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CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Parenthetical) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Distribution of shares of treasury stock for directors' deferred compensation (in shares)	3,467chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationShares	3,356chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationShares	3,240chmg_DistributionOfSharesOfTreasuryStockForDirectorsDeferredCompensationShares
Distribution of shares of treasury stock granted for employee restricted stock awards (in shares)	11,279chmg_DistributionOfTreasuryStockForEmployeeRestrictedStockAwardsShares	8,087chmg_DistributionOfTreasuryStockForEmployeeRestrictedStockAwardsShares	10,760chmg_DistributionOfTreasuryStockForEmployeeRestrictedStockAwardsShares
Forfeit of shares of restricted stock awards (in shares)		1,797us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardForfeited
Cash dividends declared (in dollars per share)	$ 1.04us-gaap_CommonStockDividendsPerShareDeclared	$ 1.04us-gaap_CommonStockDividendsPerShareDeclared	$ 1.00us-gaap_CommonStockDividendsPerShareDeclared
Distribution of shares of treasury stock for directors' compensation (in shares)	8,385chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationShares	7,969chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationShares	10,238chmg_DistributionOfSharesOfTreasuryStockForDirectorsCompensationShares
Distribution of shares of treasury stock for employee compensation (in shares)	3,595us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation	4,116us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation	3,453us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation
Sale of shares of treasury stock (in shares)		2,369us-gaap_StockIssuedDuringPeriodSharesTreasuryStockReissued	10,100us-gaap_StockIssuedDuringPeriodSharesTreasuryStockReissued
Purchase of shares of treasury stock (in shares)		3,094us-gaap_TreasuryStockSharesAcquired	25,468us-gaap_TreasuryStockSharesAcquired

X

- Definition

Number of treasury shares issued during the period for directors' compensation.

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Number of treasury shares issued during the period for directors' deferred compensation.

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- Definition

Number of shares of treasury stock issued during the period for employee restricted stock awards.

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- Definition

Aggregate dividends declared during the period for each share of common stock outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

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- Definition

Number of shares related to Restricted Stock Award forfeited during the period.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

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Number of shares (or other type of equity) issued during the period as a result of any equity-based compensation plan other than an employee stock ownership plan (ESOP), net of any shares forfeited. Shares issued could result from the issuance of restricted stock, the exercise of stock options, stock issued under employee stock purchase plans, and/or other employee benefit plans.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

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-Number 210

-Section 04

-Article 3



Reference 2: http://www.xbrl.org/2003/role/presentationRef

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Number of treasury shares (units) reissued during the period, excluding reissuance of shares (units) held in treasury used to satisfy equity-based compensation obligations exercised by the holders of such rights. Upon reissuance of shares (units) from treasury, either the common or preferred stock (unit) reissued is outstanding.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

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-Number 210

-Section 04

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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Number of shares that have been repurchased during the period and are being held in treasury.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3



Reference 2: http://www.xbrl.org/2003/role/presentationRef

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