ACQUISITIONS - Summary of Loans Acquired (Details) - PCD Loans [Member] $ in Thousands	Jun. 30, 2022 USD ($)
Certain Loans Acquired During The Period [Abstract]
Par value of loans at acquisition	$ 2,184
Allowance for credit losses at acquisition	(71)
Non-credit premium at acquisition	(81)
Fair value of acquired loans	$ 2,032

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
Balance Type:	credit
Period Type:	instant