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LEASES - Premises and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Leases [Abstract]
Buildings	$ 5,572	$ 5,572
Less: accumulated depreciation	(1,958)	(1,875)
Net book value	$ 3,614	$ 3,697

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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

X

- Definition

Finance Lease, Right-of-Use Asset, after Accumulated Amortization

+ References

No definition available.

+ Details

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

X

- Definition

Finance Lease, Right-Of-Use Asset, before Accumulated Amortization

+ References

No definition available.

+ Details

Name:	chmg_FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization
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
Period Type:	duration