Represents the period, for which commitments are generally made to make commercial draw notes.

No definition available.

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- Definition

Represents the period, for which commitments are generally made to make real estate and home equity loans.

+ References

No definition available.

+ Details

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Period Type:	duration

- Definition

Refer to interest rate on fixed-rate commitments to make commercial draw notes.

+ References

No definition available.

+ Details

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Period Type:	duration

- Definition

Refer to interest rate on the fixed-rate commitments to make loans.

+ References

No definition available.

+ Details

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Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Refers to maturity period of the fixed-rate commitments to make loans.

+ References

No definition available.

+ Details

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

- 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://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-11

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 825
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478898/942-825-50-1

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Period Type:	duration

- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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COMMITMENTS AND CONTINGENCIES - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2026	Dec. 31, 2025	Mar. 31, 2025	Dec. 31, 2024
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Allowance for credit losses	$ 24,890	$ 24,209	$ 22,522	$ 21,388
Unfunded Loan Commitment
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Allowance for credit losses	$ 600	$ 586	$ 538	$ 842
Minimum \| Fixed Rate
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Interest rate on fixed-rate commitments to make loans	5.88%
Maturity period of fixed-rate commitments to make loans (years)	5 years
Commitments period to make fixed-rate commercial draw notes (months)	3 months
Interest rate on fixed-rate commitments to make commercial draw notes	4.00%
Maximum
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments period to make real estate and home equity loans (days)	60 days
Maximum \| Fixed Rate
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Interest rate on fixed-rate commitments to make loans	7.38%
Maturity period of fixed-rate commitments to make loans (years)	30 years
Commitments period to make fixed-rate commercial draw notes (months)	24 months
Interest rate on fixed-rate commitments to make commercial draw notes	7.88%