No definition available.

Name:	chmg_FinancingReceivableAllowanceForCreditLossesIndividuallyAndCollectivelyEvaluatedForImpairmentAbstract
Namespace Prefix:	chmg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of recovery of (write-downs of) financing receivables doubtful of collection.

+ References

No definition available.

+ Details

Name:	chmg_FinancingReceivableAllowanceForCreditLossesNetChargeOff
Namespace Prefix:	chmg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

Name:	chmg_FinancingReceivableAllowanceForFinanceReceivableEvaluatedForImpairmentAbstract
Namespace Prefix:	chmg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

A valuation allowance for financing receivables that are expected to be uncollectible.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

+ Details

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

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

+ Details

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

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of recovery of financing receivables doubtful of collection that were previously charged off.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(4)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of direct write-downs of financing receivables charged against the allowance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

+ Details

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

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

+ Details

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

- 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

- Definition

Amount of expense related to estimated loss from loan and lease transactions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_UnallocatedFinancingReceivablesMember
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_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

LOANS AND ALLOWANCE FOR LOAN LOSSES - Allowances (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Allowance for loan losses, by portfolio segment [Roll Forward]
Beginning balance:	$ 13,686	$ 12,776	$ 10,433
Charge Offs	(1,631)	(4,278)	(1,493)
Recoveries	634	1,207	1,081
Net (charge offs) recoveries	(997)	(3,071)	(412)
Provision	1,571	3,981	2,755
Ending balance	14,260	13,686	12,776
Ending allowance balance attributable to loans:
Individually evaluated for impairment				$ 1,566	$ 1,235
Collectively evaluated for impairment				12,595	12,357
Allowance for loan losses	13,686	12,776	10,433	14,260	13,686
Loans:
Loans individually evaluated for impairment				14,980	15,904
Loans collectively evaluated for impairment				1,154,428	1,105,819
Total ending loans balance				1,171,503	1,124,354
Commercial, and Agricultural
Allowance for loan losses, by portfolio segment [Roll Forward]
Beginning balance:	1,460	1,979	1,708
Charge Offs	(186)	(444)	(186)
Recoveries	96	385	537
Net (charge offs) recoveries	(90)	(59)	351
Provision	461	(460)	(80)
Ending balance	1,831	1,460	1,979
Ending allowance balance attributable to loans:
Allowance for loan losses	1,460	1,979	1,708	1,831	1,460
Commercial Mortgages
Allowance for loan losses, by portfolio segment [Roll Forward]
Beginning balance:	6,326	6,243	4,428
Charge Offs	(104)	(2,229)	(44)
Recoveries	131	156	98
Net (charge offs) recoveries	27	(2,073)	54
Provision	759	2,156	1,761
Ending balance	7,112	6,326	6,243
Ending allowance balance attributable to loans:
Allowance for loan losses	6,326	6,243	4,428	7,112	6,326
Residential mortgages
Allowance for loan losses, by portfolio segment [Roll Forward]
Beginning balance:	1,572	1,517	1,565
Charge Offs	(47)	(97)	(124)
Recoveries	0	32	65
Net (charge offs) recoveries	(47)	(65)	(59)
Provision	(61)	120	11
Ending balance	1,464	1,572	1,517
Ending allowance balance attributable to loans:
Allowance for loan losses	1,572	1,517	1,565	1,464	1,572
Loans:
Total ending loans balance				196,236	197,287
Consumer Loans
Allowance for loan losses, by portfolio segment [Roll Forward]
Beginning balance:	4,328	3,037	2,706
Charge Offs	(1,294)	(1,508)	(1,139)
Recoveries	407	634	381
Net (charge offs) recoveries	(887)	(874)	(758)
