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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Goodwill and Intangible Assets (Details) $ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Nov. 30, 2013 Branch | Mar. 31, 2008 Branch | Dec. 31, 2017 USD ($)
Bank of America |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Number of branches acquired | Branch | 6 |  |
Finite-lived intangible assets |  |  | $ 0.3
Bank of America | Core Deposits |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Useful life |  |  | 7 years
Partners Trust Bank | Trust Business |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Finite-lived intangible assets |  |  | $ 1.3
Useful life |  |  | 15 years
Fort Orange Financial Corp |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Finite-lived intangible assets |  |  | $ 0.4
Useful life |  |  | 10 years
M&T Bank branch offices |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Number of branches acquired | Branch |  | 3 |
MT Branch Offices and Canton Bancorp, Inc. | Core Deposits |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Useful life |  |  | 7 years 3 months

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- Definition Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 + Details
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