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SECURITIES, OTHER THAN TEMPORARY IMPAIRMENT (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Other than temporary impairment credit losses recognized in earnings [Roll Forward]    
Beginning balance $ 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld $ 1,939us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld
Amounts related to credit loss for which an other-than-temporary impairment was not previously recognized 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment
Additions/Subtractions [Abstract]    
Amounts realized for securities sold during the period 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold
Amounts related to securities for which the Corporation intends to sell or that it will be more likely than not that the Corporation will be required to sell prior to recovery of amortized cost basis 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses
Reductions for increase in cash flows expected to be collected that are recognized over the remaining life of the security 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsCashFlows 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsCashFlows
Reductions for previous credit losses realized in securities liquidated during the period 0chmg_OtherThanTemporaryImpairmentReductionsForPreviousCreditLossesRealizedOnSecuritiesSold (1,939)chmg_OtherThanTemporaryImpairmentReductionsForPreviousCreditLossesRealizedOnSecuritiesSold
Increases to the amount related to the credit loss for which other-than-temporary impairment was previously recognized 0chmg_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsWhenImpairmentRecognizedPreviously 0chmg_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsWhenImpairmentRecognizedPreviously
Ending balance 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld
Collateralized Debt Obligations [Member]    
Additions/Subtractions [Abstract]    
Income from liquidation of debt securities   $ 500us-gaap_GainLossOnSaleOfDebtInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember