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Percentage of peak unemployment due to decrease in economic activity.

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Amount of increase in allowance for credit loss on financing receivable from recovery.

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LOANS - Allowance for Credit Losses (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Allowance for loan losses [Roll Forward]
Balance, beginning of period	$ 49,911	$ 49,284
Charge-offs		(13,727)
Recoveries		5,924
Provision	14,212	8,430	$ 10,837
Balance, end of period	60,869	49,911	49,284
Liabilities for off-balance-sheet credit exposures [Roll Forward]
Beginning balance	0
Ending balance		0
Total allowance for credit losses [Roll Forward]
Beginning balance	$ 49,911
Ending balance		$ 49,911
Allowance for Credit Losses
Allowance for credit losses to total loans ratio	0.82
Increase in allowance for credit losses to total loans ratio over the same quarter in prior year		0.10%
Minimum [Member] \| COVID-19 [Member]
Allowance for Credit Losses
Percentage of peak unemployment	6.00%
Maximum [Member] \| COVID-19 [Member]
Allowance for Credit Losses
Percentage of peak unemployment	10.00%
ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	$ 51,268
Charge-offs	(10,664)
Recoveries	5,689
Steuben acquisition	3,662
Provision	10,914
Balance, end of period	60,869	$ 51,268
Liabilities for off-balance-sheet credit exposures [Roll Forward]
Beginning balance	1,185
Charge-offs	0
Recoveries	0
Steuben acquisition	67
Provision	237
Ending balance	1,489	1,185
Total allowance for credit losses [Roll Forward]
Beginning balance	52,453
Charge-offs	(10,664)
Recoveries	5,689
Steuben acquisition	3,729
Provision	11,151
Ending balance	62,358	52,453
ASU 2016-13 [Member] \| Cumulative Effect Adjustment [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	(1,357)
Balance, end of period	(3,100)	(1,357)
Liabilities for off-balance-sheet credit exposures [Roll Forward]
Beginning balance	1,185
Ending balance		1,185
Total allowance for credit losses [Roll Forward]
Beginning balance	2,542
Ending balance		2,542
Acquired Impaired Loans [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	163	33
Charge-offs		0
Recoveries		0
Provision		130
Balance, end of period		163	33
Acquired Impaired Loans [Member] \| ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	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
Balance, end of period		163
Acquired Deteriorated
Allowance for loan losses [Roll Forward]
Balance, beginning of period	0
Balance, end of period		0
Acquired Deteriorated \| ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	3,072
Charge-offs	(91)
Recoveries	440
Steuben acquisition	668
Provision	(2,207)
Balance, end of period	1,882	3,072
Acquired Deteriorated \| ASU 2016-13 [Member] \| Cumulative Effect Adjustment [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	(3,072)
Balance, end of period		(3,072)
Consumer Mortgage [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	10,269
Balance, end of period		10,269
Consumer Mortgage [Member] \| ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	9,218
Charge-offs	(862)
Recoveries	130
Steuben acquisition	146
Provision	2,040
Balance, end of period	10,672	9,218
Consumer Mortgage [Member] \| ASU 2016-13 [Member] \| Cumulative Effect Adjustment [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	1,051
Balance, end of period		1,051
Commercial Portfolio Segment [Member] \| Business Lending [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	19,426	18,522
Charge-offs		(2,334)
Recoveries		826
Provision		2,412
Balance, end of period		19,426	18,522
Commercial Portfolio Segment [Member] \| Business Lending [Member] \| ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	19,714
Charge-offs	(1,497)
Recoveries	356
Steuben acquisition	2,343
Provision	7,274
Balance, end of period	28,190	19,714
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
Balance, end of period		288
Residential Portfolio Segment [Member] \| Consumer Mortgage [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	10,269	10,124
Charge-offs		(1,372)
Recoveries		60
Provision		1,457
Balance, end of period		10,269	10,124
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	13,712	14,366
Charge-offs		(7,631)
Recoveries		4,180
Provision		2,797
Balance, end of period		13,712	14,366
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member] \| ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	12,715
Charge-offs	(6,382)
Recoveries	3,992
Steuben acquisition	183
Provision	3,188
Balance, end of period	13,696	12,715
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
Balance, end of period		997
Consumer Portfolio Segment [Member] \| Consumer Direct [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	3,255	3,095
Charge-offs		(1,945)
Recoveries		710
Provision		1,395
Balance, end of period		3,255	3,095
Consumer Portfolio Segment [Member] \| Consumer Direct [Member] \| ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	2,612
Charge-offs	(1,633)
Recoveries	743
Steuben acquisition	87
Provision	1,398
Balance, end of period	3,207	2,612
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
Balance, end of period		643
Consumer Portfolio Segment [Member] \| Home Equity [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	2,129	2,144
Charge-offs		(445)
Recoveries		148
Provision		282
Balance, end of period		2,129	2,144
Consumer Portfolio Segment [Member] \| Home Equity [Member] \| ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	2,937
Charge-offs	(199)
Recoveries	28
Steuben acquisition	235
Provision	(779)
Balance, end of period	2,222	2,937
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)
Balance, end of period		(808)
Unallocated Financing Receivables [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	957	1,000
Charge-offs		0
Recoveries		0
Provision		(43)
Balance, end of period		957	$ 1,000
Unallocated Financing Receivables [Member] \| ASU 2016-13 [Member]
Allowance for loan losses [Roll Forward]
Balance, beginning of period	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)
Balance, end of period		$ (43)