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COMMITMENTS AND CONTINGENCIES (Details) (USD $)
3 Months Ended
Mar. 31, 2015
Proceeding
Party
Dec. 31, 2014
Settled Litigation [Member]    
Loss Contingencies [Line Items]    
Number of legal proceedings settled 2us-gaap_LossContingencyClaimsSettledAndDismissedNumber
/ us-gaap_LitigationStatusAxis
= us-gaap_SettledLitigationMember
 
Number of parties 2chmg_NumberOfParty
/ us-gaap_LitigationStatusAxis
= us-gaap_SettledLitigationMember
 
Amount of litigation settlement $ 12,100,000us-gaap_LitigationSettlementAmount
/ us-gaap_LitigationStatusAxis
= us-gaap_SettledLitigationMember
 
Accrual for legal settlement 12,100,000us-gaap_SettlementLiabilitiesCurrent
/ us-gaap_LitigationStatusAxis
= us-gaap_SettledLitigationMember
12,100,000us-gaap_SettlementLiabilitiesCurrent
/ us-gaap_LitigationStatusAxis
= us-gaap_SettledLitigationMember
Receivable for insurance proceeds 7,900,000us-gaap_InsuranceSettlementsReceivableCurrent
/ us-gaap_LitigationStatusAxis
= us-gaap_SettledLitigationMember
7,900,000us-gaap_InsuranceSettlementsReceivableCurrent
/ us-gaap_LitigationStatusAxis
= us-gaap_SettledLitigationMember
Commitments to Make Loans [Member] | Fixed Rate [Member]    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet contractual amounts of financial instruments 27,618,000us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_DebtInstrumentAxis
= chmg_FixedInterestRateMember
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommitmentsToExtendCreditMember
23,756,000us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_DebtInstrumentAxis
= chmg_FixedInterestRateMember
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommitmentsToExtendCreditMember
Commitments to Make Loans [Member] | Variable Rate [Member]    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet contractual amounts of financial instruments 10,415,000us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_DebtInstrumentAxis
= chmg_VariableInterestRateMember
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommitmentsToExtendCreditMember
11,082,000us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_DebtInstrumentAxis
= chmg_VariableInterestRateMember
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommitmentsToExtendCreditMember
Unused Line of Credit [Member] | Fixed Rate [Member]    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet contractual amounts of financial instruments 1,120,000us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_DebtInstrumentAxis
= chmg_FixedInterestRateMember
/ us-gaap_FinancialInstrumentAxis
= us-gaap_UnusedLinesOfCreditMember
812,000us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_DebtInstrumentAxis
= chmg_FixedInterestRateMember
/ us-gaap_FinancialInstrumentAxis
= us-gaap_UnusedLinesOfCreditMember
Unused Line of Credit [Member] | Variable Rate [Member]    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet contractual amounts of financial instruments 177,007,000us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_DebtInstrumentAxis
= chmg_VariableInterestRateMember
/ us-gaap_FinancialInstrumentAxis
= us-gaap_UnusedLinesOfCreditMember
185,235,000us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_DebtInstrumentAxis
= chmg_VariableInterestRateMember
/ us-gaap_FinancialInstrumentAxis
= us-gaap_UnusedLinesOfCreditMember
Standby Letters of Credit [Member] | Fixed Rate [Member]    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet contractual amounts of financial instruments 0us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_DebtInstrumentAxis
= chmg_FixedInterestRateMember
/ us-gaap_FinancialInstrumentAxis
= us-gaap_StandbyLettersOfCreditMember
0us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_DebtInstrumentAxis
= chmg_FixedInterestRateMember
/ us-gaap_FinancialInstrumentAxis
= us-gaap_StandbyLettersOfCreditMember
Standby Letters of Credit [Member] | Variable Rate [Member]    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet contractual amounts of financial instruments $ 17,053,000us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_DebtInstrumentAxis
= chmg_VariableInterestRateMember
/ us-gaap_FinancialInstrumentAxis
= us-gaap_StandbyLettersOfCreditMember
$ 16,747,000us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_DebtInstrumentAxis
= chmg_VariableInterestRateMember
/ us-gaap_FinancialInstrumentAxis
= us-gaap_StandbyLettersOfCreditMember