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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) (USD $) | 12 Months Ended | 0 Months Ended
--- | --- | ---
Dec. 31, 2014 Branch | Dec. 31, 2013 | Dec. 31, 2012 | Feb. 24, 2015 Branch
Nature of Operations [Abstract] |  |  |  |
Number of bank branches | 182 cbu_NumberOfCustomerFacilities |  |  |
Number of counties in New York where the bank has facilities | 35 cbu_NumberOfCountiesBankHasFacilities |  |  |
Number of counties in Pennsylvania where the bank has facilities | 6 cbu_NumberOfCountiesInPennsylvaniaWhereBankHasFacilities |  |  |
Impaired and Other Nonaccrual Loans [Abstract] |  |  |  |
Number of days past due for loans to be placed on nonaccrual status | 90 days |  |  |
Threshold amount of individual commercial loans evaluated for impairment | $ 500,000 cbu_ThresholdAmountOfIndividualCommercialLoansEvaluatedForImpairment |  |  |
Premises and Equipment [Abstract] |  |  |  |
Other real estate | 1,900,000 us-gaap_OtherRealEstateAndForeclosedAssets | 5,100,000 us-gaap_OtherRealEstateAndForeclosedAssets |  |
Advertising [Abstract] |  |  |  |
Advertising costs | 3,200,000 us-gaap_AdvertisingExpense | 3,000,000 us-gaap_AdvertisingExpense | 2,600,000 us-gaap_AdvertisingExpense |
Subsequent Event [Member] | Oneida Financial Corp [Member] |  |  |  |
Subsequent Event [Line Items] |  |  |  |
Total net consideration |  |  |  | 142,000,000 us-gaap_BusinessCombinationConsiderationTransferred1 / us-gaap_BusinessAcquisitionAxis = cbu_OneidaFinancialCorpMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Number of branch locations acquired |  |  |  | 12 cbu_NumberOfBranchLocationsAcquired / us-gaap_BusinessAcquisitionAxis = cbu_OneidaFinancialCorpMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Assets acquired |  |  |  | 800,000,000 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets / us-gaap_BusinessAcquisitionAxis = cbu_OneidaFinancialCorpMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Loans acquired |  |  |  | 370,000,000 cbu_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLoans / us-gaap_BusinessAcquisitionAxis = cbu_OneidaFinancialCorpMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Deposits acquired |  |  |  | $ 690,000,000 cbu_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeposits / us-gaap_BusinessAcquisitionAxis = cbu_OneidaFinancialCorpMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Minimum [Member] | Equipment [Member] |  |  |  |
Premises and Equipment [Abstract] |  |  |  |
Estimated useful life | 3 years |  |  |
Minimum [Member] | Software and Hardware [Member] |  |  |  |
Premises and Equipment [Abstract] |  |  |  |
Estimated useful life | 3 years |  |  |
Minimum [Member] | Building and Building Improvements [Member] |  |  |  |
Premises and Equipment [Abstract] |  |  |  |
Estimated useful life | 10 years |  |  |
Maximum [Member] | Equipment [Member] |  |  |  |
Premises and Equipment [Abstract] |  |  |  |
Estimated useful life | 10 years |  |  |
Maximum [Member] | Software and Hardware [Member] |  |  |  |
Premises and Equipment [Abstract] |  |  |  |
Estimated useful life | 7 years |  |  |
Maximum [Member] | Building and Building Improvements [Member] |  |  |  |
Premises and Equipment [Abstract] |  |  |  |
Estimated useful life | 40 years |  |  |
Maximum [Member] | Land Improvements [Member] |  |  |  |
Premises and Equipment [Abstract] |  |  |  |
Estimated useful life | 20 years |  |  |
Core Deposits [Member] | Minimum [Member] |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |
Intangible asset useful life (amortization period) | 7 years |  |  |
Core Deposits [Member] | Maximum [Member] |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |
Intangible asset useful life (amortization period) | 20 years |  |  |
Customer Relationships [Member] | Minimum [Member] |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |
Intangible asset useful life (amortization period) | 7 years |  |  |
Customer Relationships [Member] | Maximum [Member] |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |
Intangible asset useful life (amortization period) | 20 years |  |  |

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