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ACQUISITIONS (Details) shares in Thousands, $ in Thousands									12 Months Ended
	Oct. 04, 2021 USD ($)	Aug. 02, 2021 USD ($)	Jul. 01, 2021 USD ($)	Jun. 01, 2021 USD ($)	Jun. 12, 2020 USD ($) country Office shares	Sep. 18, 2019 USD ($)	Jul. 12, 2019 USD ($) country Office	Jan. 02, 2019 USD ($)	Dec. 31, 2021 USD ($) item country	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Acquisitions [Abstract]
Purchase price of acquisition											$ 95,034
Deposits									$ 0		568,161
Loans									0		479,877
Contingent consideration									2,900		0
Goodwill									799,109	$ 793,708	773,810
Cash						$ 500			29,329	(34,360)	4,653
Investment securities									0		39,770
Acquisition-related contingent consideration adjustment									$ 200	0	0
Upstate New York.
Acquisitions [Abstract]
Number of counties where the bank has facilities | country									42
Customer Lists [Member]
Acquisitions [Abstract]
Intangibles						$ 500
NuVantage Insurance, Inc. [Member]
Acquisitions [Abstract]
Intangibles				$ 1,400
Goodwill				1,400					$ 5,654
Revenue earned				700
Direct expenses				600
Cash				$ 2,900
NuVantage Insurance, Inc. [Member] | Minimum | Probability Adjusted Level Of Retained Revenue
Acquisitions [Abstract]
Contingent consideration, measurement input 2									5,600
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member]
Acquisitions [Abstract]
Purchase price of acquisition									16,750
Deposits									0
Loans									0
Contingent consideration			$ 1,400						1,400
Goodwill			2,000						2,046
Revenue earned			2,800
Direct expenses			2,500
Cash			$ 15,400
Increase (decrease) in other liabilities as a result of reclassification and adjustment									20
Increase (decrease) in goodwill as a result of fair value adjustment									20
Investment securities									$ 0
Contingent consideration, measurement input			1.05						1.50
Period considered for additional consideration based on future retained revenue			2 years
Consideration payable term			3 years
Acquisition-related contingent consideration adjustment									$ 200
Fair value contingent consideration adjustment									1,600
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Maximum
Acquisitions [Abstract]
Contingent consideration			$ 2,700						2,700
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Maximum | Probability Adjusted Level Of Retained Revenue
Acquisitions [Abstract]
Contingent consideration, measurement input 2			5,800						5,800
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Minimum
Acquisitions [Abstract]
Contingent consideration			0						0
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Minimum | Probability Adjusted Level Of Retained Revenue
Acquisitions [Abstract]
Contingent consideration, measurement input 2			5,600
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Customer Lists [Member]
Acquisitions [Abstract]
Intangibles			$ 14,000
Steuben Trust Corporation [Member]
Acquisitions [Abstract]
Purchase price of acquisition										98,555
Assets acquired					$ 607,800
Deposits					(516,300)					516,274
Loans					339,700					339,017
Contingent consideration										0
Goodwill					20,000					19,981	39,970
Revenue earned									13,100	7,700
Direct expenses									5,100	2,600
Cash					$ 98,600
Issuance of common shares (in shares) | shares					1,360
Issuance of common shares					$ 21,600
Increase (decrease) in other liabilities as a result of reclassification and adjustment									(300)
Increase (decrease) in goodwill as a result of fair value adjustment									(300)
Investment securities					$ 180,500					180,497
Steuben Trust Corporation [Member] | Western New York State [Member]
Acquisitions [Abstract]
Number of new counties where the bank has extended footprints | country					2
Number of counties where the bank has facilities | country					4
Number of branch locations | Office					11
Thomas Gregory Associates [Member]
Acquisitions [Abstract]
Purchase price of acquisition									13,120
Deposits									0
Loans									0
Contingent consideration		$ 1,500							1,500
Intangibles		10,900
Goodwill		2,200							2,170
Revenue earned		600
Direct expenses		600
Cash		$ 11,600
Investment securities									0
Period considered for additional consideration based on future retained revenue		3 years
Thomas Gregory Associates [Member] | Within Two Years From Acquisition [Member]
Acquisitions [Abstract]
Consideration payable term		2 years
Thomas Gregory Associates [Member] | Within Three Years From Acquisition [Member]
Acquisitions [Abstract]
Consideration payable term		3 years
Thomas Gregory Associates [Member] | Maximum
Acquisitions [Abstract]
Contingent consideration		$ 3,400							$ 3,400
Contingent consideration, measurement input		1.09							1.65
Thomas Gregory Associates [Member] | Maximum | Probability Adjusted Level Of Retained Revenue
Acquisitions [Abstract]
Contingent consideration, measurement input 2		$ 3,800							$ 3,400
Thomas Gregory Associates [Member] | Minimum
Acquisitions [Abstract]
Contingent consideration		$ 0							$ 0
Contingent consideration, measurement input		0.82							1.38
Thomas Gregory Associates [Member] | Minimum | Probability Adjusted Level Of Retained Revenue
Acquisitions [Abstract]
Contingent consideration, measurement input 2		$ 2,300							$ 3,000
Elmira Savings Bank [Member]
Acquisitions [Abstract]
Number of Branch Locations of Acquiree Bank	12
Number Of Country | item									5
Purchase price of acquisition	$ 82,800
Kinderhook Bank Corp [Member]
Acquisitions [Abstract]
Deposits							$ 568,200
Loans							479,900
Goodwill							40,000
Revenue earned									$ 11,800	16,300	10,600
Direct expenses									7,100	$ 7,400	$ 4,700
Cash							$ 93,400
Number of branch locations | Office							11
Investment securities							$ 39,800
Kinderhook Bank Corp [Member] | Upstate New York [Member]
Acquisitions [Abstract]
Assets acquired							$ 642,800
Number of counties where the bank has facilities | country							5
Wealth Resources Network, Inc [Member]
Acquisitions [Abstract]
Cash								$ 1,200
Wealth Resources Network, Inc [Member] | Customer Lists [Member]
Acquisitions [Abstract]
Intangibles								$ 1,200
Segment Total [Member]
Acquisitions [Abstract]
Purchase price of acquisition									$ 32,770

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