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ACQUISITIONS, Intangible Asset, Goodwill and Acquisition-related Expenses (Details) - USD ($) $ in Thousands		12 Months Ended
	Jul. 12, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value of Subordinated Notes Payable [Abstract]
Subordinated notes		$ 13,795	$ 0
Summary of loans acquired [Abstract]
Merger and acquisition integration related (recoveries) expenses		$ 8,608	$ (769)	$ 25,986
Kinderhook Bank Corp [Member] \| Core Deposits [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
Kinderhook Bank Corp [Member] \| Other Intangibles [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
Kinderhook Bank Corp [Member] \| Subordinated Notes Maturing in February 2028 [Member]
Fair Value of Subordinated Notes Payable [Abstract]
Subordinated notes	$ 3,000
Fixed interest rate percentage	6.00%
Maturity date	Feb. 29, 2028
Kinderhook Bank Corp [Member] \| Subordinated Notes Maturing in November 2025 [Member]
Fair Value of Subordinated Notes Payable [Abstract]
Subordinated notes	$ 10,000
Fixed interest rate percentage	6.375%
Maturity date	Nov. 30, 2025
Penna & Associates Agency, Inc. [Member] \| Core Deposits [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
Penna & Associates Agency, Inc. [Member] \| Other Intangibles [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
Styles Bridges Associates [Member] \| Core Deposits [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
Styles Bridges Associates [Member] \| Other Intangibles [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
HR Consultants (SA), LLC [Member] \| Core Deposits [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
HR Consultants (SA), LLC [Member] \| Other Intangibles [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
Merchants Bancshares, Inc. [Member] \| Core Deposits [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
Merchants Bancshares, Inc. [Member] \| Other Intangibles [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
Dryfoos [Member] \| Core Deposits [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
Dryfoos [Member] \| Other Intangibles [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
BAS [Member] \| Core Deposits [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
BAS [Member] \| Other Intangibles [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
Northeast Capital Management, Inc [Member] \| Core Deposits [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
Northeast Capital Management, Inc [Member] \| Other Intangibles [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
Gordon B. Roberts Agency, Inc [Member] \| Core Deposits [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years
Gordon B. Roberts Agency, Inc [Member] \| Other Intangibles [Member]
Intangible Assets [Abstract]
Estimated useful life		8 years