FAIR VALUE - Significant Unobservable Inputs (Details) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract]
Impaired loans	$ 6,808	$ 848
Other real estate owned	$ 1,209	$ 1,270
Other real estate owned, Valuation Technique [Extensible List]	us-gaap:MarketApproachValuationTechniqueMember	us-gaap:MarketApproachValuationTechniqueMember
Other real estate owned, Measurement Input [Extensible List]	us-gaap:MeasurementInputCostToSellMember	us-gaap:MeasurementInputCostToSellMember
Mortgage servicing rights	$ 460	$ 56
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		0.528
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		0.0300
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	7
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	0.090	0.090
Other real estate owned, measurement input	0.152	0.090
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	0.138
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	0.0122
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	0.787	0.350
Other real estate owned, measurement input	0.535	0.857
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	0.147
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	0.0198
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	0.588	0.279
Other real estate owned, measurement input	0.313	0.371
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	0.146
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	0.0192
Commitments to Originate Real Estate Loans for Sale [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	$ 2,684
Commitments to originate real estate loans for sale, Valuation Technique [Extensible List]	us-gaap:ValuationTechniqueDiscountedCashFlowMember
Commitments to originate real estate loans for sale, Measurement Input [Extensible List]	cbu:MeasurementInputEmbeddedServicingValueMember
Commitments to originate real estate loans for sale, measurement input	0.010

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.

+ Details

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Namespace Prefix:	cbu_
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Balance Type:	na
Period Type:	instant

- Definition

Value of input used to measure servicing asset.

+ References

No definition available.

+ Details

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Namespace Prefix:	cbu_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- 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

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

Indicates measurement input for 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)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	instant

- Definition

Indicates valuation technique for measuring 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)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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Namespace Prefix:	us-gaap_
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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
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

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-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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

- References

No definition available.

+ Details

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

- 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
-Subparagraph (SX 210.9-03.10(4))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

- Definition

Value of input used to measure real estate owned by financial institution by financial institution which is not directly related to its business.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

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
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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Namespace Prefix:	us-gaap_
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Period Type:	instant

- Definition

Indicates valuation technique for measuring 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
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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Namespace Prefix:	us-gaap_
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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
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122739-111746

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-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=7516967&loc=d3e66267-113978

+ Details

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Balance Type:	debit
Period Type:	instant

- Definition

Value of input used to measure servicing asset.

+ References

+ Details

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Data Type:	xbrli:decimalItemType
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Period Type:	instant

- Details

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- Details

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