LOANS AND ALLOWANCE FOR LOAN LOSSES (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Composition of loan portfolio [Abstract]
Total loans, net of deferred origination fees and costs	$ 1,216,566	$ 1,168,633
Interest receivable on loans	2,912	2,870
Total recorded investment in loans	1,219,478	1,171,503
Commercial and Agricultural [Member]
Composition of loan portfolio [Abstract]
Total recorded investment in loans	184,348	193,700
Commercial Mortgages [Member]
Composition of loan portfolio [Abstract]
Total recorded investment in loans	577,118	507,700
Commercial Mortgages [Member] \| Construction [Member]
Composition of loan portfolio [Abstract]
Total loans, net of deferred origination fees and costs	35,051	41,131
Total recorded investment in loans	35,134	41,231
Commercial Mortgages [Member] \| Commercial Mortgages Other [Member]
Composition of loan portfolio [Abstract]
Total loans, net of deferred origination fees and costs	540,710	465,347
Total recorded investment in loans	541,984	466,469
Residential Mortgages [Member]
Composition of loan portfolio [Abstract]
Total loans, net of deferred origination fees and costs	197,665	195,778
Total recorded investment in loans	198,142	196,236
Consumer Loans [Member]
Composition of loan portfolio [Abstract]
Total recorded investment in loans	259,870	273,867
Consumer Loans [Member] \| Credit Cards [Member]
Composition of loan portfolio [Abstract]
Total loans, net of deferred origination fees and costs	1,352	1,483
Total recorded investment in loans	1,352	1,482
Consumer Loans [Member] \| Home Equity Lines and Loans [Member]
Composition of loan portfolio [Abstract]
Total loans, net of deferred origination fees and costs	98,378	101,726
Total recorded investment in loans	98,629	101,977
Consumer Loans [Member] \| Indirect Consumer Loans [Member]
Composition of loan portfolio [Abstract]
Total loans, net of deferred origination fees and costs	141,489	151,327
Total recorded investment in loans	141,817	151,726
Consumer Loans [Member] \| Direct Consumer Loans [Member]
Composition of loan portfolio [Abstract]
Total loans, net of deferred origination fees and costs	18,007	18,608
Total recorded investment in loans	18,072	18,682
Commercial and Industrial Sector [Member] \| Commercial and Agricultural [Member]
Composition of loan portfolio [Abstract]
Total loans, net of deferred origination fees and costs	183,508	192,197
Total recorded investment in loans	183,941	192,661
Agricultural Sector [Member] \| Commercial and Agricultural [Member]
Composition of loan portfolio [Abstract]
Total loans, net of deferred origination fees and costs	406	1,036
Total recorded investment in loans	$ 407	$ 1,039

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

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

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=65884687&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5074-111524

+ Details

Name:	us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
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

Net amount of the investment in a contractual right to receive money on demand or on fixed or determinable dates that is recognized as an asset in the creditor's statement of financial position. Examples include, but are not limited to, credit card receivables, notes receivable and receivables relating to lessor's rights to payments from leases other than operating leases that have been recorded as assets. Excludes trade accounts receivable with contractual maturity of one year or less and arose from the sale of goods or services.

+ 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 210.5-02.3)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_NotesReceivableNet
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_CreditCardReceivablesMember
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: