LOANS AND ALLOWANCE FOR CREDIT LOSSES - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Allowance for credit loss - loans
Beginning balance	$ 63,170	$ 50,147	$ 61,059	$ 49,869
Charge-offs	(2,758)	(2,147)	(5,988)	(4,303)
Recoveries	2,052	1,764	3,771	3,381
Provision	820	5,707	4,442	6,524
Ending balance	63,284	55,542	63,284	55,542
Accrued interest receivable on loans	$ 25,000		$ 25,000
Financing Receivable, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration]	Interest Receivable		Interest Receivable
Purchase of secondary market eligible residential consumer mortgage loans				436,900
Sale of secondary market eligible residential consumer mortgage loans			$ 1,800	3,700
Liabilities for off-balance-sheet
Beginning balance	$ 1,001	892	1,123	803
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
PCD Allowance at Acquisition		0		0
Provision	(68)	331	(190)	420
Ending balance	933	1,223	933	1,223
Allowance for credit loss
Beginning balance	64,171	51,039	62,182	50,672
Charge-offs	(2,758)	(2,147)	(5,988)	(4,303)
Recoveries	2,052	1,764	3,771	3,381
PCD Allowance at Acquisition		71		71
Provision	752	6,038	4,252	6,944
Ending balance	64,217	56,765	64,217	56,765
Elmira Savings Bank ("Elmira")
Allowance for credit loss - loans
Ending balance	63,300	55,500	63,300	55,500
Commercial Loan
Allowance for credit loss - loans
Charge-offs			(479)
Consumer indirect
Allowance for credit loss - loans
Charge-offs			(4,390)
Consumer direct
Allowance for credit loss - loans
Charge-offs			(812)
Home equity
Allowance for credit loss - loans
Charge-offs			(84)
Consumer mortgage
Allowance for credit loss - loans
Charge-offs			(223)
Commercial Portfolio Segment | Commercial Loan
Allowance for credit loss - loans
Beginning balance	25,227	21,764	23,297	22,995
Charge-offs	(304)	(39)	(479)	(155)
Recoveries	113	155	285	494
Provision	255	1,290	2,188	(164)
Ending balance	25,291	23,241	25,291	23,241
Allowance for credit loss
PCD Allowance at Acquisition		71		71
Residential Portfolio Segment | Home equity
Allowance for credit loss - loans
Beginning balance	1,588	1,468	1,594	1,814
Charge-offs	(84)	(26)	(84)	(37)
Recoveries	5	28	11	121
Provision	91	0	79	(428)
Ending balance	1,600	1,470	1,600	1,470
Allowance for credit loss
PCD Allowance at Acquisition		0		0
Residential Portfolio Segment | Consumer mortgage
Allowance for credit loss - loans
Beginning balance	14,278	10,324	14,343	10,017
Charge-offs	(204)	(77)	(223)	(117)
Recoveries	25	8	32	17
Provision	454	2,376	401	2,714
Ending balance	14,553	12,631	14,553	12,631
Allowance for credit loss
PCD Allowance at Acquisition		0		0
Consumer Portfolio Segment | Consumer indirect
Allowance for credit loss - loans
Beginning balance	18,047	12,866	17,852	11,737
Charge-offs	(1,859)	(1,789)	(4,390)	(3,477)
Recoveries	1,676	1,346	3,023	2,346
Provision	(56)	1,955	1,323	3,772
Ending balance	17,808	14,378	17,808	14,378
Allowance for credit loss
PCD Allowance at Acquisition		0		0
Consumer Portfolio Segment | Consumer direct
Allowance for credit loss - loans
Beginning balance	3,030	2,725	2,973	2,306
Charge-offs	(307)	(216)	(812)	(517)
Recoveries	233	227	420	403
Provision	76	86	451	630
Ending balance	3,032	2,822	3,032	2,822
Allowance for credit loss
PCD Allowance at Acquisition		0		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		$ 0