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ACQUISITIONS (Details) shares in Thousands, $ in Thousands |  |  |  |  |  |  |  |  | 12 Months Ended
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Oct. 04, 2021 USD ($) | Aug. 02, 2021 USD ($) | Jul. 01, 2021 USD ($) | Jun. 01, 2021 USD ($) | Jun. 12, 2020 USD ($) country Office shares | Sep. 18, 2019 USD ($) | Jul. 12, 2019 USD ($) country Office | Jan. 02, 2019 USD ($) | Dec. 31, 2021 USD ($) item country | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Purchase price of acquisition |  |  |  |  |  |  |  |  |  |  | $ 95,034
Deposits |  |  |  |  |  |  |  |  | $ 0 |  | 568,161
Loans |  |  |  |  |  |  |  |  | 0 |  | 479,877
Contingent consideration |  |  |  |  |  |  |  |  | 2,900 |  | 0
Goodwill |  |  |  |  |  |  |  |  | 799,109 | $ 793,708 | 773,810
Cash |  |  |  |  |  | $ 500 |  |  | 29,329 | (34,360) | 4,653
Investment securities |  |  |  |  |  |  |  |  | 0 |  | 39,770
Acquisition-related contingent consideration adjustment |  |  |  |  |  |  |  |  | $ 200 | 0 | 0
Upstate New York. |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Number of counties where the bank has facilities | country |  |  |  |  |  |  |  |  | 42 |  |
Customer Lists [Member] |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  |  |  | $ 500 |  |  |  |  |
NuVantage Insurance, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  | $ 1,400 |  |  |  |  |  |  |
Goodwill |  |  |  | 1,400 |  |  |  |  | $ 5,654 |  |
Revenue earned |  |  |  | 700 |  |  |  |  |  |  |
Direct expenses |  |  |  | 600 |  |  |  |  |  |  |
Cash |  |  |  | $ 2,900 |  |  |  |  |  |  |
NuVantage Insurance, Inc. [Member] | Minimum | Probability Adjusted Level Of Retained Revenue |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Contingent consideration, measurement input 2 |  |  |  |  |  |  |  |  | 5,600 |  |
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Purchase price of acquisition |  |  |  |  |  |  |  |  | 16,750 |  |
Deposits |  |  |  |  |  |  |  |  | 0 |  |
Loans |  |  |  |  |  |  |  |  | 0 |  |
Contingent consideration |  |  | $ 1,400 |  |  |  |  |  | 1,400 |  |
Goodwill |  |  | 2,000 |  |  |  |  |  | 2,046 |  |
Revenue earned |  |  | 2,800 |  |  |  |  |  |  |  |
Direct expenses |  |  | 2,500 |  |  |  |  |  |  |  |
Cash |  |  | $ 15,400 |  |  |  |  |  |  |  |
Increase (decrease) in other liabilities as a result of reclassification and adjustment |  |  |  |  |  |  |  |  | 20 |  |
Increase (decrease) in goodwill as a result of fair value adjustment |  |  |  |  |  |  |  |  | 20 |  |
Investment securities |  |  |  |  |  |  |  |  | $ 0 |  |
Contingent consideration, measurement input |  |  | 1.05 |  |  |  |  |  | 1.50 |  |
Period considered for additional consideration based on future retained revenue |  |  | 2 years |  |  |  |  |  |  |  |
Consideration payable term |  |  | 3 years |  |  |  |  |  |  |  |
Acquisition-related contingent consideration adjustment |  |  |  |  |  |  |  |  | $ 200 |  |
Fair value contingent consideration adjustment |  |  |  |  |  |  |  |  | 1,600 |  |
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Maximum |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Contingent consideration |  |  | $ 2,700 |  |  |  |  |  | 2,700 |  |
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Maximum | Probability Adjusted Level Of Retained Revenue |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Contingent consideration, measurement input 2 |  |  | 5,800 |  |  |  |  |  | 5,800 |  |
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Minimum |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Contingent consideration |  |  | 0 |  |  |  |  |  | 0 |  |
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Minimum | Probability Adjusted Level Of Retained Revenue |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Contingent consideration, measurement input 2 |  |  | 5,600 |  |  |  |  |  |  |  |
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Customer Lists [Member] |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  | $ 14,000 |  |  |  |  |  |  |  |
Steuben Trust Corporation [Member] |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Purchase price of acquisition |  |  |  |  |  |  |  |  |  | 98,555 |
Assets acquired |  |  |  |  | $ 607,800 |  |  |  |  |  |
Deposits |  |  |  |  | (516,300) |  |  |  |  | 516,274 |
Loans |  |  |  |  | 339,700 |  |  |  |  | 339,017 |
Contingent consideration |  |  |  |  |  |  |  |  |  | 0 |
Goodwill |  |  |  |  | 20,000 |  |  |  |  | 19,981 | 39,970
