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FAIR VALUE, VALUATION TECHNIQUE (Details) (Non-recurring [Member], Significant Unobservable Inputs (Level 3) [Member], Third Party Appraisals [Member], USD $) In Thousands, unless otherwise specified | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2014 | Dec. 31, 2013
Impaired Loans [Member] |  |
Information related to Level 3 non-recurring fair value measurement [Abstract] |  |
Fair Value | $ 1,103 | $ 999
Unobservable Input 1 | Management discount based on underlying collateral characteristics and market conditions | Management discount based on underlying collateral characteristics and market conditions
Other real estate owned [Member] |  |
Information related to Level 3 non-recurring fair value measurement [Abstract] |  |
Fair Value | $ 3,119 | $ 538
Unobservable Input 1 | Estimated holding period | Estimated holding period
Unobservable Input 2 | Estimated closing costs | Estimated closing costs

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- Definition This item represents a description of the inputs and the information used to develop the inputs for fair value measurement using significant unobservable inputs (Level 3) as applied to the identified [major] category of asset or liability. + References No definition available. + Details
Name: | chmg_FairValueMeasurementsUnobservableInputsOneDescription
Namespace Prefix: | chmg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition This item represents a description of the inputs and the information used to develop the inputs for fair value measurement using significant unobservable inputs (Level 3) as applied to the identified major category of asset or liability. + References No definition available. + Details
Name: | chmg_FairValueMeasurementsUnobservableInputsTwoDescription
Namespace Prefix: | chmg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | chmg_InformationRelatedToLevel3NonRecurringFairValueMeasurementAbstract
Namespace Prefix: | chmg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a nonrecurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References No definition available. + Details
Name: | us-gaap_AssetsFairValueDisclosureNonrecurring
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant