Age limit applicable to forfeiture of stock, the stock is to be forfeited if the participant retires early.

No definition available.

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- Definition

Cash compensation which is paid under incentive compensation plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	chmg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

Deferred Compensation Arrangement With Individual, Cash Retainer Paid

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

Deferred Compensation Arrangement With Individual, Increase In Cash Retainer Paid

+ References

No definition available.

+ Details

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- Definition

Maximum annual installments period which can be opted by plan participants under Nonqualified Deferred Compensation Plan.

+ References

No definition available.

+ Details

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Period Type:	duration

- Definition

Number of trading days considered in valuing common stock .

+ References

No definition available.

+ Details

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- Definition

Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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- 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

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares authorized for issuance under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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Balance Type:	na
Period Type:	instant

- Definition

Percentage of vesting of award under share-based payment arrangement.

+ References

+ Details

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

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Data Type:	na
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- Details

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Data Type:	na
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Data Type:	na
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- Details

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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Stock-based compensation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)	15,000
Age limit for forfeiture of stock award	65 years
Restricted Stock \| Share-based Compensation Award, Vesting Each Year Commencing with First Anniversary Date of Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights percent	20.00%
Restricted Stock \| Share-based Compensation Award, Vesting on Fifth Anniversary Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights percent	100.00%
Incentive Compensation Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)	10,000
Cash compensation	$ 300,000
Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increase in annual cash retainer paid		$ 6,000
Annual cash retainer paid to individuals		11,500
Director \| Directors' Compensation Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)	20,000
Award requisite service period (months)	12 months
Trading days period	30 days
Board of Directors Chairman
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual cash retainer paid to individuals		19,250
Audit Committee Chairman
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increase in annual cash retainer paid		2,875
Annual cash retainer paid to individuals		$ 14,375
Deferred Compensation, Share-based Payments
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum annual installments period (years)	10 years