The amount of identifiable net tangible assets acquired at the acquisition date.

No definition available.

Name:	cbu_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedTangibleAssetsNet
Namespace Prefix:	cbu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Refers to the factor of straight-line rate of depreciation used in declining balance depreciation method.

+ References

No definition available.

+ Details

Name:	cbu_DecliningBalanceDepreciationMethodFactorOfStraightLineRateOfDepreciation
Namespace Prefix:	cbu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Refers to the number of branch locations added upon merger.

+ References

No definition available.

+ Details

Name:	cbu_NumberOfBranchLocationsAddedUponMerger
Namespace Prefix:	cbu_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares of equity interests issued or issuable to acquire entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)(4)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

+ Details

Name:	us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=109124213&loc=d3e6613-128477

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=109124213&loc=d3e6578-128477

+ Details

Name:	us-gaap_BusinessCombinationConsiderationTransferred1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Amount of assets acquired at the acquisition date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84161108&loc=d3e4845-128472

+ Details

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

- Definition

Amount of deferred tax liability attributable to taxable temporary differences assumed at the acquisition date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84176650&loc=d3e31917-109318

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84161108&loc=d3e4845-128472

+ Details

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

- Definition

The amount of identifiable intangible assets recognized as of the acquisition date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 55
-Paragraph 37
-URI http://asc.fasb.org/extlink&oid=108330185&loc=d3e2207-128464

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84161108&loc=d3e4845-128472

+ Details

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

- Definition

Amount of other liabilities due after one year or the normal operating cycle, if longer, assumed at the acquisition date.

+ References

+ Details

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

- Definition

Remaining amortization period of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

+ Details

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

- Definition

Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 16
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e961-128460

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

+ Details

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

- Definition

Amount of addition (reduction) to the amount at which a liability could be incurred (settled) in a current transaction between willing parties.

+ References

No definition available.

+ Details

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

- Definition

Generally recurring costs associated with normal operations except for the portion of these expenses which can be clearly related to production and included in cost of sales or services. Includes selling, general and administrative expense.

+ References

No definition available.

+ Details

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

- Definition

The cash outflow associated with the acquisition of a business, net of the cash acquired from the purchase.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3213-108585

+ Details

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

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8933-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

+ Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=cbu_HRConsultantsSALLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CustomerListsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=cbu_PennaAssociatesAgencyIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=cbu_StylesBridgesAssociatesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=cbu_GordonBRobertsAgencyIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_OtherIntangibleAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=cbu_NortheastCapitalManagementIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=cbu_MerchantsBancsharesIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_VT
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_MA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=cbu_DryfoosInsuranceAgencyIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=cbu_BenefitsAdvisoryServiceIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=cbu_NortheastRetirementServicesIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

ACQUISITIONS (Details) shares in Thousands, $ in Thousands									3 Months Ended				9 Months Ended
ACQUISITIONS (Details) shares in Thousands, $ in Thousands	Apr. 02, 2018 USD ($)	Jan. 02, 2018 USD ($)	Dec. 04, 2017 USD ($) shares	Nov. 17, 2017 USD ($)	May 12, 2017 USD ($) Branch Location shares	Mar. 01, 2017 USD ($)	Feb. 03, 2017 USD ($)	Jan. 01, 2017 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jan. 02, 2017 USD ($)
Acquisitions [Abstract]
Cash													$ 1,737	$ 105,402
Goodwill									$ 733,479			$ 731,505	733,479	731,505	$ 734,430
HR Consultants (SA), LLC [Member]
Acquisitions [Abstract]
Cash	$ 300
HR Consultants (SA), LLC [Member] \| Customer Lists [Member]
Acquisitions [Abstract]
Intangibles	$ 300
Penna & Associates Agency, Inc. [Member]
Acquisitions [Abstract]
Cash		$ 800
Goodwill		300
Penna & Associates Agency, Inc. [Member] \| Customer Lists [Member]
Acquisitions [Abstract]
Intangibles		300
Styles Bridges Associates [Member]
Acquisitions [Abstract]
Cash		700
Styles Bridges Associates [Member] \| Customer Lists [Member]
Acquisitions [Abstract]
Intangibles		$ 700
Gordon B. Roberts Agency, Inc [Member]
Acquisitions [Abstract]
Purchase price of acquisition			$ 3,700
Cash			$ 1,350
Issuance of common shares (in shares) \| shares			40
Other liabilities			$ 600
Goodwill			2,100
Assets acquired			600
Increase (decrease) in other liabilities as a result of reclassification and adjustment										$ (90)
Increase (decrease) in goodwill as a result of fair value adjustment										$ (90)
Gordon B. Roberts Agency, Inc [Member] \| Other Intangibles [Member]
Acquisitions [Abstract]
Intangibles			$ 1,600
Northeast Capital Management, Inc [Member]
Acquisitions [Abstract]
Cash				$ 1,200
Northeast Capital Management, Inc [Member] \| Customer Lists [Member]
Acquisitions [Abstract]
Intangibles				$ 1,200
Merchants Bancshares, Inc. [Member]
Acquisitions [Abstract]
Purchase price of acquisition					$ 345,152
Cash					$ 82,898
Issuance of common shares (in shares) \| shares					4,680
Other liabilities					$ 11,750
Goodwill					189,046
Assets acquired					$ 2,000,000
Revenues earned									14,700			16,800	46,800	25,800
Expenses incurred									7,400			7,500	22,700	11,500
Merchants Bancshares, Inc. [Member] \| VERMONT [Member]
Acquisitions [Abstract]
Number of branch locations added upon merger \| Branch					31
Merchants Bancshares, Inc. [Member] \| Massachusetts [Member]
Acquisitions [Abstract]
Number of branch locations added upon merger \| Location					1
Merchants Bancshares, Inc. [Member] \| Other Intangibles [Member]
Acquisitions [Abstract]
Intangibles					$ 2,857
Dryfoos [Member]
Acquisitions [Abstract]
Cash						$ 3,000
Goodwill						1,700
Dryfoos [Member] \| Other Intangibles [Member]
Acquisitions [Abstract]
Intangibles						$ 1,700
BAS [Member]
Acquisitions [Abstract]
Cash								$ 1,200
BAS [Member] \| Other Intangibles [Member]
Acquisitions [Abstract]
Intangibles																$ 1,200
Northeast Retirement Services, Inc. [Member]
Acquisitions [Abstract]
Purchase price of acquisition							$ 148,556
Cash							70,073
Other liabilities							26,828
Goodwill							75,256
Tangible assets, net							$ 36,100
Factor of straight-line rate of depreciation used in declining balance depreciation method							150.00%
Period of amortization							10 years
Deferred tax liability							$ 23,000
Revenues earned									10,300			8,700	30,100	22,100
Expenses incurred									$ 6,100			$ 5,800	$ 18,100	$ 15,100
Increase (decrease) in other liabilities as a result of reclassification and adjustment											$ (1,200)
Increase (decrease) in goodwill as a result of fair value adjustment											$ (1,200)
Northeast Retirement Services, Inc. [Member] \| Other Intangibles [Member]
Acquisitions [Abstract]
Intangibles							$ 60,200