PREMISES AND EQUIPMENT - Leases in Premises and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Buildings	$ 5,572	$ 5,572
Less: accumulated depreciation	(1,875)	(1,541)
Net book value	$ 3,697	$ 4,031

Finance Lease, Right-Of-Use Asset, Before accumulated amortization

No definition available.

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