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LOANS AND ALLOWANCE FOR CREDIT LOSSES - Summary of Impaired Loans, Excluding Purchased Impaired (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Impaired loans |  |  |
Loans with allowance allocation | $ 3,484 | $ 0 |
Loans without allowance allocation | 15,750 | 3,163 |
Carrying balance | 19,234 | 3,163 |
Contractual balance | 19,249 | 4,201 |
Specifically allocated allowance | 470 | 0 |
Average carrying balance of individually assessed loans | $ 20,200 | 12,200 |
Interest income on individually assessed loans |  | $ 0 | $ 0

X
- 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

X
- 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

X
- 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

X
- 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

X
- 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

X
- 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

X
- 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

X
- References No definition available. + Details
Name: | us-gaap_PurchasedFinancialAssetsWithCreditDeteriorationAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration