LOANS - Allowance for Credit Losses (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Allowance for loan losses [Roll Forward]
Balance, beginning of period	$ 51,750	$ 64,437	$ 60,869	$ 49,911	$ 49,911
Charge-offs	(2,703)	(2,720)	(5,775)
Recoveries	1,353	1,463	4,636
Steuben acquisition		1,782
Provision	(901)		(10,231)
Balance, end of period	49,499	64,962	49,499	64,962	60,869
Liabilities for off-balance-sheet credit exposures [Roll Forward]
Beginning balance	762	1,452	1,489	0	0
Charge-offs	0	0	0
Recoveries	0	0	0
Steuben acquisition		163
Provision	(43)		(770)
Ending balance	719	1,615	719	1,615	1,489
Total allowance for credit losses [Roll Forward]
Beginning balance	52,512	65,889	62,358	49,911	49,911
Charge-offs	(2,703)	(2,720)	(5,775)
Recoveries	1,353	1,463	4,636
Steuben acquisition		1,945
Provision	(944)		(11,001)
Ending balance	$ 50,218	66,577	$ 50,218	66,577	$ 62,358
Allowance for Credit Losses
Allowance for credit losses to total loans ratio	0.68		0.68
Basis points lower level			19		14
Accrued interest and fees receivable	$ 18,200		$ 18,200
Minimum [Member] | COVID-19 [Member]
Allowance for Credit Losses
Percentage of peak unemployment			2.90%
Maximum [Member] | COVID-19 [Member]
Allowance for Credit Losses
Percentage of peak unemployment			9.10%
Topic ASC 326 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				51,268	$ 51,268
Charge-offs				(7,861)
Recoveries				4,144
Steuben acquisition				3,662
Provision				13,749
Balance, end of period		64,962		64,962
Liabilities for off-balance-sheet credit exposures [Roll Forward]
Beginning balance				1,185	1,185
Charge-offs				0
Recoveries				0
Steuben acquisition				67
Provision				363
Ending balance		1,615		1,615
Total allowance for credit losses [Roll Forward]
Beginning balance				52,453	52,453
Charge-offs				(7,861)
Recoveries				4,144
Steuben acquisition				3,729
Provision				14,112
Ending balance		66,577		66,577
Topic ASC 326 [Member] | Cumulative Effect Adjustment [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				1,357	1,357
Liabilities for off-balance-sheet credit exposures [Roll Forward]
Beginning balance				1,185	1,185
Total allowance for credit losses [Roll Forward]
Beginning balance				2,542	2,542
Acquired Impaired Loans [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				163	163
Acquired Impaired Loans [Member] | Topic ASC 326 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				0	0
Charge-offs				0
Recoveries				0
Steuben acquisition				0
Provision				0
Balance, end of period		0		0
Acquired Impaired Loans [Member] | Topic ASC 326 [Member] | Cumulative Effect Adjustment [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				(163)	(163)
Acquired Impaired [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	1,885	3,561	$ 1,882	0	0
Charge-offs	0	(91)	0
Recoveries	35	32	95
Steuben acquisition		45
Provision	373		316
Balance, end of period	2,293	3,547	2,293	3,547	1,882
Acquired Impaired [Member] | Topic ASC 326 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				3,072	3,072
Charge-offs				(91)
Recoveries				80
Steuben acquisition				528
Provision				(42)
Balance, end of period		3,547		3,547
Acquired Impaired [Member] | Topic ASC 326 [Member] | Cumulative Effect Adjustment [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				3,072	3,072
Commercial Portfolio Segment [Member] | Business Lending [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	23,417	24,204	28,190	19,426	19,426
Charge-offs	(872)	(736)	(925)
Recoveries	169	67	491
Steuben acquisition		3,539
Provision	(2,688)		(7,730)
Balance, end of period	20,026	27,074	20,026	27,074	28,190
Commercial Portfolio Segment [Member] | Business Lending [Member] | Topic ASC 326 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				19,714	19,714
Charge-offs				(919)
Recoveries				289
Steuben acquisition				2,483
Provision				5,507
Balance, end of period		27,074		27,074
Commercial Portfolio Segment [Member] | Business Lending [Member] | Topic ASC 326 [Member] | Cumulative Effect Adjustment [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				288	288
Residential Portfolio Segment [Member] | Consumer Mortgage [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	10,001	13,088	10,672	10,269	10,269
Charge-offs	(127)	(248)	(369)
Recoveries	3	23	22
Steuben acquisition		599
Provision	(132)		(580)
Balance, end of period	9,745	12,264	9,745	12,264	10,672
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Topic ASC 326 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				9,218	9,218
Charge-offs				(668)
Recoveries				67
Steuben acquisition				146
Provision				3,501
Balance, end of period		12,264		12,264
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Topic ASC 326 [Member] | Cumulative Effect Adjustment [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				(1,051)	(1,051)
Consumer Portfolio Segment [Member] | Consumer Indirect [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	11,103	15,866	13,696	13,712	13,712
Charge-offs	(1,365)	(1,281)	(3,514)
Recoveries	973	1,111	3,402
Steuben acquisition		963
Provision	1,334		(1,539)
Balance, end of period	12,045	14,733	12,045	14,733	13,696
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Topic ASC 326 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				12,715	12,715
Charge-offs				(4,791)
Recoveries				3,107
Steuben acquisition				183
Provision				3,519
Balance, end of period		14,733		14,733
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Topic ASC 326 [Member] | Cumulative Effect Adjustment [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				(997)	(997)
Consumer Portfolio Segment [Member] | Consumer Direct [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	2,548	4,028	3,207	3,255	3,255
Charge-offs	(309)	(340)	(822)
Recoveries	163	225	607
Steuben acquisition		120
Provision	268		(322)
Balance, end of period	2,670	3,793	2,670	3,793	3,207
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Topic ASC 326 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				2,612	2,612
Charge-offs				(1,214)
Recoveries				578
Steuben acquisition				87
Provision				1,730
Balance, end of period		3,793		3,793
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Topic ASC 326 [Member] | Cumulative Effect Adjustment [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				(643)	(643)
Consumer Portfolio Segment [Member] | Home Equity [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	1,796	2,690	2,222	2,129	2,129
Charge-offs	(30)	(24)	(145)
Recoveries	10	5	19
Steuben acquisition		124
Provision	(56)		(376)
Balance, end of period	1,720	2,547	1,720	2,547	2,222
Consumer Portfolio Segment [Member] | Home Equity [Member] | Topic ASC 326 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				2,937	2,937
Charge-offs				(178)
Recoveries				23
Steuben acquisition				235
Provision				(470)
Balance, end of period		2,547		2,547
Consumer Portfolio Segment [Member] | Home Equity [Member] | Topic ASC 326 [Member] | Cumulative Effect Adjustment [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				808	808
Unallocated Financing Receivables [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	1,000	1,000	1,000	957	957
Charge-offs	0	0	0
Recoveries	0	0	0
Steuben acquisition		4
Provision	0		0
Balance, end of period	$ 1,000	1,004	$ 1,000	1,004	1,000
Unallocated Financing Receivables [Member] | Topic ASC 326 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				1,000	1,000
Charge-offs				0
Recoveries				0
Steuben acquisition				0
Provision				4
Balance, end of period		$ 1,004		1,004
Unallocated Financing Receivables [Member] | Topic ASC 326 [Member] | Cumulative Effect Adjustment [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				$ 43	$ 43