Provision	412	2,165	1,089
Ending balance	3,853	4,328	3,037
Ending allowance balance attributable to loans:
Allowance for loan losses	4,328	3,037	2,706	3,853	4,328
Loans:
Total ending loans balance				18,682	20,066
Unallocated
Allowance for loan losses, by portfolio segment [Roll Forward]
Beginning balance:	0	0	26
Charge Offs	0	0	0
Recoveries	0	0	0
Net (charge offs) recoveries	0	0	0
Provision	0	0	(26)
Ending balance	0	0	0
Ending allowance balance attributable to loans:
Allowance for loan losses	0	0	$ 26	0	0
Consumer Loans
Allowance for loan losses, by portfolio segment [Roll Forward]
Beginning balance:	4,328
Ending balance	3,853	4,328
Ending allowance balance attributable to loans:
Individually evaluated for impairment				77	1
Collectively evaluated for impairment				3,776	4,327
Allowance for loan losses	4,328	4,328		3,853	4,328
Loans:
Loans individually evaluated for impairment				474	486
Loans collectively evaluated for impairment				273,393	306,042
Total ending loans balance				273,867	306,528
Commercial mortgages:
Allowance for loan losses, by portfolio segment [Roll Forward]
Beginning balance:	6,326
Ending balance	7,112	6,326
Ending allowance balance attributable to loans:
Individually evaluated for impairment				1,481	1,145
Collectively evaluated for impairment				5,572	5,145
Allowance for loan losses	6,326	6,326		7,112	6,326
Loans:
Loans individually evaluated for impairment				12,773	13,712
Loans collectively evaluated for impairment				493,102	438,246
Total ending loans balance				507,700	453,719
Commercial, and Agricultural
Allowance for loan losses, by portfolio segment [Roll Forward]
Beginning balance:	1,460
Ending balance	1,831	1,460
Ending allowance balance attributable to loans:
Individually evaluated for impairment				8	89
Collectively evaluated for impairment				1,823	1,335
Allowance for loan losses	1,460	1,460		1,831	1,460
Loans:
Loans individually evaluated for impairment				1,498	1,452
Loans collectively evaluated for impairment				192,202	164,748
Total ending loans balance				193,700	166,820
Residential mortgages
Allowance for loan losses, by portfolio segment [Roll Forward]
Beginning balance:	1,572
Ending balance	1,464	1,572
Ending allowance balance attributable to loans:
Individually evaluated for impairment				0	0
Collectively evaluated for impairment				1,424	1,550
Allowance for loan losses	1,572	1,572		1,464	1,572
Loans:
Loans individually evaluated for impairment				235	254
Loans collectively evaluated for impairment				195,731	196,783
Total ending loans balance				196,236	197,287
Loans acquired with deteriorated credit quality
Allowance for loan losses, by portfolio segment [Roll Forward]
Beginning balance:	94
Ending balance	99	94
Ending allowance balance attributable to loans:
Allowance for loan losses	94	94		99	94
Loans:
Total ending loans balance				2,095	2,631
Loans acquired with deteriorated credit quality \| Consumer Loans
Allowance for loan losses, by portfolio segment [Roll Forward]
Beginning balance:	0
Ending balance	0	0
Ending allowance balance attributable to loans:
Allowance for loan losses	0	0		0	0
Loans:
Total ending loans balance				0	0
Loans acquired with deteriorated credit quality \| Commercial mortgages:
Allowance for loan losses, by portfolio segment [Roll Forward]
Beginning balance:	36
Ending balance	59	36
Ending allowance balance attributable to loans:
Allowance for loan losses	36	36		59	36
Loans:
Total ending loans balance				1,825	1,761
Loans acquired with deteriorated credit quality \| Commercial, and Agricultural
Allowance for loan losses, by portfolio segment [Roll Forward]
Beginning balance:	36
Ending balance	0	36
Ending allowance balance attributable to loans:
Allowance for loan losses	36	36		0	36
Loans:
Total ending loans balance				0	620
Loans acquired with deteriorated credit quality \| Residential mortgages
Allowance for loan losses, by portfolio segment [Roll Forward]
Beginning balance:	22
Ending balance	40	22
Ending allowance balance attributable to loans:
Allowance for loan losses	$ 22	$ 22		40	22
Loans:
Total ending loans balance				$ 270	$ 250