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SECURITIES, SECURITIES HELD TO MATURITY (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Amortized cost and estimated fair value of securities held to maturity [Abstract]    
Amortized Cost $ 5,831us-gaap_HeldToMaturitySecurities $ 6,495us-gaap_HeldToMaturitySecurities
Estimated Fair Value 6,197us-gaap_HeldToMaturitySecuritiesFairValue 6,930us-gaap_HeldToMaturitySecuritiesFairValue
Unrealized Gains 366us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain 435us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Unrealized Losses 0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss 0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Amortized Cost [Abstract]    
Within One Year 2,629us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount  
After One, But Within Five Years 2,199us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount  
After Five, But Within Ten Years 1,003us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount  
After Ten Years 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount  
Amortized Cost 5,831us-gaap_HeldToMaturitySecurities 6,495us-gaap_HeldToMaturitySecurities
Fair Value [Abstract]    
Within One Year 2,658us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue  
After One, But Within Five Years 2,390us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue  
After Five, But Within Ten Years 1,149us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue  
After Ten Years 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue  
Fair Value 6,197us-gaap_HeldToMaturitySecuritiesFairValue 6,930us-gaap_HeldToMaturitySecuritiesFairValue
Obligations of States and Political Subdivisions [Member]    
Amortized cost and estimated fair value of securities held to maturity [Abstract]    
Amortized Cost 5,175us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
5,472us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Estimated Fair Value 5,535us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
5,891us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Unrealized Gains 360us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
419us-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
Amortized Cost [Abstract]    
Amortized Cost 5,175us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
5,472us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Fair Value [Abstract]    
Fair Value 5,535us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
5,891us-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 656us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember
1,023us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember
Estimated Fair Value 662us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember
1,039us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember
Unrealized Gains 6us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember
16us-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
Amortized Cost [Abstract]    
Amortized Cost 656us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember
1,023us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember
Fair Value [Abstract]    
Fair Value $ 662us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember
$ 1,039us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= chmg_TimeDepositsWithOtherFinancialInstitutionsMember