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PREMISES AND EQUIPMENT - Leases in Premises and Equipment (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Property, Plant and Equipment [Abstract] |  |
Buildings | $ 5,572 | $ 5,572
Less: accumulated depreciation | (1,875) | (1,541)
Net book value | $ 3,697 | $ 4,031

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- Definition Finance Lease, Right-Of-Use Asset, Accumulated Amortization + References No definition available. + Details
Name: | chmg_FinanceLeaseRightOfUseAssetAccumulatedAmortization
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_PropertyPlantAndEquipmentAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration