ACQUISITIONS (Details)							3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	May 13, 2022 USD ($) country	Jan. 01, 2022 USD ($)	Aug. 02, 2021 USD ($) item	Jul. 01, 2021 USD ($) item	Jun. 01, 2021 USD ($)	Jan. 01, 2021 Agency	Sep. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2022 USD ($) item	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)
Acquisitions
Purchase price of acquisition											$ 84,643,000		$ 32,770,000
Loans							$ 436,948,000				436,948,000		0
Investment securities							11,305,000				11,305,000		0
Deposits							522,295,000				522,295,000		0
Goodwill							844,984,000		$ 799,127,000		844,984,000	$ 799,127,000	799,109,000
Provision for credit losses							5,061,000		(944,000)		12,005,000	(11,001,000)
Revenue earned							200,000				800,000
Direct expenses							100,000				300,000
Number of insurance agencies asset acquired | Agency						3
Intangibles		$ 2,500,000
Cash											(345,000)	29,329,000
Contingent consideration											0		2,900,000
Acquisition-related contingent consideration adjustment							0		$ 0		400,000	$ 0
Net increase (decrease) in goodwill											45,875,000
Core deposit intangibles
Acquisitions
Deposits							(7,970,000)				(7,970,000)		0
Elmira Savings Bank ("Elmira")
Acquisitions
Purchase price of acquisition	$ 82,200,000										82,179,000
Number of country | country	5
Assets acquired	$ 579,000,000.0
Loans	437,000,000.0						436,948,000				436,948,000
Investment securities	11,300,000						11,305,000				11,305,000
Deposits	522,300,000						522,295,000				522,295,000
Goodwill	45,800,000						45,812,000				45,812,000
Provision for credit losses	3,900,000						3,900,000				3,900,000
Revenue earned							4,800,000				7,200,000
Direct expenses							1,200,000				1,800,000
Contingent consideration											0
Net increase (decrease) in goodwill							(4,900,000)
Elmira Savings Bank ("Elmira") | Core deposit intangibles
Acquisitions
Deposits	$ 8,000,000.0						(7,970,000)				(7,970,000)
Thomas Gregory Associates Insurance Brokers, Inc. ("TGA")
Acquisitions
Purchase price of acquisition													13,120,000
Loans													0
Investment securities													0
Deposits													0
Goodwill			$ 2,200,000										2,170,000
Revenue earned							1,000,000.0				3,000,000.0
Direct expenses							400,000				1,100,000
Cash			11,600,000
Contingent consideration			$ 1,500,000										1,500,000
Period considered for additional consideration based on future retained revenue			3 years
Acquisition-related contingent consideration adjustment										$ 500,000
Fair value contingent consideration adjustment							0			2,000,000.0
Thomas Gregory Associates Insurance Brokers, Inc. ("TGA") | Minimum
Acquisitions
Contingent consideration			$ 0							$ 0
Contingent consideration, measurement input | item			0.82							4.44
Thomas Gregory Associates Insurance Brokers, Inc. ("TGA") | Maximum
Acquisitions
Contingent consideration			$ 3,400,000							$ 3,400,000
Contingent consideration, measurement input | item			1.09							4.55
Thomas Gregory Associates Insurance Brokers, Inc. ("TGA") | Probability adjusted level of retained revenue | Minimum
Acquisitions
Contingent consideration, measurement input 2			$ 2,300,000							$ 3,300,000
Thomas Gregory Associates Insurance Brokers, Inc. ("TGA") | Probability adjusted level of retained revenue | Maximum
Acquisitions
Contingent consideration, measurement input 2			$ 3,800,000							$ 3,700,000
Thomas Gregory Associates Insurance Brokers, Inc. ("TGA") | Within two years from acquisition
Acquisitions
Consideration payable term			2 years
Thomas Gregory Associates Insurance Brokers, Inc. ("TGA") | Within three years from acquisition
Acquisitions
Consideration payable term			3 years
Thomas Gregory Associates Insurance Brokers, Inc. ("TGA") | Customer Lists [Member]
Acquisitions
Intangibles			$ 10,900,000
Thomas Gregory Associates Insurance Brokers, Inc. ("TGA") | Core deposit intangibles
Acquisitions
Deposits													0
Fringe Benefits Design of Minnesota, Inc ("FBD")
Acquisitions
Purchase price of acquisition													16,750,000
Loans													0
Investment securities													0
Deposits													0
Goodwill				$ 2,100,000									2,109,000
Revenue earned							800,000				3,300,000
Direct expenses							1,000,000.0				3,200,000
Cash				15,400,000
Contingent consideration				$ 1,400,000									1,400,000
Period considered for additional consideration based on future retained revenue				2 years
Consideration payable term				3 years
Contingent consideration, measurement input | item				1.05						4.51
Acquisition-related contingent consideration adjustment										$ 100,000
Fair value contingent consideration adjustment							0			1,500,000
Net increase (decrease) in goodwill							100,000	$ 100,000
Fringe Benefits Design of Minnesota, Inc ("FBD") | Minimum
Acquisitions
Contingent consideration				$ 0						0
Fringe Benefits Design of Minnesota, Inc ("FBD") | Maximum
Acquisitions
Contingent consideration				2,700,000						2,700,000
Fringe Benefits Design of Minnesota, Inc ("FBD") | Probability adjusted level of retained revenue | Minimum
Acquisitions
Contingent consideration, measurement input 2				5,600,000
Fringe Benefits Design of Minnesota, Inc ("FBD") | Probability adjusted level of retained revenue | Maximum
Acquisitions
Contingent consideration, measurement input 2				5,800,000
Fringe Benefits Design of Minnesota, Inc ("FBD") | Customer Lists [Member]
Acquisitions
Intangibles				$ 14,000,000.0
Fringe Benefits Design of Minnesota, Inc ("FBD") | Core deposit intangibles
Acquisitions
Deposits													$ 0
NuVantage Insurance, Inc. [Member]
Acquisitions
Goodwill					$ 1,400,000
Revenue earned							300,000				900,000
Direct expenses							$ 300,000				$ 900,000
Intangibles					1,400,000
Purchase price in cash					$ 2,900,000
NuVantage Insurance, Inc. [Member] | Probability adjusted level of retained revenue | Minimum
Acquisitions
Contingent consideration, measurement input 2										5,200,000
NuVantage Insurance, Inc. [Member] | Probability adjusted level of retained revenue | Maximum
Acquisitions
Contingent consideration, measurement input 2										$ 5,400,000
One Group NY, Inc.
Acquisitions
Purchase price of acquisition		$ 2,500,000