Name:	chmg_AdditionalAmountsCommittedToCustomersWithLoansClassifiedAsTroubledDebtRestructurings
Namespace Prefix:	chmg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Financing Receivable, Modifications, Extension Period

No definition available.

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

Financing Receivable, Modifications, Loan Balance Remaining in Modified Status

No definition available.

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

Financing Receivable, Modifications, Loan Balance Remaining in Modified Status, Modified More than Once

No definition available.

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

Financing Receivable, Modifications, Number of Borrowers

No definition available.

Name:	chmg_FinancingReceivableModificationsNumberOfBorrowers
Namespace Prefix:	chmg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Financing Receivable, Modifications, Number of Days Past Due

No definition available.

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

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings and are on nonaccrual status as of the balance sheet date.

No definition available.

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

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings and have been still accruing interest under the modified terms.

No definition available.

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

Financing Receivable, Number of Loans Remaining in Modified Status

No definition available.

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

Financing Receivable, Number of Loans Remaining in Modified Status, Modified More than Once

No definition available.

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

Financing Receivable, Troubled Debt Restructuring, Purchased with Credit Deterioration, Increase

No definition available.

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

Amount of increase (decrease) of allowance reserve for credit loss.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Name:	us-gaap_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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.

No definition available.

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

Number of financing receivables that have been modified by troubled debt restructurings.

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Amount, after troubled debt restructuring, of modified financing receivable.

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of financing receivable modified as troubled debt restructuring.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=118939409&loc=d3e18845-111554

Name:	us-gaap_FinancingReceivableModificationsRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953682-111524

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Reflects the amount of the write-down related to a troubled debt restructuring.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=118939409&loc=d3e18845-111554

Name:	us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_InterestRateBelowMarketReductionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_EquitySecuritiesByIndustryAxis=us-gaap_CommercialAndIndustrialSectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_InternalCreditAssessmentAxis=chmg_NonAccrualStatusMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_PrincipalForgivenessMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=chmg_CommercialMortgagesOtherLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancialInstrumentPerformanceStatusAxis=chmg_NonAccrualStatusMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables30To59DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=chmg_CommercialAndConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: