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Percentage of peak unemployment due to decrease in economic activity.

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LOANS - Allowance for Credit Losses (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Allowance for loan losses [Roll Forward]
Balance, beginning of period	$ 55,069	$ 55,652	$ 60,869	$ 49,911	$ 49,911
Charge-offs	(1,106)	(2,094)	(3,072)
Recoveries	1,698	1,184	3,283
Steuben acquisition		3,662
Provision	(3,911)	6,033	(9,330)
Balance, end of period	51,750	64,437	51,750	64,437	60,869
Liabilities for off-balance-sheet credit exposures [Roll Forward]
Beginning balance	1,189	845	1,489	0	0
Charge-offs	0	0	0
Recoveries	0	0	0
Steuben acquisition		67
Provision	(427)	540	(727)
Ending balance	762	1,452	762	1,452	1,489
Total allowance for credit losses [Roll Forward]
Beginning balance	56,258	56,497	62,358	49,911	49,911
Charge-offs	(1,106)	(2,094)	(3,072)
Recoveries	1,698	1,184	3,283
Steuben acquisition		3,729
Provision	(4,338)	6,573	(10,057)
Ending balance	$ 52,512	65,889	$ 52,512	65,889	$ 62,358
Allowance for Credit Losses
Allowance for credit losses to total loans ratio	0.71		0.71
Basis points lower level			0.15		0.11
Accrued interest and fees receivable	$ 19,000		$ 19,000
Minimum [Member] \| COVID-19 [Member]
Allowance for Credit Losses
Percentage of peak unemployment			3.00%
Maximum [Member] \| COVID-19 [Member]
Allowance for Credit Losses
Percentage of peak unemployment			9.00%
ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				51,268	$ 51,268
Charge-offs				(5,141)
Recoveries				2,681
Steuben acquisition				3,662
Provision				11,967
Balance, end of period		64,437		64,437
Liabilities for off-balance-sheet credit exposures [Roll Forward]
Beginning balance				1,185	1,185
Charge-offs				0
Recoveries				0
Steuben acquisition				67
Provision				200
Ending balance		1,452		1,452
Total allowance for credit losses [Roll Forward]
Beginning balance				52,453	52,453
Charge-offs				(5,141)
Recoveries				2,681
Steuben acquisition				3,729
Provision				12,167
Ending balance		65,889		65,889
ASU 2016-13 [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] \| ASU 2016-13 [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] \| ASU 2016-13 [Member] \| Cumulative Effect Adjustment [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				(163)	(163)
Acquired Deteriorated
Allowance for loan losses [Roll Forward]
Balance, beginning of period	1,996	3,046	$ 1,882	0	0
Charge-offs	0	0	0
Recoveries	33	48	60
Steuben acquisition		528
Provision	(144)	(61)	(57)
Balance, end of period	1,885	3,561	1,885	3,561	1,882
Acquired Deteriorated \| ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				3,072	3,072
Charge-offs				0
Recoveries				48
Steuben acquisition				528
Provision				(87)
Balance, end of period		3,561		3,561
Acquired Deteriorated \| ASU 2016-13 [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	27,042	19,489	28,190	19,426	19,426
Charge-offs	(2)	(7)	(53)
Recoveries	255	84	322
Steuben acquisition		2,483
Provision	(3,878)	2,155	(5,042)
Balance, end of period	23,417	24,204	23,417	24,204	28,190
Commercial Portfolio Segment [Member] \| Business Lending [Member] \| ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				19,714	19,714
Charge-offs				(183)
Recoveries				222
Steuben acquisition				2,483
Provision				1,968
Balance, end of period		24,204		24,204
Commercial Portfolio Segment [Member] \| Business Lending [Member] \| ASU 2016-13 [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	9,686	12,430	10,672	10,269	10,269
Charge-offs	(142)	(234)	(242)
Recoveries	9	36	19
Steuben acquisition		146
Provision	448	710	(448)
Balance, end of period	10,001	13,088	10,001	13,088	10,672
Residential Portfolio Segment [Member] \| Consumer Mortgage [Member] \| ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				9,218	9,218
Charge-offs				(420)
Recoveries				44
Steuben acquisition				146
Provision				4,100
Balance, end of period		13,088		13,088
Residential Portfolio Segment [Member] \| Consumer Mortgage [Member] \| ASU 2016-13 [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	11,120	13,694	13,696	13,712	13,712
Charge-offs	(750)	(1,431)	(2,149)
Recoveries	1,183	833	2,429
Steuben acquisition		183
Provision	(450)	2,587	(2,873)
Balance, end of period	11,103	15,866	11,103	15,866	13,696
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member] \| ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				12,715	12,715
Charge-offs				(3,510)
Recoveries				1,996
Steuben acquisition				183
Provision				4,482
Balance, end of period		15,866		15,866
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member] \| ASU 2016-13 [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	2,682	3,737	3,207	3,255	3,255
Charge-offs	(195)	(341)	(513)
Recoveries	213	171	444
Steuben acquisition		87
Provision	(152)	374	(590)
Balance, end of period	2,548	4,028	2,548	4,028	3,207
Consumer Portfolio Segment [Member] \| Consumer Direct [Member] \| ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				2,612	2,612
Charge-offs				(874)
Recoveries				353
Steuben acquisition				87
Provision				1,850
Balance, end of period		4,028		4,028
Consumer Portfolio Segment [Member] \| Consumer Direct [Member] \| ASU 2016-13 [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	1,543	2,484	2,222	2,129	2,129
Charge-offs	(17)	(81)	(115)
Recoveries	5	12	9
Steuben acquisition		235
Provision	265	40	(320)
Balance, end of period	1,796	2,690	1,796	2,690	2,222
Consumer Portfolio Segment [Member] \| Home Equity [Member] \| ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				2,937	2,937
Charge-offs				(154)
Recoveries				18
Steuben acquisition				235
Provision				(346)
Balance, end of period		2,690		2,690
Consumer Portfolio Segment [Member] \| Home Equity [Member] \| ASU 2016-13 [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	772	1,000	957	957
Charge-offs	0	0	0
Recoveries	0	0	0
Steuben acquisition		0
Provision	0	228	0
Balance, end of period	$ 1,000	1,000	$ 1,000	1,000	1,000
Unallocated Financing Receivables [Member] \| ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				1,000	1,000
Charge-offs				0
Recoveries				0
Steuben acquisition				0
Provision				0
Balance, end of period		$ 1,000		1,000
Unallocated Financing Receivables [Member] \| ASU 2016-13 [Member] \| Cumulative Effect Adjustment [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period				$ 43	$ 43