Name:	chmg_EBPEmployerContributionVestingPercentage
Namespace Prefix:	chmg_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

Minimum Vesting service period for company contribution and/ or investment earning to the participant in Company Matching Contributions, Fixed Rate Contributions, Discretionary Profit-Sharing Contributions, Qualified Non-Elective Contributions, and Disability Contributions.

No definition available.

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

EBP, Financial Holdings

No definition available.

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

EBP, Hours Of Service During Calendar Year

No definition available.

Name:	chmg_EBPHoursOfServiceDuringCalendarYear
Namespace Prefix:	chmg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Annual increase in percentage of eligible compensation contributed for participant automatically enrolled in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) until the contribution reaches maximum threshold.

No definition available.

Name:	chmg_EBPParticipantContributionAutomaticAnnualIncreaseDeferralRate
Namespace Prefix:	chmg_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

EBP, Requisite Service Period

No definition available.

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

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478416/962-205-50-1

Name:	us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems
Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Percentage of employer matching contribution on eligible participant contribution to employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

Name:	us-gaap-ebp_EmployeeBenefitPlanEmployerContributionMatchingPercentage
Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

Percentage of participant compensation eligible for employer matching contribution for participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

Name:	us-gaap-ebp_EmployeeBenefitPlanEmployerContributionParticipantCompensationMatchedPercentage
Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

Amount of plan asset, in forfeited nonvested account, no longer allocated to participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 962
-SubTopic 325
-Name Accounting Standards Codification
-Section 55
-Paragraph 17
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478873/962-325-55-17

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (j)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478416/962-205-50-1

Name:	us-gaap-ebp_EmployeeBenefitPlanForfeitedNonvestedAccount
Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Percentage of eligible compensation contributed for participant automatically enrolled in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

Name:	us-gaap-ebp_EmployeeBenefitPlanParticipantContributionAutomaticDeferralRate
Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

Name:	dei_LegalEntityAxis=chmg_EBP002Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	chmg_EBPEmployerContributionTypeAxis=chmg_EBPDiscretionaryProfitSharingContributionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	chmg_EBPEmployerContributionTypeAxis=chmg_EmployerNonDiscretionaryContributionsPlansPriorTo2017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	chmg_EBPEmployerContributionTypeAxis=chmg_EBPRolloverContributionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	chmg_EBPEmployerContributionTypeAxis=chmg_DeferralRateOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: