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ACQUISITIONS, Summary of Loans Acquired (Details) - USD ($) $ in Thousands | Jun. 12, 2020 | Jul. 12, 2019
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Kinderhook Bank Corp [Member] |  |
Summary of loans acquired [Abstract] |  |
Contractually required principal and interest at acquisition |  | $ 649,734
Contractual cash flows not expected to be collected |  | (9,648)
Expected cash flows at acquisition |  | 640,086
Interest component of expected cash flows |  | (160,209)
Fair value of acquired loans |  | 479,877
Kinderhook Bank Corp [Member] | Acquired Impaired Loans [Member] |  |
Summary of loans acquired [Abstract] |  |
Contractually required principal and interest at acquisition |  | 13,350
Contractual cash flows not expected to be collected |  | (4,176)
Expected cash flows at acquisition |  | 9,174
Interest component of expected cash flows |  | (551)
Fair value of acquired loans |  | 8,623
Kinderhook Bank Corp [Member] | Acquired Non Impaired Loans [Member] |  |
Summary of loans acquired [Abstract] |  |
Contractually required principal and interest at acquisition |  | 636,384
Contractual cash flows not expected to be collected |  | (5,472)
Expected cash flows at acquisition |  | 630,912
Interest component of expected cash flows |  | (159,658)
Fair value of acquired loans |  | $ 471,254
Steuben [Member] | PCD Loans [Member] |  |
Summary of loans acquired [Abstract] |  |
Par value of loans at acquisition | $ 24,198 |
Allowance for credit losses at acquisition | (528) |
Non-credit discount at acquisition | (80) |
Fair value of acquired loans | 23,590 |
Steuben [Member] | Non-PCD Loans [Member] |  |
Summary of loans acquired [Abstract] |  |
Contractually required principal and interest at acquisition | 416,446 |
Contractual cash flows not expected to be collected | (3,177) |
Expected cash flows at acquisition | 413,269 |
Interest component of expected cash flows | (97,695) |
Fair value of acquired loans | $ 315,574 |

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