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LOANS, Credit Quality By Past Due Status (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Aged analysis of the company's loans [Abstract] |  |  |  |
Total Loans | $ 4,236,206 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount |  | $ 4,109,083 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount |
Legacy Loan [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Past Due 30 - 89 Days | 34,436 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 35,689 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
90 + Days Past Due and Still Accruing | 2,828 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 2,418 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Nonaccrual | 17,676 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 16,065 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Total Past Due | 54,940 us-gaap_FinancingReceivableRecordedInvestmentPastDue / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 54,172 us-gaap_FinancingReceivableRecordedInvestmentPastDue / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Current | 3,908,458 us-gaap_FinancingReceivableRecordedInvestmentCurrent / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 3,717,703 us-gaap_FinancingReceivableRecordedInvestmentCurrent / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Total Loans | 3,963,398 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 3,771,875 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Acquired Loans [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Past Due 30 - 89 Days | 3,388 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 3,522 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
90 + Days Past Due and Still Accruing | 278 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 137 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Nonaccrual | 3,055 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 3,408 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Total Past Due | 6,721 us-gaap_FinancingReceivableRecordedInvestmentPastDue / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 7,067 us-gaap_FinancingReceivableRecordedInvestmentPastDue / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Acquired Impaired | 5,312 cbu_FinancingReceivableRecordedInvestmentPurchasedImpaired / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember | [1] | 7,090 cbu_FinancingReceivableRecordedInvestmentPurchasedImpaired / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember | [1]
Current | 260,775 us-gaap_FinancingReceivableRecordedInvestmentCurrent / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 323,051 us-gaap_FinancingReceivableRecordedInvestmentCurrent / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Total Loans | 272,808 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 337,208 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Consumer Mortgage [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Total Loans | 1,613,384 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember |  | 1,582,058 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember |
Consumer Mortgage [Member] | Legacy Loan [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Past Due 30 - 89 Days | 13,978 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 16,589 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
90 + Days Past Due and Still Accruing | 2,165 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 1,253 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Nonaccrual | 13,201 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 11,097 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Total Past Due | 29,344 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 28,939 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Current | 1,515,057 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 1,473,320 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Total Loans | 1,544,401 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 1,502,259 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Consumer Mortgage [Member] | Acquired Loans [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Past Due 30 - 89 Days | 1,892 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 1,857 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
90 + Days Past Due and Still Accruing | 232 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 85 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Nonaccrual | 2,122 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 1,463 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Total Past Due | 4,246 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 3,405 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Acquired Impaired | 0 cbu_FinancingReceivableRecordedInvestmentPurchasedImpaired / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember | [1] | 0 cbu_FinancingReceivableRecordedInvestmentPurchasedImpaired / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember | [1]
Current | 64,737 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 76,394 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Total Loans | 68,983 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 79,799 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Business Lending [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Total Loans | 1,262,484 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember |  | 1,260,364 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember |
Business Lending [Member] | Legacy Loan [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Past Due 30 - 89 Days | 6,738 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 2,960 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
90 + Days Past Due and Still Accruing | 350 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 164 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Nonaccrual | 2,291 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 3,083 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Total Past Due | 9,379 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 6,207 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Current | 1,115,215 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 1,079,818 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Total Loans | 1,124,594 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 1,086,025 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Business Lending [Member] | Acquired Loans [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Past Due 30 - 89 Days | 608 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 531 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
90 + Days Past Due and Still Accruing | 0 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 0 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Nonaccrual | 489 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 1,472 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Total Past Due | 1,097 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 2,003 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Acquired Impaired | 5,312 cbu_FinancingReceivableRecordedInvestmentPurchasedImpaired / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember | [1] | 7,090 cbu_FinancingReceivableRecordedInvestmentPurchasedImpaired / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember | [1]
Current | 131,481 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 165,246 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Total Loans | 137,890 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 174,339 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Consumer Indirect [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Total Loans | 833,968 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember |  | 740,002 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember |
Consumer Indirect [Member] | Legacy Loan [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Past Due 30 - 89 Days | 10,529 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 11,647 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
