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LOANS AND ALLOWANCE FOR LOAN LOSSES, TROUBLED DEBT RESTRUCTURING (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 Loan | Dec. 31, 2013 Loan | Dec. 31, 2012 Loan
Trouble debt restructurings [Abstract] |  |  |  |
Recorded investment in troubled debt restructurings | $ 9,700,000 us-gaap_FinancingReceivableModificationsRecordedInvestment |  | $ 7,900,000 us-gaap_FinancingReceivableModificationsRecordedInvestment | $ 5,700,000 us-gaap_FinancingReceivableModificationsRecordedInvestment
Specific reserves allocated for troubled debt restructuring | 300,000 us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment |  | 300,000 us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment | 0 us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
Troubled debt restructurings accruing interest under modified terms | 8,700,000 chmg_FinancingReceivableModificationsRecordedInvestmentStillAccruing |  | 6,800,000 chmg_FinancingReceivableModificationsRecordedInvestmentStillAccruing | 5,400,000 chmg_FinancingReceivableModificationsRecordedInvestmentStillAccruing
Troubled debt restructurings on non-accrual status | 1,000,000 chmg_FinancingReceivableModificationsRecordedInvestmentNonaccrualStatus |  | 1,100,000 chmg_FinancingReceivableModificationsRecordedInvestmentNonaccrualStatus | 400,000 chmg_FinancingReceivableModificationsRecordedInvestmentNonaccrualStatus
Additional amounts committed to customers with loans classified as troubled debt restructurings | 100,000 chmg_AdditionalAmountsCommittedToCustomersWithLoansClassifiedAsTroubledDebtRestructurings |  | 200,000 chmg_AdditionalAmountsCommittedToCustomersWithLoansClassifiedAsTroubledDebtRestructurings |
Loans by class modified as troubled debt restructurings [Abstract] |  |  |  |
Number of Loans | 10 us-gaap_FinancingReceivableModificationsNumberOfContracts2 |  | 10 us-gaap_FinancingReceivableModificationsNumberOfContracts2 | 8 us-gaap_FinancingReceivableModificationsNumberOfContracts2
Pre-Modification Outstanding Recorded Investment | 4,209,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 |  | 1,936,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 | 5,430,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Post-Modification Outstanding Recorded Investment | 4,167,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 |  | 1,936,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 | 5,430,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Increase in allowance for loan losses related to troubled debt restructurings | 200,000 us-gaap_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecrease |  | 100,000 us-gaap_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecrease | 0 us-gaap_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecrease
Troubled debt restructurings, charge offs |  | [1] | 0 us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown | 0 us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown
Number of days past due after which a loan is considered to be in payment default | 90 days |  |  |
Commercial and Industrial [Member] |  |  |  |
Loans by class modified as troubled debt restructurings [Abstract] |  |  |  |
Number of Loans | 4 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_CommercialAndIndustrialMember |  | 5 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_CommercialAndIndustrialMember | 4 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_CommercialAndIndustrialMember
Pre-Modification Outstanding Recorded Investment | 1,028,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_CommercialAndIndustrialMember |  | 1,343,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_CommercialAndIndustrialMember | 1,307,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_CommercialAndIndustrialMember
Post-Modification Outstanding Recorded Investment | 1,028,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_CommercialAndIndustrialMember |  | 1,343,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_CommercialAndIndustrialMember | 1,307,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_CommercialAndIndustrialMember
Construction [Member] |  |  |  |
Loans by class modified as troubled debt restructurings [Abstract] |  |  |  |
Number of Loans |  |  | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateConstructionFinancingReceivableMember | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
Pre-Modification Outstanding Recorded Investment |  |  | 326,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateConstructionFinancingReceivableMember | 251,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
Post-Modification Outstanding Recorded Investment |  |  | 326,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateConstructionFinancingReceivableMember | 251,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateConstructionFinancingReceivableMember
Commercial Mortgages [Member] |  |  |  |
Loans by class modified as troubled debt restructurings [Abstract] |  |  |  |
Number of Loans | 4 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateOtherReceivableMember |  | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateOtherReceivableMember | 3 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateOtherReceivableMember
Pre-Modification Outstanding Recorded Investment | 2,666,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateOtherReceivableMember |  | 133,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateOtherReceivableMember | 3,872,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateOtherReceivableMember
Post-Modification Outstanding Recorded Investment | 2,623,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateOtherReceivableMember |  | 133,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateOtherReceivableMember | 3,872,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateOtherReceivableMember
Residential Mortgages [Member] |  |  |  |
Loans by class modified as troubled debt restructurings [Abstract] |  |  |  |
Number of Loans | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember |  |  |
Pre-Modification Outstanding Recorded Investment | 149,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember |  |  |
Post-Modification Outstanding Recorded Investment | 150,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember |  |  |
Home Equity Lines and Loans [Member] |  |  |  |
Loans by class modified as troubled debt restructurings [Abstract] |  |  |  |
Number of Loans | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_HomeEquityLinesAndLoansMember |  | 3 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_HomeEquityLinesAndLoansMember |
Pre-Modification Outstanding Recorded Investment | 366,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_HomeEquityLinesAndLoansMember |  | 134,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_HomeEquityLinesAndLoansMember |
Post-Modification Outstanding Recorded Investment | $ 366,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_HomeEquityLinesAndLoansMember |  | $ 134,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_HomeEquityLinesAndLoansMember |
[1] | Amount less than $0.1 million

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