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

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- 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

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- References No definition available. + Details
Name: | us-gaap_AccountingPoliciesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration