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ACQUISITIONS (Details) shares in Thousands, $ in Thousands					3 Months Ended	6 Months Ended		12 Months Ended
	Aug. 02, 2021 USD ($)	Jul. 01, 2021 USD ($)	Jun. 03, 2021 USD ($)	Jun. 12, 2020 USD ($) Office country shares	Jun. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Acquisitions [Abstract]
Cash						$ 2,900	$ (34,360)
Goodwill					$ 794,892	794,892	$ 792,715	$ 793,708
NuVantage Insurance, Inc. [Member]
Acquisitions [Abstract]
Purchase price in cash			$ 2,900
Customer List Intangibles			1,400
Goodwill			$ 1,500
Steuben Trust Corporation [Member]
Acquisitions [Abstract]
Cash				$ 21,600				21,613
Issuance of common shares (in shares) | shares				1,360
Purchase price of acquisition				$ 98,600				98,555
Deposits				516,300				516,274
Goodwill				20,000				19,981
Assets acquired				607,800
Loans				339,700				339,017
Investment securities				$ 180,500				$ 180,497
Revenue earned					3,300	6,500
Direct expenses					1,200	2,500
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
Steuben Trust Corporation [Member] | Western New York State [Member]
Acquisitions [Abstract]
Number of counties where the bank has facilities | country				4
Number of new counties where the bank has extended footprints | country				2
Number of branch locations | Office				11
Subsequent event | Fringe Benefits Design of Minnesota, Inc ("FBD") [Member]
Acquisitions [Abstract]
Purchase price of acquisition		$ 15,300
Contingent consideration		$ 2,700
Subsequent event | Thomas Gregory Associates [Member]
Acquisitions [Abstract]
Purchase price of acquisition	$ 11,600
Contingent consideration	$ 3,400

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