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ACQUISITIONS (Narrative) (Details) shares in Thousands, $ in Thousands |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jun. 12, 2020 USD ($) country Office shares | Mar. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Mar. 31, 2020 USD ($)
Acquisitions [Abstract] |  |  |  |
Goodwill |  | $ 793,456 | $ 793,708 | $ 773,473
Steuben Trust Corporation [Member] |  |  |  |
Acquisitions [Abstract] |  |  |  |
Cash | $ 21,600 |  | 21,613 |
Issuance of common shares (in shares) | shares | 1,360 |  |  |
Purchase price of acquisition | $ 98,600 |  | 98,555 |
Deposits | 516,300 |  | 516,274 |
Goodwill | 20,000 |  | 19,983 |
Assets acquired | 607,800 |  |  |
Loans | 339,700 |  | 339,017 |
Investment securities | $ 180,500 |  | $ 180,497 |
Revenue earned |  | 1,300 |  |
Direct expenses |  | 3,200 |  |
Increase (decrease) in other liabilities as a result of reclassification and adjustment |  | 300 |  |
Increase (decrease) in goodwill as a result of fair value adjustment |  | $ 300 |  |
Steuben Trust Corporation [Member] | Western New York State [Member] |  |  |  |
Acquisitions [Abstract] |  |  |  |
Number of counties where the bank has facilities | country | 4 |  |  |
Number of new counties where the bank has extended footprints | country | 2 |  |  |
Number of branch locations | Office | 11 |  |  |

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