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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 $ 0us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount $ 470us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount $ 0us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount    
Bank of America Branch Offices [Member]          
Assets acquired and deposits assumed [Abstract]          
Cash, net         170,904us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents
/ us-gaap_BusinessAcquisitionAxis
= chmg_BankOfAmericaMember
Loans         1,240us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedFinancialAssets
/ us-gaap_BusinessAcquisitionAxis
= chmg_BankOfAmericaMember
Bank premises and equipment         4,081us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment
/ us-gaap_BusinessAcquisitionAxis
= chmg_BankOfAmericaMember
Core deposit intangible asset         2,155us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles
/ us-gaap_BusinessAcquisitionAxis
= chmg_BankOfAmericaMember
Other assets         350us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets
/ us-gaap_BusinessAcquisitionAxis
= chmg_BankOfAmericaMember
Total assets acquired         178,730us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets
/ us-gaap_BusinessAcquisitionAxis
= chmg_BankOfAmericaMember
Deposits assumed         177,749chmg_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeposit
/ us-gaap_BusinessAcquisitionAxis
= chmg_BankOfAmericaMember
Time deposit premium         263chmg_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedTimeDepositPremium
/ us-gaap_BusinessAcquisitionAxis
= chmg_BankOfAmericaMember
Other liabilities         718us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther
/ us-gaap_BusinessAcquisitionAxis
= chmg_BankOfAmericaMember
Total liabilities assumed         178,730us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities
/ us-gaap_BusinessAcquisitionAxis
= chmg_BankOfAmericaMember
Number of branches acquired       6chmg_NumberOfBranchesAcquired
/ us-gaap_BusinessAcquisitionAxis
= chmg_BankOfAmericaMember
 
Gain from bargain purchase $ 470us-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