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SECURITIES, SECURITIES HELD TO MATURITY (Details) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Amortized cost and estimated fair value of securities held to maturity [Abstract] |  |
Amortized Cost | $ 5,693 us-gaap_HeldToMaturitySecurities | $ 5,831 us-gaap_HeldToMaturitySecurities
Unrealized Gains | 383 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain | 366 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Unrealized Losses | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Estimated Fair Value | 6,076 us-gaap_HeldToMaturitySecuritiesFairValue | 6,197 us-gaap_HeldToMaturitySecuritiesFairValue
Amortized Cost [Abstract] |  |
Within one year | 2,639 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount |
After one, but within five years | 2,051 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount |
After five, but within ten years | 1,003 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount |
After ten years | 0 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount |
Total | 5,693 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasis |
Amortized Cost | 5,693 us-gaap_HeldToMaturitySecurities | 5,831 us-gaap_HeldToMaturitySecurities
Fair Value [Abstract] |  |
Within one year | 2,667 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue |
After one, but within five years | 2,243 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue |
After five, but within ten years | 1,166 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue |
After ten years | 0 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue |
Total | 6,076 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValue |
Fair Value | 6,076 us-gaap_HeldToMaturitySecuritiesFairValue | 6,197 us-gaap_HeldToMaturitySecuritiesFairValue
Obligations of States and Political Subdivisions [Member] |  |
Amortized cost and estimated fair value of securities held to maturity [Abstract] |  |
Amortized Cost | 5,253 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 5,175 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Unrealized Gains | 379 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 360 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Unrealized Losses | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Estimated Fair Value | 5,632 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 5,535 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Amortized Cost [Abstract] |  |
Amortized Cost | 5,253 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 5,175 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Fair Value [Abstract] |  |
Fair Value | 5,632 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 5,535 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Time Deposits with Other Financial Institutions [Member] |  |
Amortized cost and estimated fair value of securities held to maturity [Abstract] |  |
Amortized Cost | 440 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = chmg_TimeDepositsWithOtherFinancialInstitutionsMember | 656 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = chmg_TimeDepositsWithOtherFinancialInstitutionsMember
Unrealized Gains | 4 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = chmg_TimeDepositsWithOtherFinancialInstitutionsMember | 6 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_DebtSecurityAxis = chmg_TimeDepositsWithOtherFinancialInstitutionsMember
Unrealized Losses | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = chmg_TimeDepositsWithOtherFinancialInstitutionsMember | 0 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_DebtSecurityAxis = chmg_TimeDepositsWithOtherFinancialInstitutionsMember
Estimated Fair Value | 444 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = chmg_TimeDepositsWithOtherFinancialInstitutionsMember | 662 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = chmg_TimeDepositsWithOtherFinancialInstitutionsMember
Amortized Cost [Abstract] |  |
Amortized Cost | 440 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = chmg_TimeDepositsWithOtherFinancialInstitutionsMember | 656 us-gaap_HeldToMaturitySecurities / us-gaap_DebtSecurityAxis = chmg_TimeDepositsWithOtherFinancialInstitutionsMember
Fair Value [Abstract] |  |
Fair Value | 444 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = chmg_TimeDepositsWithOtherFinancialInstitutionsMember | 662 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_DebtSecurityAxis = chmg_TimeDepositsWithOtherFinancialInstitutionsMember
Mortgage-Backed Securities, Residential [Member] |  |
Amortized Cost [Abstract] |  |
Without Single Maturity Date | 0 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateNetCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |
Fair Value [Abstract] |  |
Without Single Maturity Date | 0 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_DebtSecurityAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |
Collateralized Mortgage Obligations [Member] |  |
Amortized Cost [Abstract] |  |
Without Single Maturity Date | 0 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateNetCarryingAmount / us-gaap_DebtSecurityAxis = us-gaap_CollateralizedMortgageObligationsMember |
Fair Value [Abstract] |  |
Without Single Maturity Date | 0 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_DebtSecurityAxis = us-gaap_CollateralizedMortgageObligationsMember |
SBA Loans Pools [Member] |  |
Amortized Cost [Abstract] |  |
Without Single Maturity Date | 0 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateNetCarryingAmount / us-gaap_DebtSecurityAxis = chmg_SbaLoansMember |
Fair Value [Abstract] |  |
Without Single Maturity Date | $ 0 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_DebtSecurityAxis = chmg_SbaLoansMember |

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- Definition Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.6) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 + Details
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- Definition Fair value of held-to-maturity debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62586-112803 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 + Details
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