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ACQUISITIONS (Details) shares in Thousands, $ in Thousands |  |  |  |  |  |  | 3 Months Ended | 12 Months Ended |
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Jun. 12, 2020 USD ($) Office shares | Sep. 18, 2019 USD ($) | Jul. 12, 2019 USD ($) Office country | Jan. 02, 2019 USD ($) | Apr. 02, 2018 USD ($) | Jan. 02, 2018 USD ($) | Dec. 31, 2020 USD ($) country | Sep. 30, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2020 USD ($) country | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Oct. 21, 2019 country
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  | $ 500 | $ 95,034 |  |  |  |  |  |  |  |  | $ (34,360) | $ 4,653 | $ 1,737 |
Purchase price of acquisition |  |  | 95,034 |  |  |  |  |  |  |  |  |  |  |  |
Other liabilities |  |  | 2,922 |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  |  | 39,970 |  |  |  | $ 793,708 |  | $ 773,810 | $ 733,503 |  | 793,708 | 773,810 | 733,503 |
Loans |  |  | 479,877 |  |  |  |  |  |  |  |  |  |  |  |
Investment securities |  |  | 39,770 |  |  |  |  |  |  |  |  |  |  |  |
Deposits |  |  | (568,161) |  |  |  |  |  |  |  |  |  |  |  |
Increase in deposits |  |  |  |  |  |  |  |  |  |  |  | $ 1,713,733 | $ 104,435 | $ (122,049) |
Vermont [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of counties where the bank has facilities | country |  |  |  |  |  |  | 12 |  |  |  |  | 12 |  |  |
Massachusetts [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of counties where the bank has facilities | country |  |  |  |  |  |  | 1 |  |  |  |  | 1 |  |  |
Upstate New York [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of counties where the bank has facilities | country |  |  |  |  |  |  | 42 |  |  |  |  | 42 |  |  |
Customer Lists [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  | $ 500 |  |  |  |  |  |  |  |  |  |  |  |  |
Other Intangibles [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  | 1,650 |  |  |  |  |  |  |  |  |  |  |  |
Steuben Trust Corporation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash | $ 98,600 |  |  |  |  |  |  |  |  |  |  | $ 21,613 |  |  |
Purchase price of acquisition |  |  |  |  |  |  |  |  |  |  |  | 98,555 |  |  |
Number of branch locations | Office | 11 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common shares (in shares) | shares | 1,360 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common shares | $ 21,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets acquired | 607,800 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other liabilities |  |  |  |  |  |  | $ 5,095 |  |  |  |  | 5,095 |  |  |
Goodwill | 20,200 |  |  |  |  |  | 20,235 |  |  |  |  | 20,235 |  |  |
Loans | 339,700 |  |  |  |  |  | 339,017 |  |  |  |  | 339,017 |  |  |
Investment securities | 180,500 |  |  |  |  |  | 180,497 |  |  |  |  | 180,497 |  |  |
Deposits | 516,300 |  |  |  |  |  | (516,274) |  |  |  |  | (516,274) |  |  |
Revenue earned | 2,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Direct expenses | $ 7,700 |  |  |  |  |  |  |  |  |  |  |  |  |  |
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
Steuben Trust Corporation [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  |  |  |  | 1,196 |  |  |  |  | 1,196 |  |  |
Kinderhook Bank Corp [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  |  | 93,384 |  |  |  |  |  |  |  |  |  |  |  |
Purchase price of acquisition |  |  | $ 93,384 |  |  |  |  |  |  |  |  |  |  |  |
Number of branch locations | Office |  |  | 11 |  |  |  |  |  |  |  |  |  |  |  |
Other liabilities |  |  | $ 2,922 |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  |  | 39,970 |  |  |  |  |  |  |  |  |  |  |  |
Loans |  |  | 479,877 |  |  |  |  |  |  |  |  |  |  |  |
Investment securities |  |  | 39,770 |  |  |  |  |  |  |  |  |  |  |  |
Deposits |  |  | $ (568,161) |  |  |  |  |  |  |  |  |  |  |  |
Revenue earned |  |  |  |  |  |  |  |  |  |  |  | 16,300 |  |  |
Direct expenses |  |  |  |  |  |  |  |  |  |  |  | $ 7,400 |  |  |
Increase (decrease) in other liabilities as a result of reclassification and adjustment |  |  |  |  |  |  |  | $ 1,300 | 40 |  |  |  |  |  |
Increase (decrease) in goodwill as a result of fair value adjustment |  |  |  |  |  |  |  |  | 40 |  |  |  |  |  |
Decrease in loans |  |  |  |  |  |  |  | 200 |  |  |  |  |  |  |
Decrease in other assets |  |  |  |  |  |  |  | 1,700 |  |  |  |  |  |  |
Increased in accrued interest and fees receivable |  |  |  |  |  |  | $ 10 |  | $ 10 |  |  |  |  |  |
Increase decrease in premises and equipment |  |  |  |  |  |  |  | $ 500 |  |  |  |  |  |  |
Kinderhook Bank Corp [Member] | Upstate New York [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of counties where the bank has facilities | country |  |  | 5 |  |  |  |  |  |  |  |  |  |  |  |
Assets acquired |  |  | $ 642,800 |  |  |  |  |  |  |  |  |  |  |  |
Revenue earned |  |  | 10,600 |  |  |  |  |  |  |  |  |  |  |  |
Direct expenses |  |  | 4,700 |  |  |  |  |  |  |  |  |  |  |  |
Kinderhook Bank Corp [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |
Wealth Resources Network, Inc [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  |  |  | $ 1,200 |  |  |  |  |  |  |  |  |  |  |
Wealth Resources Network, Inc [Member] | Customer Lists [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  | $ 1,200 |  |  |  |  |  |  |  |  |  |  |
HR Consultants (SA), LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  |  |  |  | $ 300 |  |  |  |  |  |  |  |  |  |
HR Consultants (SA), LLC [Member] | Customer Lists [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  |  | $ 300 |  |  |  |  |  |  |  |  |  |
Penna & Associates Agency, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  |  |  |  |  | $ 800 |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  |  | 300 |  |  |  |  |  |  |  |  |
Penna & Associates Agency, Inc. [Member] | Customer Lists [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  |  |  | 300 |  |  |  |  |  |  |  |  |
Styles Bridges Associates [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  |  |  |  |  | 700 |  |  |  |  |  |  |  |  |
Styles Bridges Associates [Member] | Customer Lists [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  |  |  | $ 700 |  |  |  |  |  |  |  |  |
Gordon B. Roberts Agency, Inc [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in goodwill as a result of fair value adjustment |  |  |  |  |  |  |  |  |  | $ 1,100 |  |  |  |  |
NRS [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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) |  |  |  |

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