XML 96 R81.htm IDEA: XBRL DOCUMENT
v3.8.0.1

BENEFIT PLANS, Weighted-average Assumptions Used to Determine Benefit Obligations (Details) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | May 12, 2017
Pension Benefits [Member] |  |  |
Weighed average assumption used in calculating benefit obligations [Abstract] |  |  |
Discount rate | 4.00% | 4.50% | 4.40%
Expected return on plan assets | 7.00% | 7.00% |
Rate of compensation increase | 3.50% | 3.50% |
Post-retirement Benefits [Member] |  |  |
Weighed average assumption used in calculating benefit obligations [Abstract] |  |  |
Discount rate | 4.00% | 4.40% |

X
- Definition An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation. + References No definition available. + Details
Name: | cbu_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationExpectedReturnOnPlanAssets
Namespace Prefix: | cbu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingBenefitObligationAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |