeBondsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 0 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_InvestmentGradeBondsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_InvestmentGradeBondsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  |  |
International bonds [Member] |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |
Fair value of plan assets | 5,085,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_InvestmentBondsMember |  | 2,712,000 us-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 | 5,085,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_InvestmentBondsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 2,712,000 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_InvestmentBondsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 0 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_InvestmentBondsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_InvestmentBondsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  |  |
High Yield [Member] |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |
Fair value of plan assets | 7,197,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_HighYieldMember | [2] | 14,243,000 us-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 | 7,197,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_HighYieldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | [2] | 14,243,000 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_HighYieldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | [2] | 0 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = cbu_HighYieldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | [2] | 0 us-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 | 8,224,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_OtherInvestmentsMember | [3] | 77,000 us-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 | 8,154,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_OtherInvestmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | [3] | 0 us-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 | 70,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_OtherInvestmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | [3] | 77,000 us-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 | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_OtherInvestmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | [3] | 0 us-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 | 168,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = cbu_DeferredCompensationPlanMember / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember |  | 162,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = cbu_DeferredCompensationPlanMember / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember |  | 148,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = cbu_DeferredCompensationPlanMember / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember |
Liability recorded for compensation plan | 3,525,000 us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = cbu_DeferredCompensationPlanMember / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember |  | 3,424,000 us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = cbu_DeferredCompensationPlanMember / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember |  |  |
Deferred compensation arrangement with individual, shares credited (in shares) | 147,934 us-gaap_DeferredCompensationArrangementWithIndividualSharesIssued / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = cbu_DeferredCompensationPlanMember / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember |  | 153,396 us-gaap_DeferredCompensationArrangementWithIndividualSharesIssued / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = cbu_DeferredCompensationPlanMember / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember |  |  |
Pension Benefits [Member] |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |
Fair value of plan assets | 177,865,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 173,416,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 143,661,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Employer contributions | 616,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 10,622,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  |  | 10,000,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember
Projected benefit obligation | 127,513,000 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 111,174,000 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 123,739,000 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Post-retirement Benefits [Member] |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |
Employer contributions | 241,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  | 498,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  |  |
Projected benefit obligation | 2,256,000 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  | 2,340,000 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  | 3,051,000 us-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 |  |  | 3,000,000,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cbu_PentegraDbPlanMember / dei_LegalEntityAxis = cbu_TupperLakeNationalBankMember |  |  |
Projected benefit obligation |  |  | 3,000,000,000 us-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 | 59,000 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cbu_PentegraDbPlanMember / dei_LegalEntityAxis = cbu_TupperLakeNationalBankMember |  | 22,000 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cbu_PentegraDbPlanMember / dei_LegalEntityAxis = cbu_TupperLakeNationalBankMember |  | 53,000 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cbu_PentegraDbPlanMember / dei_LegalEntityAxis = cbu_TupperLakeNationalBankMember |
Unfunded Supplementary Pension Plans [Member] |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |
Projected benefit obligation | 11,000,000 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cbu_UnfundedSupplementaryPensionPlansMember |  | 11,400,000 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cbu_UnfundedSupplementaryPensionPlansMember |  |  |
Unfunded Stock Balance [Member] | Nonemployee Directors [Member] |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |
Projected benefit obligation | $ 100,000 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = cbu_UnfundedStockBalanceMember / us-gaap_TitleOfIndividualAxis = cbu_NonemployeeDirectorsMember |  | $ 100,000 us-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.

X
- Definition The assumed health care cost trend rate for participants older than 65 used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status. + References No definition available. + Details
Name: | cbu_AssumedHealthCareCostTrendRateForParticipantsOlderThan65
Namespace Prefix: | cbu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The assumed health care cost trend rate for participants younger than 65 used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status. + References No definition available. + Details
Name: | cbu_AssumedHealthCareCostTrendRateForParticipantsYoungerThan65
Namespace Prefix: | cbu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The assumed health care cost trend rate for prescription drugs used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status. + References No definition available. + Details
Name: | cbu_AssumedHealthCareCostTrendRateForPrescriptionDrugs
Namespace Prefix: | cbu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The maximum percentage of contribution made by employees from eligible compensation. + References No definition available. + Details
Name: | cbu_ContributionFromEligibleCompensationMaximum
Namespace Prefix: | cbu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The minimum percentage of contribution made by employees from eligible compensation. + References No definition available. + Details
Name: | cbu_ContributionFromEligibleCompensationMinimum
Namespace Prefix: | cbu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The percentage of the plan's projected benefit obligation funded by plan's assets at the measurement date. + References No definition available. + Details
Name: | cbu_DefinedBenefitPlanPercentagePlanFunded
Namespace Prefix: | cbu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Target allowed allocation for alternative investments to total plan assets presented on a weighted average basis. + References No definition available. + Details
Name: | cbu_DefinedBenefitPlanTargetPlanAssetAllocationsAlternativeInvestments
Namespace Prefix: | cbu_
Data Type: | num:percentItemType
Balance Type: | na
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- Definition The effect of a one-percentage-point increase in the assumed health care cost trend rates on the aggregate of the service and interest cost components of net periodic postretirement health care benefit costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 + Details
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