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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Loans (Details)	12 Months Ended
	Dec. 31, 2024
Accounting Policies [Abstract]
Period of delinquency after which a loan is placed on non-accrual status (days)	90 days

X

- Definition

Refers to the period of delinquency after which a loan is placed on non-accrual status.

+ References

No definition available.

+ Details

Name:	chmg_PeriodOfDelinquencyAfterWhichALoanIsPlacedOnNonAccrualStatus
Namespace Prefix:	chmg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

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