LOANS AND ALLOWANCE FOR CREDIT LOSSES - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Allowance for credit loss - loans
Beginning balance	$ 61,059	$ 50,672
Charge-offs	(784)		$ (8,965)
Recoveries			6,118
Provision			(8,153)
Ending balance	66,669	61,059	50,672
Accrued interest receivable on loans	$ 31,200	$ 25,100
Financing Receivable, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration]	Interest Receivable	Interest Receivable
Purchase of loans		$ 436,900
Amount pledged	$ 9,637,929	8,748,335
Amounts outstanding	406,900
Foreclosures in progress	5,800	3,500
Sale of secondary market eligible residential consumer mortgage loans	6,100	5,300
Provision	11,203	14,773	(8,839)
Liabilities for off-balance-sheet
Beginning balance	1,123	803
Charge-offs	0	0	0
Recoveries	0	0	0
PCD Allowance at Acquisition		0
Provision	(210)		(686)
Ending balance	913	1,123	803
Allowance for credit loss
Beginning balance		49,869
Charge-offs			(8,965)
Recoveries			6,118
Provision			(8,839)
Ending balance			49,869
Assets pledged as collateral | Borrowings from FHLB and FRB
Allowance for credit loss - loans
Amount pledged	4,390,000
Topic ASC 326 [Member]
Liabilities for off-balance-sheet
Provision		320
Elmira Savings Bank ("Elmira")
Allowance for credit loss - loans
Beginning balance	61,100
Ending balance	66,700	61,100
Commercial Loan
Allowance for credit loss - loans
Purchase of loans		125,300
Consumer indirect
Allowance for credit loss - loans
Charge-offs	(9,352)
Purchase of loans		9,400
Consumer direct
Allowance for credit loss - loans
Charge-offs	(2,009)
Purchase of loans		12,500
Home equity
Allowance for credit loss - loans
Charge-offs	(119)
Purchase of loans		18,300
Consumer mortgage
Allowance for credit loss - loans
Charge-offs	(669)
Purchase of loans		271,400
Unallocated
Allowance for credit loss - loans
Beginning balance		1,000
Charge-offs			0
Recoveries			0
Provision			0
Ending balance			1,000
Commercial Portfolio Segment
Allowance for credit loss - loans
Beginning balance			62,358
Liabilities for off-balance-sheet
Beginning balance			1,489
Allowance for credit loss
Beginning balance			60,869
Commercial Portfolio Segment | Commercial Loan
Allowance for credit loss - loans
Beginning balance	23,297	22,995	30,072
Charge-offs			(1,922)
Recoveries			796
Provision			(5,951)
Ending balance		23,297	22,995
Allowance for credit loss
Charge-offs		(824)
Recoveries		1,374
PCD Allowance at Acquisition		71
Commercial Portfolio Segment | Commercial Loan | Topic ASC 326 [Member]
Allowance for credit loss - loans
Provision		(319)
Commercial Portfolio Segment | Unallocated
Allowance for credit loss - loans
Beginning balance			1,000
Residential Portfolio Segment | Home equity
Allowance for credit loss - loans
Beginning balance	1,594	1,814	2,222
Charge-offs	(119)	(86)	(225)
Recoveries	14	163	92
Provision	139		(275)
Ending balance	1,628	1,594	1,814
Allowance for credit loss
PCD Allowance at Acquisition		0
Residential Portfolio Segment | Home equity | Topic ASC 326 [Member]
Allowance for credit loss - loans
Provision		(297)
Residential Portfolio Segment | Consumer mortgage
Allowance for credit loss - loans
Beginning balance	14,343	10,017	10,672
Charge-offs	(669)	(313)	(426)
Recoveries	48	62	91
Provision	1,611		(320)
Ending balance	15,333	14,343	10,017
Allowance for credit loss
PCD Allowance at Acquisition		0
Residential Portfolio Segment | Consumer mortgage | Topic ASC 326 [Member]
Allowance for credit loss - loans
Provision		4,577
Consumer Portfolio Segment | Commercial Loan
Allowance for credit loss - loans
Beginning balance	23,297
Charge-offs	(784)
Recoveries	523
Provision	3,818
Ending balance	26,854	23,297
Consumer Portfolio Segment | Consumer indirect
Allowance for credit loss - loans
Beginning balance	17,852	11,737	13,696
Charge-offs	(9,352)	(7,986)	(5,160)
Recoveries	5,719	4,756	4,346
Provision	4,366		(1,145)
Ending balance	18,585	17,852	11,737
Allowance for credit loss
PCD Allowance at Acquisition		0
Consumer Portfolio Segment | Consumer indirect | Topic ASC 326 [Member]
Allowance for credit loss - loans
Provision		9,345
Consumer Portfolio Segment | Consumer direct
Allowance for credit loss - loans
Beginning balance	2,973	2,306	3,207
Charge-offs	(2,009)	(1,252)	(1,232)
Recoveries	826	772	793
Provision	1,479		(462)
Ending balance	3,269	2,973	2,306
Allowance for credit loss
PCD Allowance at Acquisition		0
Consumer Portfolio Segment | Consumer direct | Topic ASC 326 [Member]
Allowance for credit loss - loans
Provision		1,147
Unallocated
Allowance for credit loss - loans
Beginning balance	1,000	1,000
Charge-offs	0	0
Recoveries	0	0
Provision	0
Ending balance	1,000	1,000	1,000
Allowance for credit loss
PCD Allowance at Acquisition		0
Unallocated | Topic ASC 326 [Member]
Allowance for credit loss - loans
Provision		0
Allowances For Credit Losses [Member]
Allowance for credit loss
Beginning balance	61,059	49,869
Charge-offs	(12,933)	(10,461)
Recoveries	7,130	7,127
PCD Allowance at Acquisition		71
Provision	11,413
Ending balance	66,669	61,059	49,869
Allowances For Credit Losses [Member] | Topic ASC 326 [Member]
Allowance for credit loss
Provision		14,453
Total Allowances For Credit Losses [Member]
Allowance for credit loss - loans
Beginning balance	62,182	50,672
Charge-offs	(12,933)	(10,461)
Recoveries	7,130	7,127
Provision	11,203
Ending balance	$ 67,582	62,182	$ 50,672
Allowance for credit loss
PCD Allowance at Acquisition		71
Total Allowances For Credit Losses [Member] | Topic ASC 326 [Member]
Allowance for credit loss - loans
Provision		$ 14,773