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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

X

- 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

X

- 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

X

- 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

X

- References

No definition available.

+ Details

Name:	us-gaap_LeasesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration