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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Goodwill and Intangible Assets (Details) $ in Thousands | 1 Months Ended | 12 Months Ended
--- | --- | ---
Nov. 30, 2013 branch | Mar. 31, 2008 branch | Dec. 31, 2020 USD ($) segment
Finite-Lived Intangible Assets [Line Items] |  |  |
Number of operating segments | segment |  |  | 2
Bank of America |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Number of branches acquired | branch | 6 |  |
Partners Trust Bank | Trust Business |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Finite-lived intangible assets | $ |  |  | $ 200
Useful life (years) |  |  | 15 years
Fort Orange Financial Corp |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Finite-lived intangible assets | $ |  |  | $ 12
Useful life (years) |  |  | 10 years
M&T Bank branch offices |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Number of branches acquired | branch |  | 3 |

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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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 928 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6473545&loc=d3e61844-108004 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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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- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References No definition available. + Details
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