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SECURITIES, SECURITIES HELD TO MATURITY (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Amortized cost and estimated fair value of securities held to maturity [Abstract]    
Amortized Cost $ 5,693us-gaap_HeldToMaturitySecurities $ 5,831us-gaap_HeldToMaturitySecurities
Unrealized Gains 383us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain 366us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Unrealized Losses 0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss 0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Estimated Fair Value 6,076us-gaap_HeldToMaturitySecuritiesFairValue 6,197us-gaap_HeldToMaturitySecuritiesFairValue
Amortized Cost [Abstract]    
Within one year 2,639us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount  
After one, but within five years 2,051us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount  
After five, but within ten years 1,003us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount  
After ten years 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount  
Total 5,693us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasis  
Amortized Cost 5,693us-gaap_HeldToMaturitySecurities 5,831us-gaap_HeldToMaturitySecurities
Fair Value [Abstract]    
Within one year 2,667us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue  
After one, but within five years 2,243us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue  
After five, but within ten years 1,166us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue  
After ten years 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue  
Total 6,076us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValue  
Fair Value 6,076us-gaap_HeldToMaturitySecuritiesFairValue 6,197us-gaap_HeldToMaturitySecuritiesFairValue
Obligations of States and Political Subdivisions [Member]    
Amortized cost and estimated fair value of securities held to maturity [Abstract]    
Amortized Cost 5,253us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 5,175us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Unrealized Gains 379us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 360us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Unrealized Losses 0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Estimated Fair Value 5,632us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 5,535us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Amortized Cost [Abstract]    
Amortized Cost 5,253us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 5,175us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Fair Value [Abstract]    
Fair Value 5,632us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 5,535us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Time Deposits with Other Financial Institutions [Member]    
Amortized cost and estimated fair value of securities held to maturity [Abstract]    
Amortized Cost 440us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember		 656us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember
Unrealized Gains 4us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember		 6us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember
Unrealized Losses 0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember		 0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember
Estimated Fair Value 444us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember		 662us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember
Amortized Cost [Abstract]    
Amortized Cost 440us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember		 656us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember
Fair Value [Abstract]    
Fair Value 444us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember		 662us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember
Mortgage-Backed Securities, Residential [Member]    
Amortized Cost [Abstract]    
Without Single Maturity Date 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateNetCarryingAmount
/ us-gaap_DebtSecurityAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		  
Fair Value [Abstract]    
Without Single Maturity Date 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		  
Collateralized Mortgage Obligations [Member]    
Amortized Cost [Abstract]    
Without Single Maturity Date 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateNetCarryingAmount
/ us-gaap_DebtSecurityAxis
= us-gaap_CollateralizedMortgageObligationsMember		  
Fair Value [Abstract]    
Without Single Maturity Date 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_CollateralizedMortgageObligationsMember		  
SBA Loans Pools [Member]    
Amortized Cost [Abstract]    
Without Single Maturity Date 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateNetCarryingAmount
/ us-gaap_DebtSecurityAxis
= chmg_SbaLoansMember		  
Fair Value [Abstract]    
Without Single Maturity Date $ 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue
/ us-gaap_DebtSecurityAxis
= chmg_SbaLoansMember