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LOANS AND ALLOWANCE FOR LOAN LOSSES, TROUBLED DEBT RESTRUCTURING (Details) (USD $) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 Loan | Mar. 31, 2014 Loan | Dec. 31, 2014
Troubled Debt Restructurings [Abstract] |  |  |
Recorded investment in troubled debt restructurings | $ 10,000,000 us-gaap_FinancingReceivableModificationsRecordedInvestment |  | $ 9,700,000 us-gaap_FinancingReceivableModificationsRecordedInvestment
Troubled debt restructuring reserve | 500,000 us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment |  | 300,000 us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
Troubled debt restructurings accruing interest under modified terms | 9,100,000 chmg_FinancingReceivableModificationsRecordedInvestmentStillAccruing |  | 8,700,000 chmg_FinancingReceivableModificationsRecordedInvestmentStillAccruing
Troubled debt restructurings on non-accrual status | 900,000 chmg_FinancingReceivableModificationsRecordedInvestmentNonaccrualStatus |  | 1,000,000 chmg_FinancingReceivableModificationsRecordedInvestmentNonaccrualStatus
Additional amounts committed to customers with loans classified as troubled debt restructurings | 100,000 chmg_AdditionalAmountsCommittedToCustomersWithLoansClassifiedAsTroubledDebtRestructurings |  | 100,000 chmg_AdditionalAmountsCommittedToCustomersWithLoansClassifiedAsTroubledDebtRestructurings
Loans by class modified as troubled debt restructurings [Abstract] |  |  |
Number of Loans | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 | 3 us-gaap_FinancingReceivableModificationsNumberOfContracts2 |
Pre-Modification Outstanding Recorded Investment | 477,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 | 870,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 |
Post-Modification Outstanding Recorded Investment | 477,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 | 826,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 |
Increase in allowance for loan losses related to troubled debt restructurings | 0 us-gaap_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecrease | 0 us-gaap_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecrease |
Troubled debt restructuring, charge offs | 0 us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown | 100,000 us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown |
Number of days past due after which loan considered as payment default | 90 days |  |
Commercial and Industrial [Member] |  |  |
Loans by class modified as troubled debt restructurings [Abstract] |  |  |
Number of Loans | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_CommercialAndIndustrialMember | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_CommercialAndIndustrialMember |
Pre-Modification Outstanding Recorded Investment | 477,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_CommercialAndIndustrialMember | 503,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_CommercialAndIndustrialMember |
Post-Modification Outstanding Recorded Investment | 477,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_CommercialAndIndustrialMember | 503,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = chmg_CommercialAndIndustrialMember |
Commercial mortgages [Member] |  |  |
Loans by class modified as troubled debt restructurings [Abstract] |  |  |
Number of Loans |  | 2 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateOtherReceivableMember |
Pre-Modification Outstanding Recorded Investment |  | 367,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateOtherReceivableMember |
Post-Modification Outstanding Recorded Investment |  | $ 323,000 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateOtherReceivableMember |

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- Definition Represents additional amounts committed to customers with outstanding loans that are classified as troubled debt restructurings. + References No definition available. + Details
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- Definition The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings and are on nonaccrual status as of the balance sheet date. + References No definition available. + Details
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- Definition The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings and have been still accruing interest under the modified terms. + References No definition available. + Details
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- Definition Refers to the number of days past due after which a loan is considered to be in payment default. + References No definition available. + Details
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- Definition Amount of allowance for credit losses related to receivables which had been measured under a general allowance for credit losses methodology that are now identified as troubled debt restructurings. + References No definition available. + Details
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- Definition Amount of increase (decrease) of allowance reserve for credit loss. + 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 210.9-03.7(d)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph d -Article 9 + Details
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- Definition Number of financing receivables that have been modified by troubled debt restructurings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524 + Details
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- Definition Post-modification amount of investment in financing receivables modified by troubled debt restructurings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524 + Details
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- Definition Pre-modification amount of investment in financing receivables modified by troubled debt restructurings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524 + Details
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- Definition The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 40 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18845-111554 + Details
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- Definition Reflects the amount of the write-down related to a troubled debt restructuring. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 40 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18845-111554 + Details
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