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SEGMENT INFORMATION - Additional Information (Details)	9 Months Ended
	Sep. 30, 2025
SEGMENT INFORMATION
Number Of Reportable Segments Not Disclosed [Flag]	true

X

- Definition

Indicates number of reportable segments not disclosed.

+ References

No definition available.

+ Details

Name:	cbu_NumberOfReportableSegmentsNotDisclosedFlag
Namespace Prefix:	cbu_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

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