ACQUISITIONS (Details) $ in Thousands							12 Months Ended
	Feb. 01, 2024 USD ($)	Mar. 01, 2023 USD ($)	May 13, 2022 USD ($) item	Aug. 02, 2021 USD ($)	Jul. 01, 2021 USD ($)	Jun. 01, 2021 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Acquisitions
Goodwill							$ 845,396	$ 841,841	$ 799,109
Other intangibles								2,900
Cash							8,517	668	29,329
Certain assets of six financial services companies
Acquisitions
Aggregate consideration in contingent consideration arrangements							1,400
Purchase price of acquisition							8,100
Assets acquired							5,100
Goodwill							3,000
Purchase price in cash							$ 6,700
Certain assets of six financial services companies | Maximum
Acquisitions
Term of revenue target for contingent consideration arrangements							5 years
Certain assets of six financial services companies | Minimum
Acquisitions
Term of revenue target for contingent consideration arrangements							2 years
Certain assets of six financial services companies | Customer list intangible
Acquisitions
Other intangibles							$ 4,900
Axiom
Acquisitions
Revenue							800
Direct expenses							1,800
Purchase price of acquisition							1,819
Loans							0
Investment securities							0
Deposits							0
Goodwill		$ 600					625
Other intangibles		1,200
Contingent consideration							0
Purchase price in cash		$ 1,800
Axiom | Core deposit intangibles
Acquisitions
Deposits							0
NuVantage Insurance Corp.
Acquisitions
Revenue							1,300	1,300	700
Direct expenses							2,000	1,300	600
Cash						$ 2,900
Fringe Benefits Design of Minnesota, Inc
Acquisitions
Revenue							3,500	2,800
Direct expenses							4,100	2,500
Goodwill					$ 2,100
Period considered for additional consideration based on future retained revenue					2 years
Consideration payable term					30 months
Contingent consideration					$ 1,400
Cash					$ 15,400
Contingent consideration, measurement input					0.0105
Fair value contingent consideration adjustment							2,700
Fringe Benefits Design of Minnesota, Inc | Adjusted level of retained revenue
Acquisitions
Contingent consideration, measurement input					0.50
Fringe Benefits Design of Minnesota, Inc | Maximum
Acquisitions
Contingent consideration					$ 2,700
Fringe Benefits Design of Minnesota, Inc | Minimum
Acquisitions
Contingent consideration					0
Steuben Trust Corporation
Acquisitions
Increase (decrease) in other liabilities as a result of reclassification and adjustment							(100)
Thomas Gregory Associates Insurance Brokers, Inc.
Acquisitions
Revenue							4,400	3,100	600
Direct expenses							1,900	1,800	$ 600
Period considered for additional consideration based on future retained revenue				3 years
Contingent consideration				$ 1,500
Contingent consideration, measurement input				0.450
Fair value contingent consideration adjustment							1,000
Thomas Gregory Associates Insurance Brokers, Inc. | Within two years from acquisition
Acquisitions
Consideration payable term				2 years
Thomas Gregory Associates Insurance Brokers, Inc. | Within three years from acquisition
Acquisitions
Consideration payable term				3 years
Thomas Gregory Associates Insurance Brokers, Inc. | Maximum
Acquisitions
Cash				$ 3,400
Contingent consideration, measurement input				0.0109
Thomas Gregory Associates Insurance Brokers, Inc. | Minimum
Acquisitions
Cash				$ 0
Contingent consideration, measurement input				0.0082
Elmira Savings Bank
Acquisitions
Revenue							14,900	12,100
Direct expenses							$ 4,900	3,100
Purchase price of acquisition			$ 82,200					82,179
Number of county | item			5
Assets acquired			$ 583,600
Loans			436,800					436,796
Investment securities			11,300					11,305
Deposits			522,300					522,295
Goodwill			42,100					42,105
Contingent consideration								0
Elmira Savings Bank | Core deposit intangibles
Acquisitions
Deposits			$ 8,000					(7,970)
OneGroup
Acquisitions
Revenue								100
Direct expenses								500
Purchase price of acquisition								$ 3,500
Cash				$ 11,600
Customer list intangible | NuVantage Insurance Corp.
Acquisitions
Other intangibles						1,400
Customer list intangible | Fringe Benefits Design of Minnesota, Inc
Acquisitions
Other intangibles					$ 14,000
Customer list intangible | Thomas Gregory Associates Insurance Brokers, Inc.
Acquisitions
Other intangibles				10,900
Goodwill | NuVantage Insurance Corp.
Acquisitions
Goodwill						$ 1,400
Goodwill | Thomas Gregory Associates Insurance Brokers, Inc.
Acquisitions
Goodwill				$ 2,200
Subsequent events | Certain assets Of creative plan design limited
Acquisitions
Contingent consideration	$ 6,800
Fair value of contingent consideration at acquisition date	$ 3,900