SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Postretirement Benefits (Details)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of annual service hours required for eligibility (in hours)		1000 hours
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Normal retirement benefit as percentage of final average compensation (in hundredths)		1.20%
Number of consecutive years considered in final average compensation		5 years
Number of prior years considered in final average compensation		10 years
Maximum service period		25 years
Normal retirement additional benefit		1.00%
Maximum additional service years		10 years
Percentage of average monthly compensation in excess of covered compensation for each year of credited service		0.65%
Maximum period of credited service		35 years
Covered compensation period		35 years
Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contribution cost percentage		50.00%
Minimum age for eligibility before amendment		55 years
Contribution percentage before amendment		40.00%
Retiree premium percentage		100.00%
Cost of coverage contribution		50.00%
Premium contribution percentage after amendment		50.00%
Minimum \| Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Retiree age group range		55 years
Maximum \| Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Retiree age group range		65 years
Defined Contribution Profit Sharing, Savings and Investment Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of annual service hours required for eligibility (in hours)		1000 hours
Defined Contribution Profit Sharing, Savings and Investment Plan \| Subsequent Event
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined contribution plan, employer non-discretionary contribution amount	3.00%
Contribution cost percentage	50.00%
Defined contribution plan, employer matching contribution, percent of employees' gross pay	6.00%

Percentage of contribution cost to be made by retirees, who are going to be continued under the coverage of plan.

No definition available.

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

- Definition

Percentage of contribution made under self-insure health care plan, before amendment.

+ References

No definition available.

+ Details

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

- Definition

Percentage of contribution made under self-insure health care plan.

+ References

No definition available.

+ Details

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

- Definition

The period for covered compensation, which is average of social security taxable wage bases in effect.

+ References

No definition available.

+ Details

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

- Definition

Defined Contribution Plan, Employer Non-Discretionary Contribution Amount

+ References

No definition available.

+ Details

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

- Definition

Number of annual service hours for employees to be eligible to participate in the defined contribution plan or plans described.

+ References

No definition available.

+ Details

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

- Definition

Refers to maximum additional period considered in final average compensation, during which additional 1% of average monthly compensation for each additional year of service is added.

+ References

No definition available.

+ Details

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

- Definition

Maximum period of credited service considered to apply excess of covered compensation for each year of credited service.

+ References

No definition available.

+ Details

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

- Definition

Refers to maximum period considered in final average compensation.

+ References

No definition available.

+ Details

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

- Definition

Minimum age for eligibility for coverage under self-insure health care plan, before amendment.

+ References

No definition available.

+ Details

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

- Definition

Percentage of normal retirement benefit in terms of average monthly compensation resulting for each additional year.

+ References

No definition available.

+ Details

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

- Definition

Percentage of normal retirement benefit in terms of final average compensation resulting from maximum period of 25 years.

+ References

No definition available.

+ Details

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

- Definition

Refers consecutive period considered in final average compensation.

+ References

No definition available.

+ Details

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

- Definition

Refers to prior period considered in consecutive period of final average compensation.

+ References

No definition available.

+ Details

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

- Definition

Refers to the percentage of average monthly compensation in excess of covered compensation for each year of credited service.

+ References

No definition available.

+ Details

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

- Definition

Percentage of premium contribution made under self-insure health care plan.

+ References

No definition available.

+ Details

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

- Definition

Eligible age of current and future retirees to be continued under coverage of plan.

+ References

No definition available.

+ Details

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

- Definition

Percentage of premium to be paid by retiree under self-insure health care plan.

+ References

No definition available.

+ Details

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

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

- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

Name:	us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DefinedContributionPlanNameAxis=chmg_DefinedContributionProfitSharingSavingsAndInvestmentPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: