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LEASES - Leased Branch Properties (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Leases [Abstract]
Operating lease right-of-use asset	$ 7,145	$ 8,001
Less: accumulated amortization	(181)	(705)
Change in lease agreements	371		$ (151)
Operating lease right-of-use-assets, net	$ 7,335	$ 7,145

X

- Definition

Operating Lease, Modification In Lease Arrangement

+ References

No definition available.

+ Details

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Namespace Prefix:	chmg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- 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

X

- 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

X

- References

No definition available.

+ Details

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

X

- 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

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	instant