ACQUISITIONS (Details) $ in Thousands			3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 01, 2023 USD ($)	May 13, 2022 USD ($) country	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) company	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($) Agency
Acquisitions
Aggregate consideration					$ 9,751		$ 85,656
Goodwill			$ 845,396	$ 844,984	845,396	$ 844,984	841,841
Loans			0		0		436,796
Investment securities			0		0		11,305
Deposits			0		0		522,295
Core deposit intangibles
Acquisitions
Deposits			0		$ 0		(7,970)
Certain assets of five financial services companies
Acquisitions
Number of financial service companies | company					5
Aggregate consideration					$ 7,900
Aggregate consideration in cash					6,500
Aggregate consideration in contingent consideration arrangements					1,400
Assets acquired			4,900		4,900
Goodwill			3,000		$ 3,000
Certain assets of five financial services companies | Minimum
Acquisitions
Term of revenue target for contingent consideration arrangements					2 years
Certain assets of five financial services companies | Maximum
Acquisitions
Term of revenue target for contingent consideration arrangements					5 years
Certain assets of five financial services companies | Customer list intangible
Acquisitions
Intangibles			4,700		$ 4,700
Axiom
Acquisitions
Aggregate consideration					1,819
Aggregate consideration in cash	$ 1,800
Intangibles	1,200
Goodwill	$ 600		625		625
Loans			0		0
Investment securities			0		0
Deposits			0		0
Revenue			300		400
Direct expenses			500		1,200
Axiom | Core deposit intangibles
Acquisitions
Deposits			0		0
Elmira Savings Bank ("Elmira")
Acquisitions
Aggregate consideration		$ 82,200					82,179
Assets acquired		583,600
Goodwill		$ 42,100					42,105
Number of country | country		5
Loans		$ 436,800					436,796
Investment securities		11,300					11,305
Deposits		522,300					522,295
Revenue			3,900	4,800	11,000	7,200
Direct expenses			1,100	$ 1,200	3,500	$ 1,800
Elmira Savings Bank ("Elmira") | Core deposit intangibles
Acquisitions
Deposits		$ 8,000					(7,970)
Fringe Benefits Design of Minnesota, Inc ("FBD")
Acquisitions
Net increase (decrease) in goodwill			$ 600		$ 100
One Group NY, Inc.
Acquisitions
Aggregate consideration							3,500
Goodwill							$ 500
Number of insurance agencies asset acquired | Agency							4
One Group NY, Inc. | Noncompete agreement
Acquisitions
Intangibles							$ 100
One Group NY, Inc. | Customer list intangible
Acquisitions
Intangibles							$ 2,900