LOANS AND ALLOWANCE FOR LOAN LOSSES - Loan Portfolio (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Composition of loan portfolio [Abstract]
Loans, net of deferred loan fees	$ 1,536,463	$ 1,309,219
Interest receivable on loans	5,035	3,684
Total recorded investment in loans	1,541,498	1,312,903
Residential mortgages held for sale	170	1,185
Commercial, and Agricultural
Composition of loan portfolio [Abstract]
Total recorded investment in loans	370,252	230,922
Commercial mortgages
Composition of loan portfolio [Abstract]
Total recorded investment in loans	719,145	650,570
Commercial mortgages \| Construction
Composition of loan portfolio [Abstract]
Loans, net of deferred loan fees	61,945	43,962
Total recorded investment in loans	62,164	44,082
Commercial mortgages \| Commercial mortgages
Composition of loan portfolio [Abstract]
Loans, net of deferred loan fees	654,663	604,832
Total recorded investment in loans	656,981	606,488
Residential Mortgages
Composition of loan portfolio [Abstract]
Loans, net of deferred loan fees	239,401	188,338
Total recorded investment in loans	240,013	188,874
Loans pledged as collateral	101,900	170,000
Consumer Loans
Composition of loan portfolio [Abstract]
Total recorded investment in loans	212,088	242,537
Consumer Loans \| Home equity lines and loans
Composition of loan portfolio [Abstract]
Loans, net of deferred loan fees	78,547	91,784
Total recorded investment in loans	78,761	92,065
Consumer Loans \| Indirect Consumer Loans
Composition of loan portfolio [Abstract]
Loans, net of deferred loan fees	120,538	134,973
Total recorded investment in loans	120,853	135,366
Consumer Loans \| Direct consumer loans
Composition of loan portfolio [Abstract]
Loans, net of deferred loan fees	12,423	15,038
Total recorded investment in loans	12,474	15,106
Commercial and industrial
Composition of loan portfolio [Abstract]
Payroll Protection Program Loans - CARES Act	150,900
Commercial and industrial \| Commercial, and Agricultural
Composition of loan portfolio [Abstract]
Loans, net of deferred loan fees	368,663	230,018
Total recorded investment in loans	369,968	230,648
Agricultural \| Commercial, and Agricultural
Composition of loan portfolio [Abstract]
Loans, net of deferred loan fees	283	274
Total recorded investment in loans	$ 284	$ 274

Financing Receivable, after Interest Receivable on Loans, before Allowance on Credit Loss

No definition available.

Name:	chmg_FinancingReceivableAfterInterestReceivableOnLoansBeforeAllowanceOnCreditLoss
Namespace Prefix:	chmg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing Receivable, Payroll Protection Program Loans - CARES Act

+ References

No definition available.

+ Details

Name:	chmg_FinancingReceivablePayrollProtectionProgramLoansCARESAct
Namespace Prefix:	chmg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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_InterestReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Carrying amount as of the balance sheet date of loans that have been pledged as collateral for borrowings. If the secured party has the right by contract or custom to sell or re-pledge the collateral, then the debtor reclassifies that asset and report that asset in its statement of financial position separately from other assets not so encumbered.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5086-111524

+ Details

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

- Definition

Amount, after allowance for bad debt, of loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118952595&loc=d3e4428-111522

+ Details

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

- Definition

Amount, before allowance for credit loss, of financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919230-210447

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=chmg_CommercialAndAgriculturalPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=chmg_CommercialMortgagesOtherLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=chmg_IndirectConsumerLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=chmg_DirectConsumerLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EquitySecuritiesByIndustryAxis=us-gaap_CommercialAndIndustrialSectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EquitySecuritiesByIndustryAxis=us-gaap_AgriculturalSectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: