LOANS AND ALLOWANCE FOR CREDIT LOSSES - Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Average Amortized Basis
Average recorded investment, total	$ 7,111	$ 5,468	$ 6,769	$ 6,307
Interest Income Recognized
Total	3	7	12	25
Commercial and industrial
Average Amortized Basis
With no related allowance recorded	139	163	119	520
With an allowance recorded	1,836	1,282	1,854	1,175
Interest Income Recognized
With no related allowance recorded	0	0	4	0
With an allowance recorded	3	4	8	13
Residential Mortgages
Average Amortized Basis
With no related allowance recorded	0	0	0	359
Interest Income Recognized
With no related allowance recorded	0	0	0	0
Construction \| Commercial Mortgages
Average Amortized Basis
With no related allowance recorded	1,845	0	2,098	2
Interest Income Recognized
With no related allowance recorded	0	0	0	0
Commercial mortgages, other \| Commercial Mortgages
Average Amortized Basis
With no related allowance recorded	2,796	3,863	2,517	4,052
With an allowance recorded	495	31	181	34
Interest Income Recognized
With no related allowance recorded	0	3	0	12
With an allowance recorded	0	0	0	0
Home equity lines & loans \| Consumer Loans
Average Amortized Basis
With no related allowance recorded	0	51	0	123
With an allowance recorded	0	78	0	42
Interest Income Recognized
With no related allowance recorded	0	0	0	0
With an allowance recorded	$ 0	$ 0	$ 0	$ 0

Impaired Financing Receivable, with No Related Allowance, Interest Income (Reversal), Accrual Method

No definition available.

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Namespace Prefix:	chmg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 17
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-17

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-15

+ Details

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Period Type:	duration

- References

No definition available.

+ Details

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- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 15
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-15

+ Details

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Period Type:	duration

- References

No definition available.

+ Details

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- Definition

Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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- Definition

Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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Period Type:	duration

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.

+ References

+ Details

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