DEPOSITS (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary of deposits [Abstract]
Non-interest-bearing demand deposits	$ 366,298,000	$ 351,222,000
Interest-bearing demand deposits	110,819,000	114,679,000
Insured money market accounts	392,871,000us-gaap_DepositsMoneyMarketDeposits	361,095,000us-gaap_DepositsMoneyMarketDeposits
Savings deposits	198,183,000us-gaap_DepositsSavingsDeposits	194,768,000us-gaap_DepositsSavingsDeposits
Time deposits	211,843,000us-gaap_TimeDeposits	244,492,000us-gaap_TimeDeposits
Total deposits	1,280,014,000us-gaap_Deposits	1,266,256,000us-gaap_Deposits
Scheduled maturities of time deposits [Abstract]
2015	155,295,000us-gaap_TimeDepositMaturitiesYearOne
2016	34,803,000us-gaap_TimeDepositMaturitiesYearTwo
2017	9,486,000us-gaap_TimeDepositMaturitiesYearThree
2018	4,775,000us-gaap_TimeDepositMaturitiesYearFour
2019	5,851,000us-gaap_TimeDepositMaturitiesYearFive
2020 and thereafter	1,633,000
Time deposits	211,843,000us-gaap_TimeDeposits	244,492,000us-gaap_TimeDeposits
FDIC Insurance limit	250,000chmg_FDICInsuranceLimit	250,000chmg_FDICInsuranceLimit	250,000chmg_FDICInsuranceLimit
Time deposits meeting or exceeding FDIC Insurance limit	42,800,000	37,700,000
Interest expense on time deposits	500,000us-gaap_InterestExpenseTimeDeposits	800,000us-gaap_InterestExpenseTimeDeposits	1,200,000us-gaap_InterestExpenseTimeDeposits
Maturities of time deposits in denominations of $100,000 or more [Abstract]
3 months or less	31,484,000
Over 3 through 6 months	15,286,000
Over 6 through 12 months	17,842,000
Over 12 months	15,845,000
Time deposits, $100,000 or more	$ 80,457,000us-gaap_TimeDeposits100000OrMore	$ 88,800,000us-gaap_TimeDeposits100000OrMore

Amount of deposit with financial institutions that is insured by FDIC.

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Amount of time deposits meeting or exceeding FDIC Insurance limit.

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The amount of contractual maturities of domestic and foreign time deposits (including certificates of deposits, individual retirement accounts and open accounts) in denominations of $100,000 or more, after twelve months from the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 405

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6479006&loc=d3e64164-112818

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-Publisher SEC

-Name Industry Guide

-Number 3

-Section V

-Paragraph D

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The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

+ 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 210.9-03.12)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 12

-Article 9

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Amount of deposits in accounts that offer many of the same services as checking accounts although transactions may be somewhat more limited, share some of the characteristics of a money market fund and are insured by the Federal government, also known as money market accounts.

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Amount of deposits in interest-bearing transaction accounts at banks or savings and loans, that are restricted in regard to ownership and can usually only be held by individuals, nonprofit entities and governments. Negotiable order of withdrawal accounts differ from money market demand accounts as they typically have higher reserve requirements and no limit on the number of checks that can be written.

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Amount of interest bearing deposits with no stated maturity, which may include passbook and statement savings accounts and money-market deposit accounts (MMDAs).

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The aggregate interest expense incurred on time deposits, including certificates of deposits, in domestic offices.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.6)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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-Number 210

-Section 04

-Paragraph 6

-Article 9

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The aggregate amount of all domestic and foreign noninterest-bearing deposits liabilities held by the entity.

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Amount of time deposits maturing after the fifth fiscal year following the latest fiscal year. Time deposits include, but are not limited to, certificates of deposits. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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-Topic 470

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-Section 50

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Amount of time deposits maturing in the fifth fiscal year following the latest fiscal year. Time deposits include, but are not limited to, certificates of deposits. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of time deposits maturing in the fourth fiscal year following the latest fiscal year. Time deposits include, but are not limited to, certificates of deposits. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of time deposits maturing in the next fiscal year following the latest fiscal year. Time deposits include, but are not limited to, certificates of deposits. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of time deposits maturing in the third fiscal year following the latest fiscal year. Time deposits include, but are not limited to, certificates of deposits. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of time deposits maturing in the second fiscal year following the latest fiscal year. Time deposits include, but are not limited to, certificates of deposits. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of deposits that cannot be withdrawn before a set date without penalty or for which notice of withdrawal is required. Time deposits include, but are not limited to, certificates of deposits, individual retirement accounts and open accounts.

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The aggregate amount of domestic and foreign time deposit liabilities (including certificates of deposits, individual retirement accounts and open accounts) in denominations of $100,000 or more.

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