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GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2012
Finite-Lived Intangible Assets [Line Items] |  |  |  |
Gross Carrying Amount | $ 50,345 us-gaap_FiniteLivedIntangibleAssetsGross | $ 49,767 us-gaap_FiniteLivedIntangibleAssetsGross |  |
Accumulated Amortization | (38,546)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization | (34,259)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization |  |
Net Carrying Amount | 11,799 us-gaap_FiniteLivedIntangibleAssetsNet | 15,508 us-gaap_FiniteLivedIntangibleAssetsNet |  |
Finite-lived intangible assets, future amortization expense [Abstract] |  |  |  |
2015 | 3,408 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths |  |  |
2016 | 2,612 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo |  |  |
2017 | 1,908 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree |  |  |
2018 | 1,424 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour |  |  |
2019 | 1,002 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive |  |  |
Thereafter | 1,445 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive |  |  |
Net Carrying Amount | 11,799 us-gaap_FiniteLivedIntangibleAssetsNet | 15,508 us-gaap_FiniteLivedIntangibleAssetsNet |  |
Components of goodwill [Abstract] |  |  |  |
Goodwill | 379,998 us-gaap_GoodwillGross | 379,815 us-gaap_GoodwillGross |  | 374,527 us-gaap_GoodwillGross
Goodwill, activity | 183 us-gaap_GoodwillAcquiredDuringPeriod | 5,288 us-gaap_GoodwillAcquiredDuringPeriod |  |
Accumulated impairment | (4,824)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss | (4,824)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss | (4,824)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss |
Accumulated impairment, activity | 0 us-gaap_GoodwillImpairmentLoss | 0 us-gaap_GoodwillImpairmentLoss |  |
Goodwill, net | 375,174 us-gaap_Goodwill | 374,991 us-gaap_Goodwill |  | 369,703 us-gaap_Goodwill
Goodwill, net, activity | 183 us-gaap_GoodwillPeriodIncreaseDecrease | 5,288 us-gaap_GoodwillPeriodIncreaseDecrease |  |
Key economic assumptions used to estimate the value of the MSRs [Abstract] |  |  |  |
Weighted-average contractual life | 19 years 7 months 6 days | 19 years 2 months 12 days |  |
Weighted-average constant prepayment rate (CPR) (in hundredths) | 12.80%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValuePrepaymentSpeed | 18.00%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValuePrepaymentSpeed |  |
Weighted-average discount rate (in hundredths) | 3.20%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueDiscountRate | 4.50%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueDiscountRate |  |
Core Deposit Intangibles [Member] |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |
Gross Carrying Amount | 40,326 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CoreDepositsMember | 40,326 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CoreDepositsMember | 40,326 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CoreDepositsMember |
Accumulated Amortization | (30,303)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CoreDepositsMember | (26,866)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CoreDepositsMember | (30,303)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CoreDepositsMember |
Net Carrying Amount | 10,023 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CoreDepositsMember | 13,460 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CoreDepositsMember | 10,023 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CoreDepositsMember |
Finite-lived intangible assets, future amortization expense [Abstract] |  |  |  |
Net Carrying Amount | 10,023 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CoreDepositsMember | 13,460 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CoreDepositsMember | 10,023 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CoreDepositsMember |
Other Intangibles [Member] |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |
Gross Carrying Amount | 10,019 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember | 9,441 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember | 10,019 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember |
Accumulated Amortization | (8,243)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember | (7,393)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember | (8,243)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember |
Net Carrying Amount | 1,776 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember | 2,048 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember | 1,776 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember |
Finite-lived intangible assets, future amortization expense [Abstract] |  |  |  |
Net Carrying Amount | 1,776 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember | 2,048 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember | 1,776 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember |
Mortgage Servicing Rights [Member] |  |  |  |
Servicing Asset at Fair Value, Amount [Roll Forward] |  |  |  |
Carrying value before valuation allowance at beginning of period | 1,218 us-gaap_ServicingAssetAtAmortizedValue / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember | 1,458 us-gaap_ServicingAssetAtAmortizedValue / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember |  |
Additions | 315 us-gaap_ServicingAssetAtAmortizedValueAdditions / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember | 289 us-gaap_ServicingAssetAtAmortizedValueAdditions / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember |  |
Amortization | (444)us-gaap_ServicingAssetAtAmortizedValueAmortization1 / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember | (529)us-gaap_ServicingAssetAtAmortizedValueAmortization1 / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember |  |
Carrying value before valuation allowance at end of period | 1,089 us-gaap_ServicingAssetAtAmortizedValue / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember | 1,218 us-gaap_ServicingAssetAtAmortizedValue / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Roll Forward] |  |  |  |
Valuation allowance balance at beginning of period | 0 us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsBalance / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember | (430)us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsBalance / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember |  |
Impairment charges | 0 us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsProvisions / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember | (111)us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsProvisions / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember |  |
Impairment recoveries | 0 us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsRecoveries / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember | 541 us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsRecoveries / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember |  |
Valuation allowance balance at end of period | 0 us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsBalance / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember | 0 us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsBalance / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember |  |
Net carrying value at end of period | 1,089 cbu_ServicingAssetAtAmortizedCostNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember | 1,218 cbu_ServicingAssetAtAmortizedCostNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember |  |
Fair value of MSRs at end of period | 1,616 us-gaap_ServicingAssetAtAmortizedValueFairValue / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember | 1,495 us-gaap_ServicingAssetAtAmortizedValueFairValue / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember |  |
Principal balance of loans sold during the year | 25,728 us-gaap_MortgageLoansOnRealEstateCostOfMortgagesSold / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember | 25,179 us-gaap_MortgageLoansOnRealEstateCostOfMortgagesSold / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember |  |
Principal balance of loans serviced for others | 302,895 cbu_MortgageLoansServicedForOthers / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember | 322,030 cbu_MortgageLoansServicedForOthers / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember |  |
Custodial escrow balances maintained in connection with loans serviced for others | $ 4,320 cbu_CustodialEscrowBalancesMaintainedInConnectionWithLoansServicedForOthers / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember | $ 4,519 cbu_CustodialEscrowBalancesMaintainedInConnectionWithLoansServicedForOthers / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = cbu_MortgageServicingRightsMember |  |

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