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

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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 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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- Definition Number of shares related to Restricted Stock Award forfeited during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5 + Details
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- Definition 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. + 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 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 + Details
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