Name:	cbu_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesAcquiredDuringPeriodContractualCashFlowsNotExpectedToBeCollected
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Allowance for credit losses at acquisition at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

No definition available.

Name:	cbu_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodAtAcquisitionAllowanceForCreditLosses
Namespace Prefix:	cbu_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Par value of loans at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

No definition available.

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

Non-credit (discount) premium at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

No definition available.

Name:	cbu_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodAtAcquisitionNonCreditDiscountPremium
Namespace Prefix:	cbu_
Data Type:	xbrli:monetaryItemType
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The investor's estimate, at acquisition, of the amount and timing of the interest component of expected cash flows for loans acquired in transfer not accounted for as debt securities.

No definition available.

Name:	cbu_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodInterestComponentOfCashFlows
Namespace Prefix:	cbu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

Name:	us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesAcquiredDuringPeriodAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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Fair value at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=123409700&loc=d3e13503-111538

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=121614798&loc=d3e15032-111544

Name:	us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodAtAcquisitionAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of cash flows expected to be collected at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

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

Amount of contractually required payments receivable at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=121614798&loc=d3e15032-111544

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=123409700&loc=d3e13503-111538