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ACQUISITIONS - Supplemental Pro Forma Financial Information (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Supplemental Pro Forma Financial Information [Abstract] |  |  |
Merger and acquisition integration related (recoveries) expenses | $ 4,933 | $ 8,608 | $ (769)
Total revenue, net of interest expense, pro forma | 607,382 | 626,229 | 594,174
Net income, pro forma | 171,147 | 180,237 | 167,914
Steuben Trust Corporation [Member] |  |  |
Supplemental Pro Forma Financial Information [Abstract] |  |  |
Acquisitions-related expenses | 4,800 |  |
Merger and acquisition integration related (recoveries) expenses | $ 4,900 | 8,600 | $ 800
Kinderhook Bank Corp [Member] |  |  |
Supplemental Pro Forma Financial Information [Abstract] |  |  |
Acquisitions-related expenses |  | $ 8,000 |

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- Definition The pro forma acquisition-related expenses for a period as if the business combination or combinations had been completed at the beginning of the period. + References No definition available. + Details
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- Definition The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 + Details
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- Definition The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 + Details
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- Definition This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 23 -URI http://asc.fasb.org/extlink&oid=116868678&loc=d3e1043-128460 + Details
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