COMMITMENTS AND CONTINGENCIES (Details) (USD $)	12 Months Ended
Dec. 31, 2014 Stipulation Proceeding	Dec. 31, 2013 Proceeding
Pending Litigation [Member]
Loss Contingencies [Line Items]
Number of legal proceedings		2
Accrual for legal settlement	$ 12,100,000
Receivable for insurance proceeds	7,900,000
Number of stipulations submitted to Surrogate's Court	2
Pending Litigation [Member] \| Objections to Final Settlement of Trust Account One [Member]
Loss Contingencies [Line Items]
Damages and disallowed trustee's commissions claimed	13,200,000
Pending Litigation [Member] \| Objections to Final Settlement of Trust Account Two [Member]
Loss Contingencies [Line Items]
Damages and disallowed trustee's commissions claimed	24,100,000
Minimum [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments period to make fixed-rate commercial draw notes	3 months
Minimum [Member] \| Fixed Interest Rate [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Interest rate on fixed-rate commitments to make loans (in hundredths)	2.75%
Maturity period of fixed-rate commitments to make loans	5 years
Interest rate on fixed-rate commitments to make commercial draw notes (in hundredths)	1.76%
Maximum [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments period to make real estate and home equity loans	60 days
Commitments period to make fixed-rate commercial draw notes	18 months
Off-balance sheet financial instruments, standard term	12 months
Maximum [Member] \| Fixed Interest Rate [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Interest rate on fixed-rate commitments to make loans (in hundredths)	5.875%
Maturity period of fixed-rate commitments to make loans	30 years
Interest rate on fixed-rate commitments to make commercial draw notes (in hundredths)	5.25%
Commitments to Make Loans [Member] \| Fixed Interest Rate [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contractual amounts of financial instruments with off-balance sheet risk	23,756,000	21,049,000
Commitments to Make Loans [Member] \| Variable Interest Rate [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contractual amounts of financial instruments with off-balance sheet risk	11,082,000	7,893,000
Unused Line of Credit [Member] \| Fixed Interest Rate [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contractual amounts of financial instruments with off-balance sheet risk	812,000	2,190,000
Unused Line of Credit [Member] \| Variable Interest Rate [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contractual amounts of financial instruments with off-balance sheet risk	185,235,000	187,061,000
Standby Letters of Credit [Member] \| Fixed Interest Rate [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contractual amounts of financial instruments with off-balance sheet risk	0	0
Standby Letters of Credit [Member] \| Variable Interest Rate [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contractual amounts of financial instruments with off-balance sheet risk	$ 16,747,000	17,290,000

Represents the period, for which commitments are generally made to make commercial draw notes.

No definition available.

Name:	chmg_CommitmentsPeriodToMakeCommercialDrawNotes
Namespace Prefix:	chmg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the period, for which commitments are generally made to make real estate and home equity loans.

+ References

No definition available.

+ Details

Name:	chmg_CommitmentsPeriodToMakeRealEstateAndHomeEquityLoans
Namespace Prefix:	chmg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

For off-balance sheet financial instruments, the standard term for the agreement.

+ References

No definition available.

+ Details

Name:	chmg_FairValueOffBalanceSheetRisksFinancialInstrumentStandardTerm
Namespace Prefix:	chmg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Refer to interest rate on fixed-rate commitments to make commercial draw notes.

+ References

No definition available.

+ Details

Name:	chmg_InterestRateOnFixedRateCommitmentsToMakeCommercialDrawNotes
Namespace Prefix:	chmg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Refer to interest rate on the fixed-rate commitments to make loans.

+ References

No definition available.

+ Details

Name:	chmg_InterestRateOnFixedRateCommitmentsToMakeLoans
Namespace Prefix:	chmg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Refers to maturity period of the fixed-rate commitments to make loans.

+ References

No definition available.

+ Details

Name:	chmg_MaturityPeriodOfFixedRateCommitmentsToMakeLoans
Namespace Prefix:	chmg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Refers to the number of stipulations reflecting the settlements submitted to Surrogate's Court.

+ References

No definition available.

+ Details

Name:	chmg_NumberOfStipulationsSubmittedToSurrogateSCourt
Namespace Prefix:	chmg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The fair value of financial liabilities, which are not recognized in the financial statements (off-balance sheet) because they fail to meet some other criterion for recognition.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611

+ Details

Name:	us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_FairValueOffBalanceSheetRisksDisclosureInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount due in settlement of a claim for reimbursement from an insurance company when the Company has suffered a loss covered under an insurance policy. For classified balance sheets, represents the current amount receivable, that is amounts expected to be collected within one year or the normal operating cycle, if longer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.8)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_InsuranceSettlementsReceivableCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_LossContingenciesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 4

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 9

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349

+ Details

Name:	us-gaap_LossContingencyDamagesSoughtValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of pending claims pertaining to a loss contingency.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 4

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349

+ Details

Name:	us-gaap_LossContingencyPendingClaimsNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amounts payable for money transfers, money orders, and consumer payment service arrangements. Settlement liabilities include amounts payable to intermediaries for global payment transfers.

+ References

No definition available.

+ Details

Name:	us-gaap_SettlementLiabilitiesCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommitmentsToExtendCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_StandbyLettersOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_UnusedLinesOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationStatusAxis=us-gaap_PendingLitigationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: