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SECURITIES - Sales and Calls (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract] |  |  |  |
Proceeds | $ 0 | $ 545 | $ 0 | $ 1,620
Gross gains | 0 | 0 | 0 | 12
Tax expense | $ 0 | $ 0 | $ 0 | $ 4

X
- Definition Gains From Sale Of Available For Sale Securities Debt + References No definition available. + Details
Name: | chmg_GainsFromSaleOfAvailableForSaleSecuritiesDebt
Namespace Prefix: | chmg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Proceeds from Sale and Maturity of Debt Securities, Available-for-sale + References No definition available. + Details
Name: | chmg_ProceedsfromSaleandMaturityofDebtSecuritiesAvailableforsale
Namespace Prefix: | chmg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition This item represents the tax expense on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
Name: | chmg_TaxExpenseFromSaleOfAvailableForSaleSecuritiesDebt
Namespace Prefix: | chmg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_InvestmentsDebtAndEquitySecuritiesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration