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LEASES - Premises and Equipment (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Leases [Abstract] |  |
Buildings | $ 5,572 | $ 5,572
Less: accumulated depreciation | (1,958) | (1,875)
Net book value | $ 3,614 | $ 3,697

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- Definition Finance Lease, Right-Of-Use Asset, Accumulated Depreciation + References No definition available. + Details
Name: | chmg_FinanceLeaseRightOfUseAssetAccumulatedDepreciation
Namespace Prefix: | chmg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Finance Lease, Right-of-Use Asset, after Accumulated Amortization + References No definition available. + Details
Name: | chmg_FinanceLeaseRightOfUseAssetAfterAccumulatedAmortization
Namespace Prefix: | chmg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Finance Lease, Right-Of-Use Asset, before Accumulated Amortization + References No definition available. + Details
Name: | chmg_FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization
Namespace Prefix: | chmg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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