90 + Days Past Due and Still Accruing | 82 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 738 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Nonaccrual | 10 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 14 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Total Past Due | 10,621 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 12,399 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Current | 822,124 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 723,878 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Total Loans | 832,745 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 736,277 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Consumer Indirect [Member] | Acquired Loans [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Past Due 30 - 89 Days | 40 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 157 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
90 + Days Past Due and Still Accruing | 0 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 17 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Nonaccrual | 0 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 0 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Total Past Due | 40 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 174 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Acquired Impaired | 0 cbu_FinancingReceivableRecordedInvestmentPurchasedImpaired / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember | [1] | 0 cbu_FinancingReceivableRecordedInvestmentPurchasedImpaired / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember | [1]
Current | 1,183 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 3,551 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Total Loans | 1,223 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 3,725 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentIndirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Consumer Direct [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Total Loans | 184,028 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember |  | 180,139 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember |
Consumer Direct [Member] | Legacy Loan [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Past Due 30 - 89 Days | 1,389 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 1,858 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
90 + Days Past Due and Still Accruing | 36 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 90 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Nonaccrual | 2 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 4 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Total Past Due | 1,427 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 1,952 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Current | 177,158 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 169,452 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Total Loans | 178,585 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 171,404 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Consumer Direct [Member] | Acquired Loans [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Past Due 30 - 89 Days | 174 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 385 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
90 + Days Past Due and Still Accruing | 0 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 27 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Nonaccrual | 18 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 0 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Total Past Due | 192 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 412 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Acquired Impaired | 0 cbu_FinancingReceivableRecordedInvestmentPurchasedImpaired / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember | [1] | 0 cbu_FinancingReceivableRecordedInvestmentPurchasedImpaired / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember | [1]
Current | 5,251 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 8,323 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Total Loans | 5,443 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 8,735 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_ConsumerInstallmentDirectMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Home Equity [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Total Loans | 342,342 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember |  | 346,520 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember |
Home Equity [Member] | Legacy Loan [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Past Due 30 - 89 Days | 1,802 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 2,635 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
90 + Days Past Due and Still Accruing | 195 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 173 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Nonaccrual | 2,172 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 1,867 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Total Past Due | 4,169 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 4,675 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Current | 278,904 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 271,235 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Total Loans | 283,073 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |  | 275,910 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_LegacyLoanMember |
Home Equity [Member] | Acquired Loans [Member] |  |  |  |
Aged analysis of the company's loans [Abstract] |  |  |  |
Past Due 30 - 89 Days | 674 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 592 cbu_FinancingReceivableRecordedInvestment30To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
90 + Days Past Due and Still Accruing | 46 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 8 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Nonaccrual | 426 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 473 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Total Past Due | 1,146 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 1,073 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Acquired Impaired | 0 cbu_FinancingReceivableRecordedInvestmentPurchasedImpaired / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember | [1] | 0 cbu_FinancingReceivableRecordedInvestmentPurchasedImpaired / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember | [1]
Current | 58,123 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | 69,537 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
Total Loans | $ 59,269 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |  | $ 70,610 us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = cbu_HomeEquityReceivablesMember / cbu_FinancingReceivableRecordedInvestmentByTypeOfFinancingReceivableAxis = cbu_AcquiredLoansMember |
[1] | Acquired impaired loans were not classified as nonperforming assets as the loans are considered to be performing under ASC 310-30. As a result interest income, through the accretion of the difference between the carrying amount of the loans and the expected cashflows, is being recognized on all acquired impaired loans.

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- Definition Financing receivables that are less than 90 days past due but more than 29 days past due. + References No definition available. + Details
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- Definition Loans acquired after January 1, 2009 that have evidence of deterioration in credit quality since origination and for which it is probable, at acquisition, that the company will be unable to collect all contractually required payments. + References No definition available. + Details
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- Definition Recorded investment in financing receivables that are 90 days or more past due and still accruing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=28368275&loc=d3e5111-111524 + Details
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- Definition Financing receivables that are current. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=28368275&loc=SL6953401-111524 + Details
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- Definition Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=28368275&loc=d3e5111-111524 + Details
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- Definition Recorded Investment in financing receivables that are past due at the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=28368275&loc=SL6953401-111524 + Details
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- Definition Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522 + Details
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