Name:	cbu_AllowanceForLoanLossesAbstract
Namespace Prefix:	cbu_
Data Type:	xbrli:stringItemType
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Period Type:	duration

The minimum threshold amount of loans which are individually evaluated for impairment.

No definition available.

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

Refers to the number of broad components used in the entity's allowance methodology.

No definition available.

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

Refers to the number of calculations used for general loan loss allocation.

No definition available.

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

The number of counties where the bank operates or has branches in New York.

No definition available.

Name:	cbu_NumberOfCountiesBankHasFacilities
Namespace Prefix:	cbu_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

The number of counties where the bank operates or has branches in Pennsylvania.

No definition available.

Name:	cbu_NumberOfCountiesInPennsylvaniaWhereBankHasFacilities
Namespace Prefix:	cbu_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

The number of customer locations or branches operated by the bank.

No definition available.

Name:	cbu_NumberOfCustomerFacilities
Namespace Prefix:	cbu_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Refers to the number of days past due after which loans are charged off to the extent outstanding principal balance exceeds fair value of collateral/property, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	cbu_NumberOfDaysPastDueAfterWhichLoansAreChargedOffToTheExtentOutstandingPrincipalBalanceExceedsFairValueOfCollateral
Namespace Prefix:	cbu_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

The number of days according to the entity's policy that a loan must be past due before being placed on nonaccrual status, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	cbu_NumberOfDaysPastDueForLoansToBePlacedOnNonaccrualStatus
Namespace Prefix:	cbu_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Refers to the number of main components of economic risk.

No definition available.

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

Refers to the number of main loan segments of the entity.

No definition available.

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

Refers to the number of qualitative environmental factors used in qualitative calculation for loan loss allocation.

No definition available.

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

Refers to the number of consolidated subsidiary owned by the entity.

No definition available.

Name:	cbu_NumberOfSignificantConsolidatedSubsidiariesOwned
Namespace Prefix:	cbu_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

The number of unconsolidated subsidiary trust created for the issuance of debt. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer.

No definition available.

Name:	cbu_NumberOfUnconsolidatedSubsidiaryBusinessTrustsOwned
Namespace Prefix:	cbu_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Refers to the period for which a nonaccrual TDR loan remains on a nonaccrual status months to demonstrate that the borrower is able to meet the terms of the modified loan, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	cbu_PeriodForWhichNonaccrualTDRLoanRemainsOnNonaccrualStatus
Namespace Prefix:	cbu_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Refers to the period of historical net charge-off data for each loan class used in determining allowance level, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	cbu_PeriodOfHistoricalNetChargeOffDataForEachLoanClassUsedInDeterminingAllowanceLevel
Namespace Prefix:	cbu_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Amount charged to advertising expense for the period, which are expenses incurred with the objective of increasing revenue for a specified brand, product or product line.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 720
-SubTopic 35
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6420018&loc=d3e36677-107848

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

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.

No definition available.

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

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

No definition available.

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

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.

No definition available.

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

- References

No definition available.

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

Carrying amount as of the balance sheet date of other real estate and foreclosed assets. Other real estate may include 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. Foreclosed assets include all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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=6876686&loc=d3e534808-122878

Name:	us-gaap_OtherRealEstateAndForeclosedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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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.

No definition available.

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

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

No definition available.

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

- References

No definition available.

Name:	us-gaap_RisksAndUncertaintiesAbstract
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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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.

No definition available.

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

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cbu_ConsumerInstallmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cbu_HomeEquityReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_EquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_SoftwareAndSoftwareDevelopmentCostsMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_ComputerEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingAndBuildingImprovementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_LandImprovementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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

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

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CustomerRelationshipsMember
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Data Type:	na
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