FAIR VALUE, Significant Unobservable Inputs (Details)	12 Months Ended
Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Fair value of MSRs at end of period	$ 1,362,000	$ 1,384,000
Significant Unobservable Inputs, Level 3 [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Impaired loans	848,000	25,063,000
Other real estate owned	1,270,000	883,000
Fair value of MSRs at end of period	$ 56,000	$ 682,000
Significant Unobservable Inputs, Level 3 [Member] \| Weighted Average Discount Rate [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Mortgage servicing rights, Measurement Input [Extensible List]	us-gaap:MeasurementInputDiscountRateMember	us-gaap:MeasurementInputDiscountRateMember
Significant Unobservable Inputs, Level 3 [Member] \| Adequate Compensation [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Mortgage servicing rights, Measurement Input [Extensible List]	cbu:MeasurementInputAdequateCompensationMember	cbu:MeasurementInputAdequateCompensationMember
Significant Unobservable Inputs, Level 3 [Member] \| Fair Value of Collateral [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Impaired loans, Valuation Technique [Extensible List]		cbu:ValuationTechniqueFairValueOfCollateralMember
Other real estate owned, Valuation Technique [Extensible List]		cbu:ValuationTechniqueFairValueOfCollateralMember
Significant Unobservable Inputs, Level 3 [Member] \| Fair Value of Collateral [Member] \| Estimated Cost of Disposal/Market Adjustment [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Impaired loans, Valuation Technique [Extensible List]	cbu:ValuationTechniqueFairValueOfCollateralMember
Impaired loans, Measurement Input [Extensible List]	cbu:MeasurementInputEstimatedCostOfDisposalOrMarketAdjustmentMember	cbu:MeasurementInputEstimatedCostOfDisposalOrMarketAdjustmentMember
Other real estate owned, Valuation Technique [Extensible List]	cbu:ValuationTechniqueFairValueOfCollateralMember
Other real estate owned, Measurement Input [Extensible List]	cbu:MeasurementInputEstimatedCostOfDisposalOrMarketAdjustmentMember	us-gaap:MeasurementInputCostToSellMember
Other real estate owned, measurement input	9.0
Significant Unobservable Inputs, Level 3 [Member] \| Discounted Cash Flow [Member] \| Weighted Average Constant Prepayment Rate [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Mortgage servicing rights, measurement input	52.8
Mortgage servicing rights, Valuation Technique [Extensible List]	us-gaap:ValuationTechniqueDiscountedCashFlowMember	us-gaap:ValuationTechniqueDiscountedCashFlowMember
Mortgage servicing rights, Measurement Input [Extensible List]	us-gaap:MeasurementInputConstantPrepaymentRateMember	us-gaap:MeasurementInputConstantPrepaymentRateMember
Significant Unobservable Inputs, Level 3 [Member] \| Discounted Cash Flow [Member] \| Weighted Average Discount Rate [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Mortgage servicing rights, measurement input	3.00
Significant Unobservable Inputs, Level 3 [Member] \| Discounted Cash Flow [Member] \| Adequate Compensation [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Mortgage servicing rights, measurement input		7
Mortgage servicing rights, measurement input	7,000
Significant Unobservable Inputs, Level 3 [Member] \| Minimum [Member] \| Fair Value of Collateral [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Impaired loans, measurement input		9.0
Significant Unobservable Inputs, Level 3 [Member] \| Minimum [Member] \| Fair Value of Collateral [Member] \| Estimated Cost of Disposal/Market Adjustment [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Impaired loans, measurement input	9.0
Other real estate owned, measurement input		11.3
Significant Unobservable Inputs, Level 3 [Member] \| Minimum [Member] \| Discounted Cash Flow [Member] \| Weighted Average Constant Prepayment Rate [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Mortgage servicing rights, measurement input		9.8
Significant Unobservable Inputs, Level 3 [Member] \| Minimum [Member] \| Discounted Cash Flow [Member] \| Weighted Average Discount Rate [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Mortgage servicing rights, measurement input		1.7
Significant Unobservable Inputs, Level 3 [Member] \| Maximum [Member] \| Fair Value of Collateral [Member] \| Estimated Cost of Disposal/Market Adjustment [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Impaired loans, measurement input	35.0	78.4
Other real estate owned, measurement input	85.7	52.9
Significant Unobservable Inputs, Level 3 [Member] \| Maximum [Member] \| Discounted Cash Flow [Member] \| Weighted Average Constant Prepayment Rate [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Mortgage servicing rights, measurement input		18.8
Significant Unobservable Inputs, Level 3 [Member] \| Maximum [Member] \| Discounted Cash Flow [Member] \| Weighted Average Discount Rate [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Mortgage servicing rights, measurement input		2.2
Significant Unobservable Inputs, Level 3 [Member] \| Weighted Average [Member] \| Fair Value of Collateral [Member] \| Estimated Cost of Disposal/Market Adjustment [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Impaired loans, measurement input	(27.9)	(52.7)
Other real estate owned, measurement input	(37.1)	(34.0)
Significant Unobservable Inputs, Level 3 [Member] \| Weighted Average [Member] \| Discounted Cash Flow [Member] \| Weighted Average Constant Prepayment Rate [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Mortgage servicing rights, measurement input		(17.6)
Significant Unobservable Inputs, Level 3 [Member] \| Weighted Average [Member] \| Discounted Cash Flow [Member] \| Weighted Average Discount Rate [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Mortgage servicing rights, measurement input		(2.1)
Commitments to Originate Real Estate Loans for Sale [Member] \| Significant Unobservable Inputs, Level 3 [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Commitments to originate real estate loans for sale		$ 14,000
Commitments to originate real estate loans for sale, measurement input		1.0
Commitments to Originate Real Estate Loans for Sale [Member] \| Significant Unobservable Inputs, Level 3 [Member] \| Discounted Cash Flow [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Commitments to originate real estate loans for sale, Valuation Technique [Extensible List]		us-gaap:ValuationTechniqueDiscountedCashFlowMember
Commitments to Originate Real Estate Loans for Sale [Member] \| Significant Unobservable Inputs, Level 3 [Member] \| Discounted Cash Flow [Member] \| Embedded Servicing Value [Member]
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Commitments to originate real estate loans for sale, Measurement Input [Extensible List]		cbu:MeasurementInputEmbeddedServicingValueMember

This is the amount of impaired loans carried at fair value.

No definition available.

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- Definition

Value of input used to measure impaired loans.

+ References

No definition available.

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- Definition

Value of input used to measure servicing asset.

+ References

No definition available.

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- Definition

Value of input used to measure derivative asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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Indicates measurement input for derivative asset.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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- Definition

Indicates valuation technique for measuring derivative asset.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
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-Publisher FASB
-Name Accounting Standards Codification
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- References

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- Definition

Indicates measurement input for loan held for sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Indicates valuation technique for measuring loan held for sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

Carrying amount of other real estate not separately disclosed, including, but not limited to, real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. This does not include real estate assets taken in settlement of troubled loans through surrender or foreclosure.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
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Value of input used to measure real estate owned by financial institution by financial institution which is not directly related to its business.

+ References

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Indicates measurement input for real estate owned by financial institution which is not directly related to its business.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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-Section 50
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Indicates valuation technique for measuring real estate owned by financial institution which is not directly related to its business.

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- Definition

Fair value of contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Value of input used to measure servicing asset.

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Indicates measurement input for servicing asset.

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-Name Accounting Standards Codification
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Indicates valuation technique for measuring servicing asset.

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