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SECURITIES SOLD UNDER AGREEMENTS TO REPURCHASE (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Summary of securities sold under agreements to repurchase [Abstract]      
Balance at December 31 $ 29,652us-gaap_SecuritiesSoldUnderAgreementsToRepurchase $ 32,701us-gaap_SecuritiesSoldUnderAgreementsToRepurchase $ 32,711us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
Maximum month-end balance 31,914chmg_SecuritiesSoldUnderAgreementsToRepurchaseMaximumMonthEndBalance 32,701chmg_SecuritiesSoldUnderAgreementsToRepurchaseMaximumMonthEndBalance 38,284chmg_SecuritiesSoldUnderAgreementsToRepurchaseMaximumMonthEndBalance
Average balance during year 30,667chmg_SecuritiesSoldUnderRepurchaseAgreementsAverageBalanceDuringYear 31,102chmg_SecuritiesSoldUnderRepurchaseAgreementsAverageBalanceDuringYear 34,534chmg_SecuritiesSoldUnderRepurchaseAgreementsAverageBalanceDuringYear
Weighted-average interest rate, period end (in hundredths) 2.82%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate [1] 2.93%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate 3.02%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
Average interest rate paid during period (in hundredths) 2.77%chmg_AssetsSoldUnderAgreementsToRepurchaseAverageInterestRatePaidDuringYear 2.76%chmg_AssetsSoldUnderAgreementsToRepurchaseAverageInterestRatePaidDuringYear 2.88%chmg_AssetsSoldUnderAgreementsToRepurchaseAverageInterestRatePaidDuringYear
Repurchase Liability 29,652us-gaap_AssetsSoldUnderAgreementsToRepurchaseRepurchaseLiability [1]    
Accrued Interest Payable 63chmg_AssetsSoldUnderAgreementsToRepurchaseAccruedInterestPayable [1]    
Weighted Average Rate (in hundredths) 2.82%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate [1] 2.93%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate 3.02%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
Estimated Fair Value of Collateral Securities 40,212us-gaap_SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure [1],[2]    
Accrued interest receivable included in fair value of repurchase agreements 342chmg_SecuritiesSoldUnderRepurchaseAgreementsAccruedInterestReceivable    
Within 90 Days [Member]      
Summary of securities sold under agreements to repurchase [Abstract]      
Weighted-average interest rate, period end (in hundredths) 0.10%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= chmg_MaturityWithin90DaysMember		 [1]    
Repurchase Liability 9,652us-gaap_AssetsSoldUnderAgreementsToRepurchaseRepurchaseLiability
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= chmg_MaturityWithin90DaysMember		 [1]    
Accrued Interest Payable 0chmg_AssetsSoldUnderAgreementsToRepurchaseAccruedInterestPayable
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= chmg_MaturityWithin90DaysMember		 [1]    
Weighted Average Rate (in hundredths) 0.10%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= chmg_MaturityWithin90DaysMember		 [1]    
Estimated Fair Value of Collateral Securities 16,682us-gaap_SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= chmg_MaturityWithin90DaysMember		 [1],[2]    
After 90 Days But Within One Year [Member]      
Summary of securities sold under agreements to repurchase [Abstract]      
Weighted-average interest rate, period end (in hundredths) 0.00%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= us-gaap_MaturityOver90DaysMember		 [1]    
Repurchase Liability 0us-gaap_AssetsSoldUnderAgreementsToRepurchaseRepurchaseLiability
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= us-gaap_MaturityOver90DaysMember		 [1]    
Accrued Interest Payable 0chmg_AssetsSoldUnderAgreementsToRepurchaseAccruedInterestPayable
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= us-gaap_MaturityOver90DaysMember		 [1]    
Weighted Average Rate (in hundredths) 0.00%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= us-gaap_MaturityOver90DaysMember		 [1]    
Estimated Fair Value of Collateral Securities 0us-gaap_SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= us-gaap_MaturityOver90DaysMember		 [1],[2]    
After One Year But Within Five Years [Member]      
Summary of securities sold under agreements to repurchase [Abstract]      
Weighted-average interest rate, period end (in hundredths) 4.13%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= chmg_AfterOneYearButWithinFiveYearsMember		 [1]    
Repurchase Liability 20,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseRepurchaseLiability
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= chmg_AfterOneYearButWithinFiveYearsMember		 [1]    
Accrued Interest Payable 63chmg_AssetsSoldUnderAgreementsToRepurchaseAccruedInterestPayable
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= chmg_AfterOneYearButWithinFiveYearsMember		 [1]    
Weighted Average Rate (in hundredths) 4.13%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= chmg_AfterOneYearButWithinFiveYearsMember		 [1]    
Estimated Fair Value of Collateral Securities 23,530us-gaap_SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= chmg_AfterOneYearButWithinFiveYearsMember		 [1],[2]    
After Five Years But Within Ten Years [Member]      
Summary of securities sold under agreements to repurchase [Abstract]      
Weighted-average interest rate, period end (in hundredths) 0.00%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= chmg_AfterFiveYearsButWithinTenYearsMember		 [1]    
Repurchase Liability 0us-gaap_AssetsSoldUnderAgreementsToRepurchaseRepurchaseLiability
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= chmg_AfterFiveYearsButWithinTenYearsMember		 [1]    
Accrued Interest Payable 0chmg_AssetsSoldUnderAgreementsToRepurchaseAccruedInterestPayable
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= chmg_AfterFiveYearsButWithinTenYearsMember		 [1]    
Weighted Average Rate (in hundredths) 0.00%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= chmg_AfterFiveYearsButWithinTenYearsMember		 [1]    
Estimated Fair Value of Collateral Securities $ 0us-gaap_SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= chmg_AfterFiveYearsButWithinTenYearsMember		 [1],[2]    
Without Call Provisions [Member]      
Summary of securities sold under agreements to repurchase [Abstract]      
Weighted-average interest rate, period end (in hundredths) 3.10%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseAxis
= chmg_WithoutCallProvisionsMember		    
Weighted Average Rate (in hundredths) 3.10%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseAxis
= chmg_WithoutCallProvisionsMember		    
Weight average remaining term to maturity 1 year 10 months 24 days    
[1] At December 31, 2014, the securities repurchase agreements were non-callable with a weighted-average rate of 3.10%, and a weighted-average term to maturity of approximately 1.9 years.
[2] Represents the estimated fair value of the securities subject to the repurchase agreements, including accrued interest receivable, of approximately $342 thousand at December 31, 2014.