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ACQUISITIONS - Intangible Asset, Goodwill and Acquisition-related Expenses (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Jul. 12, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Fair Value of Subordinated Notes Payable [Abstract]
Subordinated notes payable		$ 13,735		$ 13,735		$ 13,795
Summary of loans acquired [Abstract]
Acquisition-related expenses		796	$ 6,061	$ 4,537	$ 7,789
Subordinated Notes Maturing in February 2028
Fair Value of Subordinated Notes Payable [Abstract]
Subordinated notes payable	$ 3,000
Fixed interest rate percentage	6.00%
Maturity date	Feb. 29, 2028
Subordinated Notes Maturing in November 2025
Fair Value of Subordinated Notes Payable [Abstract]
Subordinated notes payable	$ 10,000
Fixed interest rate percentage	6.375%
Maturity date	Nov. 30, 2025
Kinderhook Bank Corp \| Core Deposit Intangibles [Member]
Intangible Assets [Abstract]
Estimated useful life				8 years
Kinderhook Bank Corp \| Other Intangibles [Member]
Intangible Assets [Abstract]
Estimated useful life				8 years
Steuben
Summary of loans acquired [Abstract]
Acquisition-related expenses		$ 800		$ 4,400
Steuben \| Core Deposit Intangibles [Member]
Intangible Assets [Abstract]
Estimated useful life				8 years
Steuben \| Other Intangibles [Member]
Intangible Assets [Abstract]
Estimated useful life				8 years