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SECURITIES SOLD UNDER AGREEMENTS TO REPURCHASE (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Summary of securities sold under agreements to repurchase [Abstract] |  |  |  |
Balance at December 31 | $ 29,652 us-gaap_SecuritiesSoldUnderAgreementsToRepurchase |  | $ 32,701 us-gaap_SecuritiesSoldUnderAgreementsToRepurchase | $ 32,711 us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
Maximum month-end balance | 31,914 chmg_SecuritiesSoldUnderAgreementsToRepurchaseMaximumMonthEndBalance |  | 32,701 chmg_SecuritiesSoldUnderAgreementsToRepurchaseMaximumMonthEndBalance | 38,284 chmg_SecuritiesSoldUnderAgreementsToRepurchaseMaximumMonthEndBalance
Average balance during year | 30,667 chmg_SecuritiesSoldUnderRepurchaseAgreementsAverageBalanceDuringYear |  | 31,102 chmg_SecuritiesSoldUnderRepurchaseAgreementsAverageBalanceDuringYear | 34,534 chmg_SecuritiesSoldUnderRepurchaseAgreementsAverageBalanceDuringYear
Weighted-average interest rate, period end (in hundredths) | 2.82%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate | [1] | 2.93%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate | 3.02%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
Average interest rate paid during period (in hundredths) | 2.77%chmg_AssetsSoldUnderAgreementsToRepurchaseAverageInterestRatePaidDuringYear |  | 2.76%chmg_AssetsSoldUnderAgreementsToRepurchaseAverageInterestRatePaidDuringYear | 2.88%chmg_AssetsSoldUnderAgreementsToRepurchaseAverageInterestRatePaidDuringYear
Repurchase Liability | 29,652 us-gaap_AssetsSoldUnderAgreementsToRepurchaseRepurchaseLiability | [1] |  |
Accrued Interest Payable | 63 chmg_AssetsSoldUnderAgreementsToRepurchaseAccruedInterestPayable | [1] |  |
Weighted Average Rate (in hundredths) | 2.82%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate | [1] | 2.93%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate | 3.02%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
Estimated Fair Value of Collateral Securities | 40,212 us-gaap_SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure | [1],[2] |  |
Accrued interest receivable included in fair value of repurchase agreements | 342 chmg_SecuritiesSoldUnderRepurchaseAgreementsAccruedInterestReceivable |  |  |
Within 90 Days [Member] |  |  |  |
Summary of securities sold under agreements to repurchase [Abstract] |  |  |  |
Weighted-average interest rate, period end (in hundredths) | 0.10%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = chmg_MaturityWithin90DaysMember | [1] |  |
Repurchase Liability | 9,652 us-gaap_AssetsSoldUnderAgreementsToRepurchaseRepurchaseLiability / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = chmg_MaturityWithin90DaysMember | [1] |  |
Accrued Interest Payable | 0 chmg_AssetsSoldUnderAgreementsToRepurchaseAccruedInterestPayable / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = chmg_MaturityWithin90DaysMember | [1] |  |
Weighted Average Rate (in hundredths) | 0.10%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = chmg_MaturityWithin90DaysMember | [1] |  |
Estimated Fair Value of Collateral Securities | 16,682 us-gaap_SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = chmg_MaturityWithin90DaysMember | [1],[2] |  |
After 90 Days But Within One Year [Member] |  |  |  |
Summary of securities sold under agreements to repurchase [Abstract] |  |  |  |
Weighted-average interest rate, period end (in hundredths) | 0.00%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOver90DaysMember | [1] |  |
Repurchase Liability | 0 us-gaap_AssetsSoldUnderAgreementsToRepurchaseRepurchaseLiability / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOver90DaysMember | [1] |  |
Accrued Interest Payable | 0 chmg_AssetsSoldUnderAgreementsToRepurchaseAccruedInterestPayable / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOver90DaysMember | [1] |  |
Weighted Average Rate (in hundredths) | 0.00%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOver90DaysMember | [1] |  |
Estimated Fair Value of Collateral Securities | 0 us-gaap_SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOver90DaysMember | [1],[2] |  |
After One Year But Within Five Years [Member] |  |  |  |
Summary of securities sold under agreements to repurchase [Abstract] |  |  |  |
Weighted-average interest rate, period end (in hundredths) | 4.13%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = chmg_AfterOneYearButWithinFiveYearsMember | [1] |  |
Repurchase Liability | 20,000 us-gaap_AssetsSoldUnderAgreementsToRepurchaseRepurchaseLiability / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = chmg_AfterOneYearButWithinFiveYearsMember | [1] |  |
Accrued Interest Payable | 63 chmg_AssetsSoldUnderAgreementsToRepurchaseAccruedInterestPayable / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = chmg_AfterOneYearButWithinFiveYearsMember | [1] |  |
Weighted Average Rate (in hundredths) | 4.13%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = chmg_AfterOneYearButWithinFiveYearsMember | [1] |  |
Estimated Fair Value of Collateral Securities | 23,530 us-gaap_SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = chmg_AfterOneYearButWithinFiveYearsMember | [1],[2] |  |
After Five Years But Within Ten Years [Member] |  |  |  |
Summary of securities sold under agreements to repurchase [Abstract] |  |  |  |
