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FAIR VALUE, Unobservable Input Reconciliation (Details) (Commitments to Originate Real Estate Loans for Sale [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Commitments to Originate Real Estate Loans for Sale [Member]
Level 3 Assets Measured at Fair Value on a Recurring Basis [Abstract]
Beginning balance	$ 185us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = cbu_RealEstateLoanOriginationAndSaleCommitmentsMember		$ 44us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = cbu_RealEstateLoanOriginationAndSaleCommitmentsMember
Total (losses)/gains included in earnings	(185)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByAssetClassAxis = cbu_RealEstateLoanOriginationAndSaleCommitmentsMember	[1]	(44)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByAssetClassAxis = cbu_RealEstateLoanOriginationAndSaleCommitmentsMember	[1]
Commitments to originate real estate loans held for sale, net	152cbu_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLoanOriginationCommitmentsRealEstate / us-gaap_FairValueByAssetClassAxis = cbu_RealEstateLoanOriginationAndSaleCommitmentsMember		67cbu_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLoanOriginationCommitmentsRealEstate / us-gaap_FairValueByAssetClassAxis = cbu_RealEstateLoanOriginationAndSaleCommitmentsMember
Ending balance	$ 152us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = cbu_RealEstateLoanOriginationAndSaleCommitmentsMember		$ 67us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = cbu_RealEstateLoanOriginationAndSaleCommitmentsMember
[1] Amounts included in earnings associated with the commitments to originate real estate loans for sale are reported as a component of other banking services in the Consolidated Statement of Income.

X

- Definition

Commitments to borrowers or third party investors for the origination of real estate loans for sale classified as an asset measured and using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

No definition available.

+ Details

Name:	cbu_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLoanOriginationCommitmentsRealEstate
Namespace Prefix:	cbu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

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

X

- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

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

X

- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

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