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FAIR VALUES, Quantitative Information (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] |  |  |
Impaired loan valuation allowance | $ 1,235 us-gaap_ImpairedFinancingReceivableRelatedAllowance | $ 1,046 us-gaap_ImpairedFinancingReceivableRelatedAllowance |
Provision for loan losses | 3,981 us-gaap_ProvisionForLoanAndLeaseLosses | 2,755 us-gaap_ProvisionForLoanAndLeaseLosses | 828 us-gaap_ProvisionForLoanAndLeaseLosses
Write down of OREO | 4,278 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs | 1,493 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs | 1,273 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs
Impaired Loans [Member] |  |  |
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] |  |  |
Principal balance of impaired loans | 4,900 us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember | 2,000 us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember |
Impaired loan valuation allowance | 1,200 us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember | 1,000 us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember |
Provision for loan losses | 232 us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember | 853 us-gaap_ProvisionForLoanAndLeaseLosses / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember |
OREO [Member] |  |  |
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] |  |  |
Net carrying amount of OREO | 3,100 chmg_OtherRealEstateOwnedCarryingValueNet / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember | 538 chmg_OtherRealEstateOwnedCarryingValueNet / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember |
Outstanding balance of OREO | 3,100 us-gaap_OtherRealEstateAndForeclosedAssets / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember | 732 us-gaap_OtherRealEstateAndForeclosedAssets / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember |
Other real estate owned valuation allowance | 2 us-gaap_RealEstateOwnedValuationAllowanceComponent / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember | 194 us-gaap_RealEstateOwnedValuationAllowanceComponent / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember |
Write down of OREO |  | 2 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FairValueByAssetClassAxis = chmg_OtherRealEstateOwnedMember |
Non-recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Third Party Real Estate and a 100% Discount of Personal Property [Member] | Impaired Loans [Member] |  |  |
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] |  |  |
Fair value | 3,645 us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember / us-gaap_ValuationTechniqueAxis = chmg_ThirdPartyRealEstateAndA100DiscountOfPersonalPropertyMember | 999 us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByAssetClassAxis = chmg_ImpairedLoansMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember / us-gaap_ValuationTechniqueAxis = chmg_ThirdPartyRealEstateAndA100DiscountOfPersonalPropertyMember |
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] | OREO [Member] |  |  |
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] |  |  |
Fair value | $ 3,065 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 | $ 538 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 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 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 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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