SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Loans (Details)	12 Months Ended
Dec. 31, 2025
Accounting Policies [Abstract]
Period of delinquency after which a loan is placed on non-accrual status (days)	90 days

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

No definition available.

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