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ACQUISITIONS (Details) shares in Thousands, $ in Thousands |  |  |  |  |  |  | 3 Months Ended | 9 Months Ended |
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Oct. 04, 2021 USD ($) | Aug. 02, 2021 USD ($) | Jul. 01, 2021 USD ($) | Jun. 03, 2021 USD ($) | Jun. 01, 2021 USD ($) | Jun. 12, 2020 USD ($) country Office shares | Sep. 30, 2021 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Dec. 31, 2020 USD ($)
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |
Purchase price of acquisition |  |  |  |  |  |  |  | $ 32,770 |  |
Deposits |  |  |  |  | $ 0 |  |  |  |  |
Loans |  |  |  |  | 0 |  |  |  |  |
Contingent consideration |  |  |  |  | 2,900 |  |  |  |  |
Goodwill |  |  |  |  | 5,669 |  | $ 799,127 | 799,127 | $ 793,749 | $ 793,708
Cash |  |  |  |  | 29,870 |  |  | 29,329 | $ (34,360) |
Investment securities |  |  |  |  | $ 0 |  |  |  |  |
Maximum [Member] |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |
Contingent consideration |  | $ 3,400 |  |  |  |  |  |  |  |
NuVantage Insurance, Inc. [Member] |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  | $ 1,400 |  |  |  |  |  |
Goodwill |  |  |  | 1,400 |  |  |  |  |  |
Revenue earned |  |  |  |  |  |  | 300 | 400 |  |
Direct expenses |  |  |  |  |  |  | 300 | 300 |  |
Purchase price in cash |  |  |  | $ 2,900 |  |  |  |  |  |
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |
Purchase price of acquisition |  |  | $ 16,750 |  |  |  |  |  |  |
Deposits |  |  | 0 |  |  |  |  |  |  |
Loans |  |  | 0 |  |  |  |  |  |  |
Contingent consideration |  |  | 1,400 |  |  |  |  |  |  |
Goodwill |  |  | 2,061 |  |  |  |  |  |  |
Revenue earned |  |  | 1,400 |  |  |  |  |  |  |
Direct expenses |  |  | 1,100 |  |  |  |  |  |  |
Cash |  |  | 15,350 |  |  |  |  |  |  |
Investment securities |  |  | $ 0 |  |  |  |  |  |  |
Business Combination, Contingent Consideration, Liability, Measurement Input |  |  | 1.05 |  |  |  |  |  |  |
Period considered for additional consideration based on future retained revenue |  |  | 2 years |  |  |  |  |  |  |
Consideration payable term |  |  | 3 years |  |  |  |  |  |  |
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |
Contingent consideration |  |  | $ 2,700 |  |  |  |  |  |  |
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Maximum [Member] | Probability Adjusted Level Of Retained Revenue [Member] |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |
Business Combination, Contingent Consideration Liability Measurement Input2 |  |  | 5,800 |  |  |  |  |  |  |
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |
Contingent consideration |  |  | 0 |  |  |  |  |  |  |
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Minimum [Member] | Probability Adjusted Level Of Retained Revenue [Member] |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |
Business Combination, Contingent Consideration Liability Measurement Input2 |  |  | 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 |  |  |  |  |  | 516,274 |  |  |  |
Loans |  |  |  |  |  | 339,017 |  |  |  |
Loans acquired |  |  |  |  |  | 339,700 |  |  |  |
Contingent consideration |  |  |  |  |  | 0 |  |  |  |
Goodwill |  |  |  |  |  | 19,981 |  |  |  |
Revenue earned |  |  |  |  |  |  | 3,300 | 9,800 |  |
Direct expenses |  |  |  |  |  |  | 1,400 | 3,900 |  |
Cash |  |  |  |  |  | $ 21,613 |  |  |  |
Issuance of common shares (in shares) | shares |  |  |  |  |  | 1,360 |  |  |  |
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 |  |  |  |  |  | $ 180,497 |  |  |  |
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 |  |  |  |  |  |  |  |
Goodwill |  | 2,170 |  |  |  |  |  |  |  |
Revenue earned |  | 300 |  |  |  |  |  |  |  |
Direct expenses |  | 200 |  |  |  |  |  |  |  |
Cash |  | 11,620 |  |  |  |  |  |  |  |
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 [Member] |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |
Business Combination, Contingent Consideration, Liability, Measurement Input |  | 1.09 |  |  |  |  |  |  |  |
Thomas Gregory Associates [Member] | Maximum [Member] | Probability Adjusted Level Of Retained Revenue [Member] |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |
Business Combination, Contingent Consideration Liability Measurement Input2 |  | $ 3,800 |  |  |  |  |  |  |  |
Thomas Gregory Associates [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |
Contingent consideration |  | $ 0 |  |  |  |  |  |  |  |
Business Combination, Contingent Consideration, Liability, Measurement Input |  | 0.82 |  |  |  |  |  |  |  |
Thomas Gregory Associates [Member] | Minimum [Member] | Probability Adjusted Level Of Retained Revenue [Member] |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |
Business Combination, Contingent Consideration Liability Measurement Input2 |  | $ 2,300 |  |  |  |  |  |  |  |
Thomas Gregory Associates [Member] | Customer Lists [Member] |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |
Intangibles |  | $ 10,900 |  |  |  |  |  |  |  |
Subsequent event | Elmira Savings Bank [Member] |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |
Number of Branch Locations of Acquiree Bank | 12 |  |  |  |  |  |  |  |  |
Purchase price of acquisition | $ 82,800 |  |  |  |  |  |  |  |  |
Assets acquired | 648,700 |  |  |  |  |  |  |  |  |
Deposits | 551,200 |  |  |  |  |  |  |  |  |
Loans | $ 465,300 |  |  |  |  |  |  |  |  |

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