LOANS AND ALLOWANCE FOR CREDIT LOSSES - Summary of Impaired Loans, Excluding Purchased Impaired (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021
Impaired loans
Loans with allowance allocation	$ 0	$ 7,102
Loans without allowance allocation	3,163	7,417
Carrying balance	3,163	14,519
Contractual balance	4,201	16,963
Specifically allocated allowance	0	566
Average carrying balance of individually assessed loans	12,200	33,400
Interest income on individually assessed loans	$ 0	$ 0

- Definition

Amount of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

No definition available.

+ Details

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

- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

No definition available.

+ Details

Name:	cbu_ImpairedFinancingReceivableAverageBalances
Namespace Prefix:	cbu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of contractual balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

No definition available.

+ Details

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

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

No definition available.

+ Details

Name:	cbu_ImpairedFinancingReceivableInterestIncome
Namespace Prefix:	cbu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of allowance for credit losses related to specifically allocated.

+ References

No definition available.

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

No definition available.

+ Details

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

- Definition

Amount of impaired financing receivables for which there is a related allowance for credit losses.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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