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ACQUISITIONS - Summary of Loans Acquired (Details) - USD ($) $ in Thousands	Jun. 12, 2020	Jul. 12, 2019
Kinderhook Bank Corp
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 | Acquired Impaired Loans
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 | Acquired Non Impaired Loans
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 | Purchased Credit Deteriorated
Summary of loans acquired [Abstract]
Par value of loans at acquisition	$ 24,415
Allowance for credit losses at acquisition	(524)
Non-credit discount at acquisition	(115)
Fair value of acquired loans	23,776
Steuben | Non-PCD Loans
Summary of loans acquired [Abstract]
Contractually required principal and interest at acquisition	416,230
Contractual cash flows not expected to be collected	(3,131)
Expected cash flows at acquisition	413,099
Interest component of expected cash flows	(97,858)
Fair value of acquired loans	$ 315,241

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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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