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v3.20.2

LEASES - Premises and Equipment (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Leases [Abstract] |  |
Buildings | $ 5,572 | $ 5,572
Less: accumulated depreciation | (1,791) | (1,541)
Net book value | $ 3,781 | $ 4,031

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

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

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- Definition Amount of lessee's right to use underlying asset under finance 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_FinanceLeaseRightOfUseAsset
Namespace Prefix: | us-gaap_
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