LOANS AND ALLOWANCE FOR CREDIT LOSSES - Allowance for Credit Losses (Details) $ in Thousands				3 Months Ended		9 Months Ended
	May 13, 2022 USD ($)	Dec. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)
Allowance for credit losses - loans
Beginning Balance				$ 55,542	$ 51,750	$ 49,869	$ 60,869
Charge-offs				(2,688)	(2,703)	(6,991)	(5,775)
Recoveries				2,330	1,353	5,711	4,636
PCD Allowance at Acquisition						71
Provision				5,179	(901)	11,703	(10,231)
Ending Balance		$ 49,869	$ 49,499	60,363	49,499	60,363	49,499
Liabilities for off-balance-sheet credit exposures
Beginning balance				1,223	762	803	1,489
Charge-offs				0	0	0	0
Recoveries				0	0	0	0
PCD allowance at acquisition					(43)	0
Provision				(118)		302	(770)
Ending balance		$ 803	$ 719	$ 1,105	719	$ 1,105	719
Allowance for Credit Losses
Allowance for credit losses to total loans ratio				0.71		0.71
Provision for credit losses				$ 5,061	(944)	$ 12,005	(11,001)
Basis points lower level		3	3
Accrued interest and fees receivable				19,800		19,800
Total allowance for credit losses
Beginning balance				56,765	52,512	50,672	62,358
Charge-offs				(2,688)	(2,703)	(6,991)	(5,775)
Recoveries				2,330	1,353	5,711	4,636
PCD Allowance at Acquisition						71
Provision				5,061	(944)	12,005	(11,001)
Ending balance		$ 50,672	$ 50,218	61,468	50,218	61,468	50,218
Elmira Savings Bank
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				23,241	25,302	22,995	30,072
Charge-offs				(495)	(872)	(650)	(925)
Recoveries				755	204	1,249	586
PCD Allowance at Acquisition						71
Provision				430	(2,315)	266	(7,414)
Ending Balance		22,995	22,319	23,931	22,319	23,931	22,319
Total allowance for credit losses
Charge-offs				(495)	(872)	(650)	(925)
Recoveries				755	204	1,249	586
PCD Allowance at Acquisition						71
Residential Portfolio Segment | Consumer mortgage
Allowance for credit losses - loans
Beginning Balance				12,631	10,001	10,017	10,672
Charge-offs				(113)	(127)	(230)	(369)
Recoveries				4	3	21	22
PCD Allowance at Acquisition						0
Provision				1,296	(132)	4,010	(580)
Ending Balance		10,017	9,745	13,818	9,745	13,818	9,745
Total allowance for credit losses
Charge-offs				(113)	(127)	(230)	(369)
Recoveries				4	3	21	22
PCD Allowance at Acquisition						0
Residential Portfolio Segment | Home equity
Allowance for credit losses - loans
Beginning Balance				1,470	1,796	1,814	2,222
Charge-offs				(32)	(30)	(69)	(145)
Recoveries				11	10	132	19
PCD Allowance at Acquisition						0
Provision				132	(56)	(296)	(376)
Ending Balance		1,814	1,720	1,581	1,720	1,581	1,720
Total allowance for credit losses
Charge-offs				(32)	(30)	(69)	(145)
Recoveries				11	10	132	19
PCD Allowance at Acquisition						0
Consumer Portfolio Segment | Consumer indirect
Allowance for credit losses - loans
Beginning Balance				14,378	11,103	11,737	13,696
Charge-offs				(1,706)	(1,365)	(5,183)	(3,514)
Recoveries				1,386	973	3,732	3,402
PCD Allowance at Acquisition						0
Provision				2,960	1,334	6,732	(1,539)
Ending Balance		11,737	12,045	17,018	12,045	17,018	12,045
Total allowance for credit losses
Charge-offs				(1,706)	(1,365)	(5,183)	(3,514)
Recoveries				1,386	973	3,732	3,402
PCD Allowance at Acquisition						0
Consumer Portfolio Segment | Consumer direct
Allowance for credit losses - loans
Beginning Balance				2,822	2,548	2,306	3,207
Charge-offs				(342)	(309)	(859)	(822)
Recoveries				174	163	577	607
PCD Allowance at Acquisition						0
Provision				361	268	991	(322)
Ending Balance		2,306	2,670	3,015	2,670	3,015	2,670
Total allowance for credit losses
Charge-offs				(342)	(309)	(859)	(822)
Recoveries				174	163	577	607
PCD Allowance at Acquisition						0
Unallocated
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
Provision				0	0	0	0
Ending Balance		$ 1,000	$ 1,000	1,000	1,000	1,000	1,000
Total allowance for credit losses
Charge-offs				0	0	0	0
Recoveries				$ 0	$ 0	0	$ 0
PCD Allowance at Acquisition						$ 0