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LOANS AND ALLOWANCE FOR CREDIT LOSSES - Allowance for Credit Losses (Details) $ in Thousands		3 Months Ended		6 Months Ended
	May 13, 2022 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)
Allowance for credit losses - loans
Beginning Balance		$ 50,147	$ 55,069	$ 49,869	$ 60,869
Charge-offs		(2,147)	(1,106)	(4,303)	(3,072)
Recoveries		1,764	1,698	3,381	3,283
PCD Allowance at Acquisition		71		71
Provision		6,038	(4,338)	6,944	(10,057)
Provision		5,707	(3,911)	6,524	(9,330)
Ending Balance		55,542	51,750	55,542	51,750
Liabilities for off-balance-sheet credit exposures [Roll Forward]
Beginning balance		892	1,189	803	1,489
Charge-offs		0	0	0	0
Recoveries		0	0	0	0
PCD allowance at acquisition		0	(427)	0
Provision		331		420	(727)
Ending balance		$ 1,223	762	$ 1,223	762
Allowance for Credit Losses
Allowance for credit losses to total loans ratio		0.68		0.68
Provision for credit losses		$ 6,038	(4,338)	$ 6,944	$ (10,057)
Basis points lower level					3
Accrued interest and fees receivable		19,700		19,700
Total allowance for credit losses [Roll Forward]
Beginning balance		51,039	56,258	50,672	$ 62,358
Charge-offs		(2,147)	(1,106)	(4,303)	(3,072)
Recoveries		1,764	1,698	3,381	3,283
PCD Allowance at Acquisition		71		71
Provision		6,038	(4,338)	6,944	(10,057)
Ending balance		56,765	52,512	56,765	52,512
Elmira Savings Bank [Member]
Allowance for Credit Losses
Provision for credit losses	$ 3,900	3,900		3,900
Commercial Portfolio Segment | Business Lending
Allowance for credit losses - loans
Beginning Balance		21,764	29,038	22,995	30,072
Charge-offs		(39)	(2)	(155)	(53)
Recoveries		155	288	494	382
PCD Allowance at Acquisition		71		71
Provision		1,290	(4,022)	(164)	(5,099)
Ending Balance		23,241	25,302	23,241	25,302
Total allowance for credit losses [Roll Forward]
Charge-offs		(39)	(2)	(155)	(53)
Recoveries		155	288	494	382
PCD Allowance at Acquisition		71		71
Residential Portfolio Segment | Consumer Mortgage
Allowance for credit losses - loans
Beginning Balance		10,324	9,686	10,017	10,672
Charge-offs		(77)	(142)	(117)	(242)
Recoveries		8	9	17	19
PCD Allowance at Acquisition		0		0
Provision		2,376	448	2,714	(448)
Ending Balance		12,631	10,001	12,631	10,001
Total allowance for credit losses [Roll Forward]
Charge-offs		(77)	(142)	(117)	(242)
Recoveries		8	9	17	19
PCD Allowance at Acquisition		0		0
Consumer Portfolio Segment | Consumer Indirect
Allowance for credit losses - loans
Beginning Balance		12,866	11,120	11,737	13,696
Charge-offs		(1,789)	(750)	(3,477)	(2,149)
Recoveries		1,346	1,183	2,346	2,429
PCD Allowance at Acquisition		0		0
Provision		1,955	(450)	3,772	(2,873)
Ending Balance		14,378	11,103	14,378	11,103
Total allowance for credit losses [Roll Forward]
Charge-offs		(1,789)	(750)	(3,477)	(2,149)
Recoveries		1,346	1,183	2,346	2,429
PCD Allowance at Acquisition		0		0
Consumer Portfolio Segment | Consumer Direct
Allowance for credit losses - loans
Beginning Balance		2,725	2,682	2,306	3,207
Charge-offs		(216)	(195)	(517)	(513)
Recoveries		227	213	403	444
PCD Allowance at Acquisition		0		0
Provision		86	(152)	630	(590)
Ending Balance		2,822	2,548	2,822	2,548
Total allowance for credit losses [Roll Forward]
Charge-offs		(216)	(195)	(517)	(513)
Recoveries		227	213	403	444
PCD Allowance at Acquisition		0		0
Consumer Portfolio Segment | Home Equity
Allowance for credit losses - loans
Beginning Balance		1,468	1,543	1,814	2,222
Charge-offs		(26)	(17)	(37)	(115)
Recoveries		28	5	121	9
PCD Allowance at Acquisition		0		0
Provision		0	265	(428)	(320)
Ending Balance		1,470	1,796	1,470	1,796
Total allowance for credit losses [Roll Forward]
Charge-offs		(26)	(17)	(37)	(115)
Recoveries		28	5	121	9
PCD Allowance at Acquisition		0		0
Unallocated Financing Receivables [Member]
Allowance for credit losses - loans
Beginning Balance		1,000	1,000	1,000	1,000
Charge-offs		0	0	0	0
Recoveries		0	0	0	0
PCD Allowance at Acquisition		0		0
Provision		0	0	0	0
Ending Balance		1,000	1,000	1,000	1,000
Total allowance for credit losses [Roll Forward]
Charge-offs		0	0	0	0
Recoveries		0	$ 0	0	$ 0
PCD Allowance at Acquisition		$ 0		$ 0

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