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INVESTMENT SECURITIES (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Position
Dec. 31, 2014
Position
Available-for-Sale Portfolio [Abstract]    
Amortized Cost $ 2,516,351us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 2,397,886us-gaap_AvailableForSaleSecuritiesAmortizedCost
Gross Unrealized Gains 107,531cbu_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax 77,293cbu_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Gross Unrealized Losses 1,103cbu_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax 2,254cbu_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Fair Value 2,622,779us-gaap_AvailableForSaleSecurities 2,472,925us-gaap_AvailableForSaleSecurities
Available For Sale Debt Securities Amortized Cost Basis [Abstract]    
Fair Value 2,622,353us-gaap_AvailableForSaleSecuritiesDebtSecurities  
Other Securities [Abstract]    
Amortized Cost 33,645us-gaap_OtherInvestmentsAndSecuritiesAtCost 40,049us-gaap_OtherInvestmentsAndSecuritiesAtCost
Fair Value 33,645us-gaap_OtherInvestmentNotReadilyMarketableFairValue 40,049us-gaap_OtherInvestmentNotReadilyMarketableFairValue
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]    
Less than 12 months 35us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear 28us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
12 months or longer 24us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear 71us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
Total 59us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 99us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Available-for-Sale Portfolio in Unrealized Loss Positions, Fair Value [Abstract]    
Less than 12 months 18,428us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue 16,458us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
12 months or longer 38,308us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 166,383us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Total 56,736us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue 182,841us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]    
Less than 12 months 273cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss 35cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
12 months or longer 830cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss 2,219cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Total 1,103cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss 2,254cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]    
Due in one year or less 40,781us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost  
Due in one to five years 163,322us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost  
Due in five through ten years 1,834,409us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost  
Due after ten years 241,003us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost  
Subtotal 2,279,515us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasis  
Amortized cost 2,516,101us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAmortizedCost  
Available-for-Sale, Debt Maturities, Fair Value [Abstract]    
Due in one year or less 41,170us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue  
Due in one to five years 167,984us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue  
Due in five through ten years 1,914,623us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue  
Due after ten years 252,070us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue  
Subtotal 2,375,847us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDate  
Fair value 2,622,353us-gaap_AvailableForSaleSecuritiesDebtSecurities  
U.S. Treasury and Agency Obligations [Member]    
Available For Sale Debt Securities Amortized Cost Basis [Abstract]    
Amortized Cost 1,606,766us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
1,479,134us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Gross Unrealized Gains 69,346cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
39,509cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Gross Unrealized Losses 0cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
910cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Fair Value 1,676,112us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
1,517,733us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]    
Less than 12 months   0us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
12 months or longer   4us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Total   4us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Available-for-Sale Portfolio in Unrealized Loss Positions, Fair Value [Abstract]    
Less than 12 months   0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
12 months or longer   102,363us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Total   102,363us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]    
Less than 12 months   0cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
12 months or longer   910cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Total   910cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Available-for-Sale, Debt Maturities, Fair Value [Abstract]    
Fair value 1,676,112us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
1,517,733us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Obligations of State and Political Subdivisions [Member]    
Available For Sale Debt Securities Amortized Cost Basis [Abstract]    
Amortized Cost 645,979us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
645,398us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Gross Unrealized Gains 27,041cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
26,749cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Gross Unrealized Losses 311cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
244cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Fair Value 672,709us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
671,903us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]    
Less than 12 months 31us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
23us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
12 months or longer 3us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
46us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Total 34us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
69us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Available-for-Sale Portfolio in Unrealized Loss Positions, Fair Value [Abstract]    
Less than 12 months 18,327us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
13,413us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
12 months or longer 1,209us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
26,490us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Total 19,536us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
39,903us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]    
Less than 12 months 271cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
34cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
12 months or longer 40cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
210cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Total 311cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
244cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Available-for-Sale, Debt Maturities, Fair Value [Abstract]    
Fair value 672,709us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
671,903us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Government Agency Mortgage-Backed Securities [Member]    
Available For Sale Debt Securities Amortized Cost Basis [Abstract]    
Amortized Cost 220,279us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
228,971us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Gross Unrealized Gains 10,016cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
9,782cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Gross Unrealized Losses 769cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
1,025cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Fair Value 229,526us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
237,728us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]    
Less than 12 months 2us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
3us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
12 months or longer 19us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
19us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Total 21us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
22us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Available-for-Sale Portfolio in Unrealized Loss Positions, Fair Value [Abstract]    
Less than 12 months 2us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
5us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
12 months or longer 34,302us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
34,770us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Total 34,304us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
34,775us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]    
Less than 12 months 0cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
0cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
12 months or longer 769cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
1,025cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Total 769cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
1,025cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]    
Without single maturity date 220,279us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
 
Available-for-Sale, Debt Maturities, Fair Value [Abstract]    
Without single maturity date 229,526us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
 
Fair value 229,526us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
237,728us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Corporate Debt Securities [Member]    
Available For Sale Debt Securities Amortized Cost Basis [Abstract]    
Amortized Cost 26,770us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
26,803us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Gross Unrealized Gains 277cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
363cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Gross Unrealized Losses 21cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
75cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Fair Value 27,026us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
27,091us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]    
Less than 12 months 0us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
1us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
12 months or longer 1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Total 1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
2us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Available-for-Sale Portfolio in Unrealized Loss Positions, Fair Value [Abstract]    
Less than 12 months 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
3,040us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
12 months or longer 2,792us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
2,755us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Total 2,792us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
5,795us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]    
Less than 12 months 0cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
1cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
12 months or longer 21cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
74cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Total 21cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
75cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Available-for-Sale, Debt Maturities, Fair Value [Abstract]    
Fair value 27,026us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
27,091us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Government Agency Collateralized Mortgage Obligations [Member]    
Available For Sale Debt Securities Amortized Cost Basis [Abstract]    
Amortized Cost 16,307us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
17,330us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
Gross Unrealized Gains 673cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
695cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
Gross Unrealized Losses 0cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
0cbu_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
Fair Value 16,980us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
18,025us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]    
Less than 12 months 1us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
1us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
12 months or longer 1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
Total 2us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
2us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
Available-for-Sale Portfolio in Unrealized Loss Positions, Fair Value [Abstract]    
Less than 12 months 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
12 months or longer 5us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
5us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
Total 5us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
5us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]    
Less than 12 months 0cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
0cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
12 months or longer 0cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
0cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
Total 0cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
0cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]    
Without single maturity date 16,307us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
 
Available-for-Sale, Debt Maturities, Fair Value [Abstract]    
Without single maturity date 16,980us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
 
Fair value 16,980us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
18,025us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_GovernmentAgencyCollateralizedMortgageObligationsMember
Marketable Equity Securities [Member]    
Available For Sale Equity Securities Amortized Cost Basis [Abstract]    
Amortized Cost 250us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
250us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
Gross Unrealized Gains 178cbu_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
195cbu_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
Gross Unrealized Losses 2cbu_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
0cbu_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
Fair Value 426us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
445us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract]    
Less than 12 months 1us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
 
12 months or longer 0us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
 
Total 1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
 
Available-for-Sale Portfolio in Unrealized Loss Positions, Fair Value [Abstract]    
Less than 12 months 99us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
 
12 months or longer 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
 
Total 99us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
 
Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract]    
Less than 12 months 2cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
 
12 months or longer 0cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
 
Total 2cbu_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_MarketableEquitySecurityMember
 
Federal Home Loan Bank Common Stock [Member]    
Other Securities [Abstract]    
Amortized Cost 13,149us-gaap_OtherInvestmentsAndSecuritiesAtCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InvestmentInFederalHomeLoanBankStockMember
19,553us-gaap_OtherInvestmentsAndSecuritiesAtCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InvestmentInFederalHomeLoanBankStockMember
Fair Value 13,149us-gaap_OtherInvestmentNotReadilyMarketableFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InvestmentInFederalHomeLoanBankStockMember
19,553us-gaap_OtherInvestmentNotReadilyMarketableFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InvestmentInFederalHomeLoanBankStockMember
Federal Reserve Bank Common Stock [Member]    
Other Securities [Abstract]    
Amortized Cost 16,050us-gaap_OtherInvestmentsAndSecuritiesAtCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_FederalReserveBankStockMember
16,050us-gaap_OtherInvestmentsAndSecuritiesAtCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_FederalReserveBankStockMember
Fair Value 16,050us-gaap_OtherInvestmentNotReadilyMarketableFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_FederalReserveBankStockMember
16,050us-gaap_OtherInvestmentNotReadilyMarketableFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_FederalReserveBankStockMember
Other Equity Securities [Member]    
Other Securities [Abstract]    
Amortized Cost 4,446us-gaap_OtherInvestmentsAndSecuritiesAtCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_OtherEquitySecuritiesMember
4,446us-gaap_OtherInvestmentsAndSecuritiesAtCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_OtherEquitySecuritiesMember
Fair Value $ 4,446us-gaap_OtherInvestmentNotReadilyMarketableFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_OtherEquitySecuritiesMember
$ 4,446us-gaap_OtherInvestmentNotReadilyMarketableFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbu_OtherEquitySecuritiesMember