Revenue earned |  |  |  |  |  |  |  |  | 13,100 | 7,700 |
Direct expenses |  |  |  |  |  |  |  |  | 5,100 | 2,600 |
Cash |  |  |  |  | $ 98,600 |  |  |  |  |  |
Issuance of common shares (in shares) | shares |  |  |  |  | 1,360 |  |  |  |  |  |
Issuance of common shares |  |  |  |  | $ 21,600 |  |  |  |  |  |
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) |  |
Investment securities |  |  |  |  | $ 180,500 |  |  |  |  | 180,497 |
Steuben Trust Corporation [Member] | Western New York State [Member] |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Number of new counties where the bank has extended footprints | country |  |  |  |  | 2 |  |  |  |  |  |
Number of counties where the bank has facilities | country |  |  |  |  | 4 |  |  |  |  |  |
Number of branch locations | Office |  |  |  |  | 11 |  |  |  |  |  |
Thomas Gregory Associates [Member] |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Purchase price of acquisition |  |  |  |  |  |  |  |  | 13,120 |  |
Deposits |  |  |  |  |  |  |  |  | 0 |  |
Loans |  |  |  |  |  |  |  |  | 0 |  |
Contingent consideration |  | $ 1,500 |  |  |  |  |  |  | 1,500 |  |
Intangibles |  | 10,900 |  |  |  |  |  |  |  |  |
Goodwill |  | 2,200 |  |  |  |  |  |  | 2,170 |  |
Revenue earned |  | 600 |  |  |  |  |  |  |  |  |
Direct expenses |  | 600 |  |  |  |  |  |  |  |  |
Cash |  | $ 11,600 |  |  |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |  |  | 0 |  |
Period considered for additional consideration based on future retained revenue |  | 3 years |  |  |  |  |  |  |  |  |
Thomas Gregory Associates [Member] | Within Two Years From Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consideration payable term |  | 2 years |  |  |  |  |  |  |  |  |
Thomas Gregory Associates [Member] | Within Three Years From Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Consideration payable term |  | 3 years |  |  |  |  |  |  |  |  |
Thomas Gregory Associates [Member] | Maximum |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Contingent consideration |  | $ 3,400 |  |  |  |  |  |  | $ 3,400 |  |
Contingent consideration, measurement input |  | 1.09 |  |  |  |  |  |  | 1.65 |  |
Thomas Gregory Associates [Member] | Maximum | Probability Adjusted Level Of Retained Revenue |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Contingent consideration, measurement input 2 |  | $ 3,800 |  |  |  |  |  |  | $ 3,400 |  |
Thomas Gregory Associates [Member] | Minimum |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Contingent consideration |  | $ 0 |  |  |  |  |  |  | $ 0 |  |
Contingent consideration, measurement input |  | 0.82 |  |  |  |  |  |  | 1.38 |  |
Thomas Gregory Associates [Member] | Minimum | Probability Adjusted Level Of Retained Revenue |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Contingent consideration, measurement input 2 |  | $ 2,300 |  |  |  |  |  |  | $ 3,000 |  |
Elmira Savings Bank [Member] |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Number of Branch Locations of Acquiree Bank | 12 |  |  |  |  |  |  |  |  |  |
Number Of Country | item |  |  |  |  |  |  |  |  | 5 |  |
Purchase price of acquisition | $ 82,800 |  |  |  |  |  |  |  |  |  |
Kinderhook Bank Corp [Member] |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Deposits |  |  |  |  |  |  | $ 568,200 |  |  |  |
Loans |  |  |  |  |  |  | 479,900 |  |  |  |
Goodwill |  |  |  |  |  |  | 40,000 |  |  |  |
Revenue earned |  |  |  |  |  |  |  |  | $ 11,800 | 16,300 | 10,600
Direct expenses |  |  |  |  |  |  |  |  | 7,100 | $ 7,400 | $ 4,700
Cash |  |  |  |  |  |  | $ 93,400 |  |  |  |
Number of branch locations | Office |  |  |  |  |  |  | 11 |  |  |  |
Investment securities |  |  |  |  |  |  | $ 39,800 |  |  |  |
Kinderhook Bank Corp [Member] | Upstate New York [Member] |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Assets acquired |  |  |  |  |  |  | $ 642,800 |  |  |  |
Number of counties where the bank has facilities | country |  |  |  |  |  |  | 5 |  |  |  |
Wealth Resources Network, Inc [Member] |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Cash |  |  |  |  |  |  |  | $ 1,200 |  |  |
Wealth Resources Network, Inc [Member] | Customer Lists [Member] |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  |  |  |  |  | $ 1,200 |  |  |
Segment Total [Member] |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |
Purchase price of acquisition |  |  |  |  |  |  |  |  | $ 32,770 |  |

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