LOANS AND ALLOWANCE FOR CREDIT LOSSES - Modifications (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023
Financing Receivable, Modifications [Line Items]
Amortized cost basis	$ 1,200	$ 3,277
Commercial and industrial
Financing Receivable, Modifications [Line Items]
Amortized cost basis	$ 384	$ 1,011
% of Loan Class	0.13%	0.38%
Weighted average interest rate reduction (%)	0.00%	0.00%
Weighted average term extension (in months)	60 months	12 months
Weighted-average payment delay (in months)	0 months	0 months
Commercial Mortgages \| Commercial mortgages, other
Financing Receivable, Modifications [Line Items]
Amortized cost basis	$ 376	$ 2,150
% of Loan Class	0.03%	0.22%
Weighted average interest rate reduction (%)	0.00%	0.00%
Weighted average term extension (in months)	0 months	60 months
Weighted-average payment delay (in months)	101 months	4 months
Residential Mortgages
Financing Receivable, Modifications [Line Items]
Amortized cost basis	$ 440
% of Loan Class	0.16%
Weighted average interest rate reduction (%)	0.00%
Weighted average term extension (in months)	0 months
Weighted-average payment delay (in months)	6 months
Consumer Loans \| Home equity lines and loans
Financing Receivable, Modifications [Line Items]
Amortized cost basis		$ 116
% of Loan Class		0.13%
Weighted average interest rate reduction (%)		0.00%
Weighted average term extension (in months)		180 months
Weighted-average payment delay (in months)		0 months
Principal Reduction
Financing Receivable, Modifications [Line Items]
Amortized cost basis	$ 0	$ 0
Principal Reduction \| Commercial and industrial
Financing Receivable, Modifications [Line Items]
Amortized cost basis	0	0
Principal reduction	0	0
Principal Reduction \| Commercial Mortgages \| Commercial mortgages, other
Financing Receivable, Modifications [Line Items]
Amortized cost basis	0	0
Principal reduction	0	0
Principal Reduction \| Residential Mortgages
Financing Receivable, Modifications [Line Items]
Amortized cost basis	0
Principal reduction	0
Principal Reduction \| Consumer Loans \| Home equity lines and loans
Financing Receivable, Modifications [Line Items]
Amortized cost basis		0
Principal reduction		0
Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Amortized cost basis	0	0
Interest Rate Reduction \| Commercial and industrial
Financing Receivable, Modifications [Line Items]
Amortized cost basis	0	0
Interest Rate Reduction \| Commercial Mortgages \| Commercial mortgages, other
Financing Receivable, Modifications [Line Items]
Amortized cost basis	0	0
Interest Rate Reduction \| Residential Mortgages
Financing Receivable, Modifications [Line Items]
Amortized cost basis	0
Interest Rate Reduction \| Consumer Loans \| Home equity lines and loans
Financing Receivable, Modifications [Line Items]
Amortized cost basis		0
Term Extension
Financing Receivable, Modifications [Line Items]
Amortized cost basis	384	1,399
Term Extension \| Commercial and industrial
Financing Receivable, Modifications [Line Items]
Amortized cost basis	384	1,011
Term Extension \| Commercial Mortgages \| Commercial mortgages, other
Financing Receivable, Modifications [Line Items]
Amortized cost basis	0	272
Term Extension \| Residential Mortgages
Financing Receivable, Modifications [Line Items]
Amortized cost basis	0
Term Extension \| Consumer Loans \| Home equity lines and loans
Financing Receivable, Modifications [Line Items]
Amortized cost basis		116
Payment Delay
Financing Receivable, Modifications [Line Items]
Amortized cost basis	816	1,878
Payment Delay \| Commercial and industrial
Financing Receivable, Modifications [Line Items]
Amortized cost basis	0	0
Payment Delay \| Commercial Mortgages \| Commercial mortgages, other
Financing Receivable, Modifications [Line Items]
Amortized cost basis	376	1,878
Payment Delay \| Residential Mortgages
Financing Receivable, Modifications [Line Items]
Amortized cost basis	440
Payment Delay \| Consumer Loans \| Home equity lines and loans
Financing Receivable, Modifications [Line Items]
Amortized cost basis		0
Combination
Financing Receivable, Modifications [Line Items]
Amortized cost basis	0	0
Combination \| Commercial and industrial
Financing Receivable, Modifications [Line Items]
Amortized cost basis	0	0
Combination \| Commercial Mortgages \| Commercial mortgages, other
Financing Receivable, Modifications [Line Items]
Amortized cost basis	0	0
Combination \| Residential Mortgages
Financing Receivable, Modifications [Line Items]
Amortized cost basis	$ 0
Combination \| Consumer Loans \| Home equity lines and loans
Financing Receivable, Modifications [Line Items]
Amortized cost basis		$ 0

Financing Receivable, Excluding Accrued Interest, Modified in Period, Amount

No definition available.

Name:	chmg_FinancingReceivableExcludingAccruedInterestModifiedInPeriodAmount
Namespace Prefix:	chmg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Financing Receivable, Excluding Accrued Interest, Modified in Period, Payment Delay

+ References

No definition available.

+ Details

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

- Definition

Financing Receivable, Excluding Accrued Interest, Modified in Period, Term Extension

+ References

No definition available.

+ Details

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

- Definition

Financing Receivable, Excluding Accrued Interest, Modified in Period, to Total Financing Receivables, Percentage

+ References

No definition available.

+ Details

Name:	chmg_FinancingReceivableExcludingAccruedInterestModifiedInPeriodToTotalFinancingReceivablesPercentage
Namespace Prefix:	chmg_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Accumulated amortized cost excluding accrued interest, of financing receivable modified for debtor experiencing financial difficulty.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

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

- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-34

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-33

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

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

- Definition

Percentage of decrease in weighted average contractual interest rate of financing receivable modified for debtor experiencing financial difficulty.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageInterestRateDecreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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