XML 32 R40.htm IDEA: XBRL DOCUMENT v2.4.1.9
LOANS AND ALLOWANCE FOR LOAN LOSSES, CREDIT QUALITY INDICATOR (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Finance receivable by credit quality indicator [Abstract]    
Loans $ 1,146,245us-gaap_NotesReceivableNet $ 1,124,354us-gaap_NotesReceivableNet
Residential and consumer finance receivable [Abstract]    
Residential and consumer loans receivable 18,836chmg_ResidentialAndConsumerLoansReceivable 20,066chmg_ResidentialAndConsumerLoansReceivable
Deteriorated Loans Transferred in [Abstract]    
Contractually required principal and interest, opening balance 3,621us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodContractuallyRequiredPaymentsReceivableAtAcquisition  
Contractually required principal and interest, income accretion 0chmg_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesContractuallyRequiredPaymentsReceivableIncomeAccretion  
Contractually required principal and interest, all other adjustments (676)chmg_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodContractuallyRequiredPaymentsReceivableOtherAdjustments  
Contractually required principal and interest, closing balance 2,945us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodContractuallyRequiredPaymentsReceivableAtAcquisition  
Contractual cash flows not expected to be collected (nonaccretable discount), opening balance (570)us-gaap_CertainLoansAcquiredInTransferNonaccretableDifference  
Contractual cash flows not expected to be collected (nonaccretable discount), income accretion 0chmg_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCashFlowsNotExpectedToBeCollectedIncomeAccretion  
Contractual cash flows not expected to be collected (nonaccretable discount), all other adjustments (25)chmg_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCashFlowsNotExpectedToBeCollectedOtherAdjustments  
Contractual cash flows not expected to be collected (nonaccretable discount), closing balance (595)us-gaap_CertainLoansAcquiredInTransferNonaccretableDifference  
Cash flows expected to be collected, opening balance 3,051us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodCashFlowsExpectedToBeCollectedAtAcquisition  
Cash flows expected to be collected, income accretion 0chmg_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCashFlowsExpectedToBeCollectedIncomeAccretion  
Cash flows expected to be collected, all other adjustments (701)chmg_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodCashFlowsExpectedToBeCollectedOtherAdjustments  
Cash flows expected to be collected, closing balance 2,350us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodCashFlowsExpectedToBeCollectedAtAcquisition  
Interest component of expected cash flows (accretable yield), opening balance (420)us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableYield  
Interest component of expected cash flows (accretable yield), income accretion 63us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableYieldAccretion  
Interest component of expected cash flows (accretable yield), all other adjustments (24)chmg_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableYieldOtherAdjustments  
Interest component of expected cash flows (accretable yield), closing balance (333)us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableYield  
Fair value of loans acquired with deteriorating credit quality, opening balance 2,631us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodAtAcquisitionAtFairValue  
Fair value of loans acquired with deteriorating credit quality, income accretion 63chmg_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesContractuallyRequiredPaymentsReceivableAtFairValueIncomeAccretion  
Fair value of loans acquired with deteriorating credit quality, all other adjustments (677)chmg_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesContractuallyRequiredPaymentsReceivableAtFairValueOtherAdjustments  
Fair value of loans acquired with deteriorating credit quality, closing balance 2,017us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodAtAcquisitionAtFairValue  
Commercial and Agricultural [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 172,227us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndAgriculturalPortfolioSegmentMember		 166,820us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndAgriculturalPortfolioSegmentMember
Commercial and Industrial [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 171,069us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndIndustrialMember		 165,797us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndIndustrialMember
Agricultural [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 1,158us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_AgriculturalMember		 1,023us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_AgriculturalMember
Commercial Mortgages [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 481,488us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 453,719us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
Construction [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 49,505us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember		 54,967us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
Commercial mortgages [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 431,983us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember		 398,752us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
Residential mortgages [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 199,127us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember		 197,287us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
Residential and consumer finance receivable [Abstract]    
Residential and consumer loans receivable 199,127chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember		 197,287chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
Consumer Loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 293,403us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember		 306,528us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Credit Cards [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 1,526us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember		 1,654us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
Residential and consumer finance receivable [Abstract]    
Residential and consumer loans receivable 1,526chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember		 1,654chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
Home equity lines and loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 98,953us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember		 99,591us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
Residential and consumer finance receivable [Abstract]    
Residential and consumer loans receivable 98,953chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember		 99,591chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
Indirect consumer loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 174,088us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember		 185,217us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
Residential and consumer finance receivable [Abstract]    
Residential and consumer loans receivable 174,088chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember		 185,217chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
Direct consumer loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 18,836us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember		 20,066us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
Not Rated [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 487,649us-gaap_NotesReceivableNet
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember		 499,137us-gaap_NotesReceivableNet
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember
Not Rated [Member] | Commercial and Industrial [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember
Not Rated [Member] | Agricultural [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_AgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_AgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember
Not Rated [Member] | Construction [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember
Not Rated [Member] | Commercial mortgages [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember
Not Rated [Member] | Residential mortgages [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 195,166us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember		 193,422us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember
Not Rated [Member] | Credit Cards [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 1,526us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember		 1,654us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember
Not Rated [Member] | Home equity lines and loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 98,298us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember		 99,076us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember
Not Rated [Member] | Indirect consumer loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 173,835us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember		 184,940us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember
Not Rated [Member] | Direct consumer loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 18,824us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember		 20,045us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_NotRatedMember
Pass [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 616,783us-gaap_NotesReceivableNet
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 576,136us-gaap_NotesReceivableNet
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Pass [Member] | Commercial and Industrial [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 164,385us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 158,140us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Pass [Member] | Agricultural [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 1,158us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_AgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 1,023us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_AgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Pass [Member] | Construction [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 47,607us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 51,525us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Pass [Member] | Commercial mortgages [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 403,633us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 365,448us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Pass [Member] | Residential mortgages [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Pass [Member] | Credit Cards [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Pass [Member] | Home equity lines and loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Pass [Member] | Indirect consumer loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Pass [Member] | Direct consumer loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Loans Acquired with Deteriorated Credit Quality [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 2,017us-gaap_NotesReceivableNet
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember		 2,631us-gaap_NotesReceivableNet
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember
Loans Acquired with Deteriorated Credit Quality [Member] | Commercial and Industrial [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember		 620us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember
Loans Acquired with Deteriorated Credit Quality [Member] | Agricultural [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_AgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_AgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember
Loans Acquired with Deteriorated Credit Quality [Member] | Construction [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember
Loans Acquired with Deteriorated Credit Quality [Member] | Commercial mortgages [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 1,761us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember		 1,761us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember
Loans Acquired with Deteriorated Credit Quality [Member] | Residential mortgages [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 256us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember		 250us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember
Loans Acquired with Deteriorated Credit Quality [Member] | Credit Cards [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember
Loans Acquired with Deteriorated Credit Quality [Member] | Home equity lines and loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember
Loans Acquired with Deteriorated Credit Quality [Member] | Indirect consumer loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember
Loans Acquired with Deteriorated Credit Quality [Member] | Direct consumer loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= chmg_LoansAcquiredWithDeterioratedCreditQualityMember
Special Mention [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 14,991us-gaap_NotesReceivableNet
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 27,858us-gaap_NotesReceivableNet
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Commercial and Industrial [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 4,220us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 3,695us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Agricultural [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_AgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_AgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Construction [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 1,750us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 3,292us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Commercial mortgages [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 9,021us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 20,871us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Residential mortgages [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Credit Cards [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Home equity lines and loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Indirect consumer loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Special Mention [Member] | Direct consumer loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Substandard [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 20,426us-gaap_NotesReceivableNet
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 18,150us-gaap_NotesReceivableNet
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Commercial and Industrial [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 2,285us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 3,306us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Agricultural [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_AgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_AgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Construction [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 148us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 150us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Commercial mortgages [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 13,368us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 10,266us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Residential mortgages [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 3,705us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 3,615us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Credit Cards [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Home equity lines and loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 655us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 515us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Indirect consumer loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 253us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 277us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Substandard [Member] | Direct consumer loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 12us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 21us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Doubtful [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 4,379us-gaap_NotesReceivableNet
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 442us-gaap_NotesReceivableNet
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Commercial and Industrial [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 179us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 36us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Agricultural [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_AgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_AgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Construction [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Commercial mortgages [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 4,200us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 406us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateOtherReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Residential mortgages [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Credit Cards [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Home equity lines and loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Indirect consumer loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Doubtful [Member] | Direct consumer loans [Member]    
Finance receivable by credit quality indicator [Abstract]    
Loans 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember		 0us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerOtherFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
Performing [Member]    
Residential and consumer finance receivable [Abstract]    
Residential and consumer loans receivable 18,824chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember		 20,036chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember
Performing [Member] | Residential mortgages [Member]    
Residential and consumer finance receivable [Abstract]    
Residential and consumer loans receivable 195,422chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember		 193,672chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember
Performing [Member] | Credit Cards [Member]    
Residential and consumer finance receivable [Abstract]    
Residential and consumer loans receivable 1,526chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember		 1,654chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember
Performing [Member] | Home equity lines and loans [Member]    
Residential and consumer finance receivable [Abstract]    
Residential and consumer loans receivable 98,298chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember		 99,076chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember
Performing [Member] | Indirect consumer loans [Member]    
Residential and consumer finance receivable [Abstract]    
Residential and consumer loans receivable 173,839chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember		 184,892chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PerformingFinancingReceivableMember
Non-Performing [Member]    
Residential and consumer finance receivable [Abstract]    
Residential and consumer loans receivable 12chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember		 30chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember
Non-Performing [Member] | Residential mortgages [Member]    
Residential and consumer finance receivable [Abstract]    
Residential and consumer loans receivable 3,705chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember		 3,615chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember
Non-Performing [Member] | Credit Cards [Member]    
Residential and consumer finance receivable [Abstract]    
Residential and consumer loans receivable 0chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember		 0chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember
Non-Performing [Member] | Home equity lines and loans [Member]    
Residential and consumer finance receivable [Abstract]    
Residential and consumer loans receivable 655chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember		 515chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_HomeEquityLinesAndLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember
Non-Performing [Member] | Indirect consumer loans [Member]    
Residential and consumer finance receivable [Abstract]    
Residential and consumer loans receivable $ 249chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember		 $ 325chmg_ResidentialAndConsumerLoansReceivable
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= chmg_IndirectConsumerLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_NonperformingFinancingReceivableMember