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FAIR VALUE, Quantitative Information (Details) (USD $) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Information related to Level 3 non-recurring fair value measurement [Abstract] |  |  |
Impaired loan valuation allowance | $ 1,581,000 us-gaap_ImpairedFinancingReceivableRelatedAllowance |  | $ 1,235,000 us-gaap_ImpairedFinancingReceivableRelatedAllowance
Increase (decrease) in provision for loan losses | 390,000 us-gaap_ProvisionForLoanAndLeaseLosses | 639,000 us-gaap_ProvisionForLoanAndLeaseLosses |
Write down of OREO | 390,000 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs | 573,000 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs |
Impaired Loans [Member] |  |  |
Information related to Level 3 non-recurring fair value measurement [Abstract] |  |  |
Carrying amount of impaired loans | 4,700,000 us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember |  | 4,900,000 us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember
Impaired loan valuation allowance | 1,600,000 us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember |  | 1,200,000 us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember
Increase (decrease) in provision for loan losses | (653,000)us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember |  | 232,000 us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember
Other Real Estate Owned [Member] |  |  |
Information related to Level 3 non-recurring fair value measurement [Abstract] |  |  |
Net carrying amount of OREO | 2,500,000 chmg_OtherRealEstateOwnedCarryingValueNet / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember |  | 3,100,000 chmg_OtherRealEstateOwnedCarryingValueNet / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember
Outstanding balance of OREO |  |  | 3,100,000 us-gaap_OtherRealEstateAndForeclosedAssets / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember
Other real estate owned valuation allowance | 0 us-gaap_RealEstateOwnedValuationAllowanceComponent / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember |  | 2,000 us-gaap_RealEstateOwnedValuationAllowanceComponent / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember
Write down of OREO | 0 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember |  |
Non-recurring [Member] | Other Real Estate Owned [Member] |  |  |
Information related to Level 3 non-recurring fair value measurement [Abstract] |  |  |
Outstanding balance of OREO | 2,506,000 us-gaap_OtherRealEstateAndForeclosedAssets / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember |  | 3,065,000 us-gaap_OtherRealEstateAndForeclosedAssets / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Non-recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other Real Estate Owned [Member] |  |  |
Information related to Level 3 non-recurring fair value measurement [Abstract] |  |  |
Outstanding balance of OREO | 0 us-gaap_OtherRealEstateAndForeclosedAssets / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember |  | 0 us-gaap_OtherRealEstateAndForeclosedAssets / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Non-recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other Real Estate Owned [Member] |  |  |
Information related to Level 3 non-recurring fair value measurement [Abstract] |  |  |
Outstanding balance of OREO | 0 us-gaap_OtherRealEstateAndForeclosedAssets / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember |  | 0 us-gaap_OtherRealEstateAndForeclosedAssets / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Non-recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other Real Estate Owned [Member] |  |  |
Information related to Level 3 non-recurring fair value measurement [Abstract] |  |  |
Outstanding balance of OREO | 2,506,000 us-gaap_OtherRealEstateAndForeclosedAssets / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember |  | 3,065,000 us-gaap_OtherRealEstateAndForeclosedAssets / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Non-recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Third Party Appraisals [Member] | Impaired Loans [Member] |  |  |
Information related to Level 3 non-recurring fair value measurement [Abstract] |  |  |
Fair Value | 2,993,000 us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember / us-gaap_ValuationTechniqueAxis = us-gaap_MarketApproachValuationTechniqueMember |  | 3,645,000 us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember / us-gaap_ValuationTechniqueAxis = us-gaap_MarketApproachValuationTechniqueMember
Unobservable Input 1 | Management discount based on underlying collateral characteristics and market conditions |  | Management discount based on underlying collateral characteristics and market conditions
Non-recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Third Party Appraisals [Member] | Other Real Estate Owned [Member] |  |  |
Information related to Level 3 non-recurring fair value measurement [Abstract] |  |  |
Fair Value | $ 2,506,000 us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember / us-gaap_ValuationTechniqueAxis = us-gaap_MarketApproachValuationTechniqueMember |  | $ 3,065,000 us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember / us-gaap_ValuationTechniqueAxis = us-gaap_MarketApproachValuationTechniqueMember
Unobservable Input 1 | Estimated holding period |  | Estimated holding period
Unobservable Input 2 | Estimated closing costs |  | Estimated closing costs

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- Definition Net carrying value of other real estate owned at balance sheet date. + References No definition available. + Details
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- Definition This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a nonrecurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References No definition available. + Details
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- Definition Amount of direct write-downs of financing receivables charged against the allowance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524 + Details
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- Definition Amount of allowance for credit losses related to recorded investment. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524 + Details
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- Definition Carrying amount as of the balance sheet date of other real estate and foreclosed assets. Other real estate may include real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. Foreclosed assets include all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.10(4)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Amount of expense related to estimated loss from loan and lease transactions. + 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 (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 11 -Article 9 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.11) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524 + Details
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- Definition The valuation allowance loss on real estate owned or any impairment losses taken against real estate investments held for use or resale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.10(4)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 35 -Paragraph 43 -URI http://asc.fasb.org/extlink&oid=29634951&loc=d3e1739-110224 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 10 -Subparagraph 4 -Article 9 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 35 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=29634951&loc=d3e1424-110224 + Details
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