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ACQUISITIONS (Details) shares in Thousands, $ in Thousands							3 Months Ended	9 Months Ended
ACQUISITIONS (Details) shares in Thousands, $ in Thousands	Oct. 04, 2021 USD ($)	Aug. 02, 2021 USD ($)	Jul. 01, 2021 USD ($)	Jun. 03, 2021 USD ($)	Jun. 01, 2021 USD ($)	Jun. 12, 2020 USD ($) country Office shares	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Acquisitions [Abstract]
Purchase price of acquisition								$ 32,770
Deposits					$ 0
Loans					0
Contingent consideration					2,900
Goodwill					5,669		$ 799,127	799,127	$ 793,749	$ 793,708
Cash					29,870			29,329	$ (34,360)
Investment securities					$ 0
Maximum [Member]
Acquisitions [Abstract]
Contingent consideration		$ 3,400
NuVantage Insurance, Inc. [Member]
Acquisitions [Abstract]
Intangibles				$ 1,400
Goodwill				1,400
Revenue earned							300	400
Direct expenses							300	300
Purchase price in cash				$ 2,900
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member]
Acquisitions [Abstract]
Purchase price of acquisition			$ 16,750
Deposits			0
Loans			0
Contingent consideration			1,400
Goodwill			2,061
Revenue earned			1,400
Direct expenses			1,100
Cash			15,350
Investment securities			$ 0
Business Combination, Contingent Consideration, Liability, Measurement Input			1.05
Period considered for additional consideration based on future retained revenue			2 years
Consideration payable term			3 years
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] \| Maximum [Member]
Acquisitions [Abstract]
Contingent consideration			$ 2,700
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] \| Maximum [Member] \| Probability Adjusted Level Of Retained Revenue [Member]
Acquisitions [Abstract]
Business Combination, Contingent Consideration Liability Measurement Input2			5,800
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] \| Minimum [Member]
Acquisitions [Abstract]
Contingent consideration			0
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] \| Minimum [Member] \| Probability Adjusted Level Of Retained Revenue [Member]
Acquisitions [Abstract]
Business Combination, Contingent Consideration Liability Measurement Input2			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						516,274
Loans						339,017
Loans acquired						339,700
Contingent consideration						0
Goodwill						19,981
Revenue earned							3,300	9,800
Direct expenses							1,400	3,900
Cash						$ 21,613
Issuance of common shares (in shares) \| shares						1,360
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						$ 180,497
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
Goodwill		2,170
Revenue earned		300
Direct expenses		200
Cash		11,620
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 [Member]
Acquisitions [Abstract]
Business Combination, Contingent Consideration, Liability, Measurement Input		1.09
Thomas Gregory Associates [Member] \| Maximum [Member] \| Probability Adjusted Level Of Retained Revenue [Member]
Acquisitions [Abstract]
Business Combination, Contingent Consideration Liability Measurement Input2		$ 3,800
Thomas Gregory Associates [Member] \| Minimum [Member]
Acquisitions [Abstract]
Contingent consideration		$ 0
Business Combination, Contingent Consideration, Liability, Measurement Input		0.82
Thomas Gregory Associates [Member] \| Minimum [Member] \| Probability Adjusted Level Of Retained Revenue [Member]
Acquisitions [Abstract]
Business Combination, Contingent Consideration Liability Measurement Input2		$ 2,300
Thomas Gregory Associates [Member] \| Customer Lists [Member]
Acquisitions [Abstract]
Intangibles		$ 10,900
Subsequent event \| Elmira Savings Bank [Member]
Acquisitions [Abstract]
Number of Branch Locations of Acquiree Bank	12
Purchase price of acquisition	$ 82,800
Assets acquired	648,700
Deposits	551,200
Loans	$ 465,300