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ACQUISITIONS - Summary of Loans Acquired (Details) - USD ($) $ in Thousands	Jun. 12, 2020	Jul. 12, 2019
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 Trust Corporation [Member] | Acquired Deteriorated
Summary of loans acquired [Abstract]
Par value of loans at acquisition	$ 35,906
Allowance for credit losses at acquisition	(668)
Non-credit discount at acquisition	103
Fair value of acquired loans	35,341
Steuben Trust Corporation [Member] | Non-PCD Loans
Summary of loans acquired [Abstract]
Contractually required principal and interest at acquisition	400,738
Contractual cash flows not expected to be collected	(2,994)
Expected cash flows at acquisition	397,744
Interest component of expected cash flows	(94,068)
Fair value of acquired loans	$ 303,676

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Fair value at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

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