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

Represents the number of parties involved in transaction.

+ References

No definition available.

+ Details

Name:	chmg_NumberOfParty
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=77997519&loc=d3e13433-108611

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

The amount of expense provided in the period for legal costs incurred on or before the balance sheet date pertaining to resolved, pending or threatened litigation, including arbitration and mediation proceedings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.3)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

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

- Definition

Amount awarded to other party in judgment or settlement of litigation.

+ References

No definition available.

+ Details

Name:	us-gaap_LitigationSettlementAmountAwardedToOtherParty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of loss contingency liability.

+ References

+ Details

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

- Definition

Amount of loss contingency liability expected to be resolved after one year or beyond the normal operating cycle, if longer.

+ References

+ Details

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

- Definition

The number of claims settled and dismissed during the period.

+ 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=82911808&loc=d3e14435-108349

+ Details

Name:	us-gaap_LossContingencyClaimsSettledAndDismissedNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
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=82911808&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 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14557-108349

Reference 3: 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=82911808&loc=d3e14435-108349

+ Details

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

- Definition

Amount of insurance proceeds for an event or transaction that is unusual in nature or infrequent in occurrence, or both.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64828382&loc=d3e15138-107781

+ Details

Name:	us-gaap_UnusualOrInfrequentItemInsuranceProceeds
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Details

Name:	chmg_InterestRateTypeAxis=chmg_FixedInterestRateMember
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_FinancialInstrumentAxis=us-gaap_CommitmentsToExtendCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	chmg_InterestRateTypeAxis=chmg_VariableInterestRateMember
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_FinancialInstrumentAxis=us-gaap_StandbyLettersOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

COMMITMENTS AND CONTINGENCIES (Details) $ in Thousands		6 Months Ended	12 Months Ended
COMMITMENTS AND CONTINGENCIES (Details) $ in Thousands	Mar. 23, 2016 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 26, 2016 USD ($) Proceeding Party	Dec. 31, 2015 USD ($)	Jun. 30, 2016 USD ($)	Sep. 30, 2014 USD ($)
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contingent obligations			$ 15,000
Legal accruals and settlements			$ 850	$ 1,200		$ 0
Settled Litigation
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contingent obligations								$ 4,300
Claims settled and dismissed \| Proceeding					2
Number of party \| Party					2
Litigation settlement amount					$ 12,100
Insurance proceeds					$ 7,900
Pending Litigation
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contingent obligations		$ 2,300					$ 1,200
Damages sought	$ 4,000
Loss contingency, accrual, noncurrent		850
Legal accruals and settlements		$ 200
Minimum
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments period to make fixed-rate commercial draw notes			3 months
Minimum \| Fixed Rate
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Interest rate on fixed-rate commitments to make loans			3.375%
Maturity period of fixed-rate commitments to make loans			5 years
Interest rate on fixed-rate commitments to make commercial draw notes (in hundredths)			4.25%
Maximum
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			24 months
Off-balance sheet financial instruments, standard term			12 months
Maximum \| Fixed Rate
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Interest rate on fixed-rate commitments to make loans			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)			5.50%
Commitments to make loans \| Fixed Rate
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contractual amounts of financial instruments with off-balance sheet risk			$ 16,019	38,246
Commitments to make loans \| Variable Rate
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contractual amounts of financial instruments with off-balance sheet risk			28,591	33,189
Unused lines of credit \| Fixed Rate
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contractual amounts of financial instruments with off-balance sheet risk			1,604	610
Unused lines of credit \| Variable Rate
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contractual amounts of financial instruments with off-balance sheet risk			200,353	208,124
Standby letters of credit \| Fixed Rate
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 \| Variable Rate
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contractual amounts of financial instruments with off-balance sheet risk			$ 15,022	$ 14,241