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BRANCH ACQUISITION (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended | 1 Months Ended |
--- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Nov. 30, 2013 Branch | Nov. 23, 2013
Assets acquired and deposits assumed [Abstract] |  |  |  |  |
Gain from bargain purchase | $ 0 us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount | $ 470 us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount | $ 0 us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount |  |
Bank of America Branch Offices [Member] |  |  |  |  |
Assets acquired and deposits assumed [Abstract] |  |  |  |  |
Cash, net |  |  |  |  | 170,904 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents / us-gaap_BusinessAcquisitionAxis = chmg_BankOfAmericaMember
Loans |  |  |  |  | 1,240 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedFinancialAssets / us-gaap_BusinessAcquisitionAxis = chmg_BankOfAmericaMember
Bank premises and equipment |  |  |  |  | 4,081 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment / us-gaap_BusinessAcquisitionAxis = chmg_BankOfAmericaMember
Core deposit intangible asset |  |  |  |  | 2,155 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles / us-gaap_BusinessAcquisitionAxis = chmg_BankOfAmericaMember
Other assets |  |  |  |  | 350 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets / us-gaap_BusinessAcquisitionAxis = chmg_BankOfAmericaMember
Total assets acquired |  |  |  |  | 178,730 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets / us-gaap_BusinessAcquisitionAxis = chmg_BankOfAmericaMember
Deposits assumed |  |  |  |  | 177,749 chmg_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeposit / us-gaap_BusinessAcquisitionAxis = chmg_BankOfAmericaMember
Time deposit premium |  |  |  |  | 263 chmg_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedTimeDepositPremium / us-gaap_BusinessAcquisitionAxis = chmg_BankOfAmericaMember
Other liabilities |  |  |  |  | 718 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther / us-gaap_BusinessAcquisitionAxis = chmg_BankOfAmericaMember
Total liabilities assumed |  |  |  |  | 178,730 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities / us-gaap_BusinessAcquisitionAxis = chmg_BankOfAmericaMember
Number of branches acquired |  |  |  | 6 chmg_NumberOfBranchesAcquired / us-gaap_BusinessAcquisitionAxis = chmg_BankOfAmericaMember |
Gain from bargain purchase | $ 470 us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount / us-gaap_BusinessAcquisitionAxis = chmg_BankOfAmericaMember |  |  |  |
Bank of America Branch Offices [Member] | Core Deposit Intangibles [Member] |  |  |  |  |
Assets acquired and deposits assumed [Abstract] |  |  |  |  |
Amortization period | 7 years |  |  |  |

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- Definition The amount of acquisition cost of a business combination allocated to time deposit. + References No definition available. + Details
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- Definition In a business combination in which the amount of net identifiable assets acquired and liabilities assumed exceeds the aggregate consideration transferred or to be transferred (as defined), this element represents the amount of gain recognized by the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (f)(1) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6571-128477 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6527-128477 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 25 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6396-128476 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6387-128476 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 25 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6393-128476 + Details
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- Definition Amount of assets acquired at the acquisition date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472 + Details
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- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions, acquired at the acquisition date. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472 + Details
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- Definition The amount of identifiable intangible assets recognized as of the acquisition date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 55 -Paragraph 37 -URI http://asc.fasb.org/extlink&oid=35744172&loc=d3e2207-128464 + Details
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- Definition Amount of other liabilities due after one year or the normal operating cycle, if longer, assumed at the acquisition date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472 + Details
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- Definition Amount of other assets expected to be realized or consumed after one year or the normal operating cycle, if longer, acquired at the acquisition date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472 + Details
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