The Tax Identification Number (TIN), also known as an Employer Identification Number (EIN), is a unique 9-digit value assigned by the IRS.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation 12B
-Number 240
-Section 12b
-Subsection 1

+ Details

Name:	dei_EntityTaxIdentificationNumber
Namespace Prefix:	dei_
Data Type:	dei:nineDigitItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of contributions made to multiemployer plan by employer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450702-114947

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 5
-Subparagraph (e)(1)
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450657-114947

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450691-114947

+ Details

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

- Definition

Three digit plan number for a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450657-114947

+ Details

Name:	us-gaap_MultiemployerPlanNumber
Namespace Prefix:	us-gaap_
Data Type:	us-types:threeDigitItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of accumulated benefit obligation attributable to the pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 7
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450678-114947

+ Details

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

- Definition

Indicate whether the multiemployer plan is less than 65 percent funded, between 65 and less than 80 percent funded, or at least 80 percent funded. The acceptable values are: (1) "less than 65 percent", (2) "between 65 and less than 80 percent", (3) "at least 80 percent", and (4) "NA". A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450657-114947

+ Details

Name:	us-gaap_MultiemployerPlansFundedStatus
Namespace Prefix:	us-gaap_
Data Type:	us-types:fundedStatusItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Indicates (true/false) whether employer contributions represent more than five percent of total contributions to multiemployer plan for most recent plan year available.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 5
-Subparagraph (e)(2)
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450657-114947

+ Details

Name:	us-gaap_MultiemployerPlansPeriodContributionsSignificanceOfContributions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of plan assets attributable to the pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

+ References

+ Details

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

- Details

Name:	us-gaap_MultiemployerPlanTypeAxis=us-gaap_MultiemployerPlansPensionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MultiemployerPlanNameAxis=cbu_PentegraDbPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=cbu_TupperLakeNationalBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

BENEFIT PLANS, Multi-employer Defined Benefit Plan (Details) - Multiemployer Plans, Pension [Member] - Pentegra DB Plan [Member] - Tupper Lake National Bank [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 30, 2016
Multiemployer Plans [Line Items]
Identification number	135645888
Plan number	333
Fair value of plan assets				$ 3,300,000
Projected benefit obligation				$ 4,800,000
Funded status	At least 80 percent
Period contributions	$ 100	$ 50	$ 30
Significance of employer contributions	false