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SECURITIES, OTHER THAN TEMPORARY IMPAIRMENT (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended | 3 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2014 | Dec. 31, 2013 Security
Other than temporary impairment [Abstract] |  |  |  |  |
Amortized cost of securities sold | $ 277,038 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis |  |  |  |
Proceeds from sale of securities | 36,258 us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt | 2,650 us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt | 26,210 us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt |  |
Other than temporary impairment credit losses recognized in earnings [Roll Forward] |  |  |  |  |
Beginning balance | 1,939 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld | 3,506 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld | 3,506 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld | 1,939 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld |
Amounts related to credit loss for which an other-than-temporary impairment was not previously recognized | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment |  |
Additions/Subtractions [Abstract] |  |  |  |  |
Amounts related to securities for which the company intends to sell or that it will be more likely than not that the company will be required to sell prior to recovery of amortized cost basis | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses |  |
Reductions for increase in cash flows expected to be collected that are recognized over the remaining life of the security | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsCashFlows | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsCashFlows | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsCashFlows |  |
Reductions for previous credit losses realized on securities sold during the year | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold | (1,596)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold |  |
Reductions for previous credit losses realized in securities liquidated during the year | (1,939)chmg_OtherThanTemporaryImpairmentReductionsForPreviousCreditLossesRealizedOnSecuritiesSold | 0 chmg_OtherThanTemporaryImpairmentReductionsForPreviousCreditLossesRealizedOnSecuritiesSold | 0 chmg_OtherThanTemporaryImpairmentReductionsForPreviousCreditLossesRealizedOnSecuritiesSold |  |
Increases to the amount related to the credit loss for which other-than-temporary impairment was previously recognized | 0 chmg_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsWhenImpairmentRecognizedPreviously | 29 chmg_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsWhenImpairmentRecognizedPreviously | 0 chmg_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsWhenImpairmentRecognizedPreviously |  |
Ending balance | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld | 1,939 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld | 3,506 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld |  | 1,939 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld
Collateralized Debt Obligations [Member] |  |  |  |  |
Other than temporary impairment [Abstract] |  |  |  |  |
Number of securities sold |  |  |  |  | 1 chmg_AvailableForSaleSecuritiesNumberOfSecuritiesSold / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedDebtObligationsMember
Amortized cost of securities sold |  | 600 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedDebtObligationsMember |  |  | 600 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedDebtObligationsMember
Proceeds from sale of securities |  |  |  |  | 600 us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedDebtObligationsMember
Additional credit loss in OTTI recognized |  |  |  |  | 29 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedDebtObligationsMember
Additions/Subtractions [Abstract] |  |  |  |  |
Income from liquidation of debt securities |  |  |  | $ 500 us-gaap_GainLossOnSaleOfDebtInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedDebtObligationsMember |

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- Definition This item represents an reduction of cumulative amount of credit losses realized on securities sold during the period. + References No definition available. + Details
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- Definition This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
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- Definition The difference between the carrying value and the sale price of debt securities, not separately or otherwise categorized as trading, available-for-sale, or held-to-maturity. This element refers to the gain (loss) included in earnings and not to the cash proceeds of the sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(h)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.3(a)) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 + Details
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- Definition This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which a previous OTTI was recognized and the investor does not intend to sell the debt security and it is not more likely than not that the investor will be required to sell the debt security before recovery of its amortized cost basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563 + Details
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- Definition This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which no such other than temporary impairment (OTTI) was previously recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563 + Details
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- Definition The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563 + Details
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- Definition This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held due to increases in cash flows expected to be collected and recognized over the remaining life of the security. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563 + Details
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- Definition The amount of other than temporary impairment losses on investments in debt and equity securities categorized as Available-for-sale which have been recognized in earnings during the period. Such impairment losses consist of other than temporary impairments (OTTI) on equity securities, credit losses on investments in debt securities, and when the entity intends to sell an impaired debt security or it is more likely than not that the entity will be required to sell the impaired debt security before recovery of its amortized cost basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
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