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Amount of deposits acquired at the acquisition date.

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Amount of loans acquired at the acquisition date.

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Represents the number of branch locations acquired by the entity.

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The number of counties where the bank operates or has branches in New York.

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The number of counties where the bank operates or has branches in Pennsylvania.

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The number of customer locations or branches operated by the bank.

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The number of days according to the entity's policy that a loan must be past due before being placed on nonaccrual status.

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The threshold amount of commercial loans which are individually evaluated for impairment.

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Amount charged to advertising expense for the period, which are expenses incurred with the objective of increasing revenue for a specified brand, product or product line.

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Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

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Amount of assets acquired at the acquisition date.

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Carrying amount as of the balance sheet date of other real estate and foreclosed assets. Other real estate may include real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. Foreclosed assets include all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings.

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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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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	182cbu_NumberOfCustomerFacilities
Number of counties in New York where the bank has facilities	35cbu_NumberOfCountiesBankHasFacilities
Number of counties in Pennsylvania where the bank has facilities	6
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
Premises and Equipment [Abstract]
Other real estate	1,900,000	5,100,000
Advertising [Abstract]
Advertising costs	3,200,000us-gaap_AdvertisingExpense	3,000,000us-gaap_AdvertisingExpense	2,600,000us-gaap_AdvertisingExpense
Subsequent Event [Member] \| Oneida Financial Corp [Member]
Subsequent Event [Line Items]
Total net consideration				142,000,000
Number of branch locations acquired				12
Assets acquired				800,000,000
Loans acquired				370,000,000
Deposits acquired				$ 690,000,000
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