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LEASES - Leased Branch Properties (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Leases [Abstract] |  |
Operating lease right-of-use asset | $ 8,001 | $ 8,713
Less: accumulated amortization | (705) | (712)
Less: Lease termination | (151) | 0
Operating lease right-of-use-assets, net | $ 7,145 | $ 8,001

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

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- Definition Operating Lease, Right-Of-Use Asset, Gross + References No definition available. + Details
Name: | chmg_OperatingLeaseRightOfUseAssetGross
Namespace Prefix: | chmg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Operating Lease, Right-of-Use Asset, Lease Termination + References No definition available. + Details
Name: | chmg_OperatingLeaseRightOfUseAssetLeaseTermination
Namespace Prefix: | chmg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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

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- Definition Amount of lessee's right to use underlying asset under operating lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977 + Details
Name: | us-gaap_OperatingLeaseRightOfUseAsset
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant