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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Postretirement Benefits (Details) | 12 Months Ended
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Dec. 31, 2015
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] |
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%
Contribution cost percentage | 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

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- Definition Percentage of contribution cost to be made by retirees, who are going to be continued under the coverage of plan. + References No definition available. + Details
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Period Type: | duration

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- Definition Percentage of contribution made under self-insure health care plan, before amendment. + References No definition available. + Details
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Namespace Prefix: | chmg_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- 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

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- Definition The period for covered compensation, which is average of social security taxable wage bases in effect. + References No definition available. + Details
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- 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
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- 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
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Period Type: | duration

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- Definition Maximum period of credited service considered to apply excess of covered compensation for each year of credited service. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Refers to maximum period considered in final average compensation. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Minimum age for eligibility for coverage under self-insure health care plan, before amendment. + References No definition available. + Details
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Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- 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

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- Definition Percentage of normal retirement benefit in terms of final average compensation resulting from maximum period of 25 years. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Refers consecutive period considered in final average compensation. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Refers to prior period considered in consecutive period of final average compensation. + References No definition available. + Details
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Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- 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
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Period Type: | duration

X
- Definition Percentage of premium contribution made under self-insure health care plan. + References No definition available. + Details
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Namespace Prefix: | chmg_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Eligible age of current and future retirees to be continued under coverage of plan. + References No definition available. + Details
Name: | chmg_RetireeAgeGroupRange
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Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage of premium to be paid by retiree under self-insure health care plan. + References No definition available. + Details
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Namespace Prefix: | chmg_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- 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
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- Details
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- Details
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- Details
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