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LEASES - Leased Branch Properties (Details) - USD ($) $ in Thousands | Jun. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Leases [Abstract] |  |
Operating lease right-of-use assets | $ 5,446 | $ 5,648
Less: accumulated amortization | (387) | (772)
Less: lease termination | 0 | 0
Add: lease modifications | 80 | 570
Operating lease right-of-use assets | $ 5,139 | $ 5,446

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- Definition Operating Lease, Right-Of-Use Asset, Accumulated Amortization + References No definition available. + Details
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Namespace Prefix: | chmg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition Operating Lease, Right-Of-Use Asset, Gross + References No definition available. + Details
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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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- Definition Operating Lease, Right-of-Use Asset, New Lease Agreement and Modifications + References No definition available. + Details
Name: | chmg_OperatingLeaseRightOfUseAssetNewLeaseAgreementAndModifications
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
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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 -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479041/842-20-45-1 + Details
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