Weighted-average interest rate, period end (in hundredths) | 0.00%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = chmg_AfterFiveYearsButWithinTenYearsMember | [1] |  |
Repurchase Liability | 0 us-gaap_AssetsSoldUnderAgreementsToRepurchaseRepurchaseLiability / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = chmg_AfterFiveYearsButWithinTenYearsMember | [1] |  |
Accrued Interest Payable | 0 chmg_AssetsSoldUnderAgreementsToRepurchaseAccruedInterestPayable / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = chmg_AfterFiveYearsButWithinTenYearsMember | [1] |  |
Weighted Average Rate (in hundredths) | 0.00%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = chmg_AfterFiveYearsButWithinTenYearsMember | [1] |  |
Estimated Fair Value of Collateral Securities | $ 0 us-gaap_SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = chmg_AfterFiveYearsButWithinTenYearsMember | [1],[2] |  |
Without Call Provisions [Member] |  |  |  |
Summary of securities sold under agreements to repurchase [Abstract] |  |  |  |
Weighted-average interest rate, period end (in hundredths) | 3.10%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseAxis = chmg_WithoutCallProvisionsMember |  |  |
Weighted Average Rate (in hundredths) | 3.10%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseAxis = chmg_WithoutCallProvisionsMember |  |  |
Weight average remaining term to maturity | 1 year 10 months 24 days |  |  |
[1] | At December 31, 2014, the securities repurchase agreements were non-callable with a weighted-average rate of 3.10%, and a weighted-average term to maturity of approximately 1.9 years.
[2] | Represents the estimated fair value of the securities subject to the repurchase agreements, including accrued interest receivable, of approximately $342 thousand at December 31, 2014.

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- Definition Amount of interest payable on n the amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets. + References No definition available. + Details
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- Definition Refers to the average interest rate paid during year on assets under repurchase agreement. + References No definition available. + Details
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- Definition Maximum month end carrying value as of the balance sheet date of securities that an institution sells and agrees to repurchase (the identical or substantially the same securities) as a seller-borrower at a specified date for a specified price, also known as a repurchase agreement, or repo. Most repos involve obligations of the federal government or its agencies, but other financial instruments, such as commercial paper, banker's acceptances, and negotiable certificates of deposit, are sometimes used in repos. + References No definition available. + Details
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- Definition For securities sold under repurchase agreements, interest accrued on interest earning assets that has not yet been received as of the balance sheet date. + References No definition available. + Details
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- Definition Average balance over period for the carrying value as of the balance sheet date of securities that an institution sells and agrees to repurchase (the identical or substantially the same securities) as a seller-borrower at a specified date for a specified price, also known as a repurchase agreement, or repo. Most repos involve obligations of the federal government or its agencies, but other financial instruments, such as commercial paper, banker's acceptances, and negotiable certificates of deposit, are sometimes used in repos. + References No definition available. + Details
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- Definition Weighted average remaining contractual term for assets sold under repurchase agreements outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Interest rate on the amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph m -Subparagraph 1 -Article 4 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(m)(1)(i)(A)(ii)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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- Definition Amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph m -Subparagraph 1 -Article 4 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(m)(1)(i)(A)(ii)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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- Definition Fair value portion of securities that an entity sells and agrees to repurchase at a specified date for a specified price. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611 + Details
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- Definition Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset. + 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 210.9-03.13(1)) -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 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph 1 -Article 9 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=45278096&loc=d3e9760-107771 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(m)(1)(a)(i)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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