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LOANS AND ALLOWANCE FOR CREDIT LOSSES - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Allowance for credit loss - loans
Beginning balance	$ 63,284	$ 55,542	$ 61,059	$ 49,869
Charge-offs	(2,942)	(2,688)	(8,930)	(6,991)
Recoveries	1,693	2,330	5,464	5,711
Provision	2,910	5,179	7,352	11,703
Ending balance	64,945	60,363	64,945	60,363
Accrued interest receivable on loans	28,400		28,400
Purchase of loans				436,900
Amount pledged	9,385,121		9,385,121		$ 8,748,335
Amounts outstanding	315,800		315,800
Sale of secondary market eligible residential consumer mortgage loans			4,600	4,500
Liabilities for off-balance-sheet
Beginning balance	933	1,223	1,123	803
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
PCD Allowance at Acquisition				0
Provision	(32)	(118)	(222)	302
Ending balance	901	1,105	901	1,105
Allowance for credit loss
Beginning balance	64,217	56,765	62,182	50,672
Charge-offs	(2,942)	(2,688)	(8,930)	(6,991)
Recoveries	1,693	2,330	5,464	5,711
PCD Allowance at Acquisition				71
Provision	2,878	5,061	7,130	12,005
Ending balance	65,846	61,468	65,846	61,468
Assets pledged as collateral | Borrowings from FHLB and FRB
Allowance for credit loss - loans
Amount pledged	4,300,000		4,300,000
Elmira Savings Bank ("Elmira")
Allowance for credit loss - loans
Ending balance	64,900	60,400	64,900	60,400
Commercial Loan
Allowance for credit loss - loans
Charge-offs			(618)
Purchase of loans				125,300
Consumer indirect
Allowance for credit loss - loans
Charge-offs			(6,490)
Purchase of loans				9,400
Consumer direct
Allowance for credit loss - loans
Charge-offs			(1,366)
Purchase of loans				12,500
Home equity
Allowance for credit loss - loans
Charge-offs			(90)
Purchase of loans				18,300
Consumer mortgage
Allowance for credit loss - loans
Charge-offs			(366)
Purchase of loans				271,400
Commercial Portfolio Segment | Commercial Loan
Allowance for credit loss - loans
Beginning balance	25,291	23,241	23,297	22,995
Charge-offs	(139)	(495)	(618)	(650)
Recoveries	152	755	437	1,249
Provision	740	430	2,928	266
Ending balance	26,044	23,931	26,044	23,931
Allowance for credit loss
PCD Allowance at Acquisition				71
Residential Portfolio Segment | Home equity
Allowance for credit loss - loans
Beginning balance	1,600	1,470	1,594	1,814
Charge-offs	(6)	(32)	(90)	(69)
Recoveries	2	11	13	132
Provision	(2)	132	77	(296)
Ending balance	1,594	1,581	1,594	1,581
Allowance for credit loss
PCD Allowance at Acquisition				0
Residential Portfolio Segment | Consumer mortgage
Allowance for credit loss - loans
Beginning balance	14,553	12,631	14,343	10,017
Charge-offs	(143)	(113)	(366)	(230)
Recoveries	3	4	35	21
Provision	558	1,296	959	4,010
Ending balance	14,971	13,818	14,971	13,818
Allowance for credit loss
PCD Allowance at Acquisition				0
Consumer Portfolio Segment | Consumer indirect
Allowance for credit loss - loans
Beginning balance	17,808	14,378	17,852	11,737
Charge-offs	(2,100)	(1,706)	(6,490)	(5,183)
Recoveries	1,319	1,386	4,342	3,732
Provision	1,263	2,960	2,586	6,732
Ending balance	18,290	17,018	18,290	17,018
Allowance for credit loss
PCD Allowance at Acquisition				0
Consumer Portfolio Segment | Consumer direct
Allowance for credit loss - loans
Beginning balance	3,032	2,822	2,973	2,306
Charge-offs	(554)	(342)	(1,366)	(859)
Recoveries	217	174	637	577
Provision	351	361	802	991
Ending balance	3,046	3,015	3,046	3,015
Allowance for credit loss
PCD Allowance at Acquisition				0
Unallocated Financing Receivables
Allowance for credit loss - loans
Beginning balance	1,000	1,000	1,000	1,000
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	0	0	0	0
Ending balance	$ 1,000	$ 1,000	$ 1,000	1,000
Allowance for credit loss
PCD Allowance at Acquisition				$ 0

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