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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			9 Months Ended		12 Months Ended
LOANS, Allowance for Credit Losses (Details) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Allowance for credit losses - loans [Roll Forward]
Beginning balance		$ 64,437	$ 49,310	$ 49,911	$ 49,284	$ 49,284
Charge-offs		(2,720)	(3,230)		(10,002)
Recoveries		1,463	1,592		4,568
Provision		1,782	1,751		5,573
Ending balance		64,962	49,423	64,962	49,423	49,911
Liabilities for off-balance-sheet credit exposures [Roll Forward]
Beginning balance		1,452		0
Charge-offs		0
Recoveries		0
Provision		163
Ending balance		1,615		1,615		0
Total allowance for credit losses [Roll Forward]
Beginning balance		65,889		49,911
Charge-offs		(2,720)
Recoveries		1,463
Provision		1,945
Ending balance		$ 66,577		$ 66,577		$ 49,911
Allowance for Credit Losses
Allowance for credit losses to total loans ratio		0.0087		0.0087
Increase in allowance for credit losses to total loans ratio over the same quarter in prior year				15.00%		15.00%
Forecast [Member] \| Minimum [Member] \| COVID-19 [Member]
Allowance for Credit Losses
Percentage of peak unemployment	7.00%
Forecast [Member] \| Maximum [Member] \| COVID-19 [Member]
Allowance for Credit Losses
Percentage of peak unemployment	11.00%
Topic ASC 326
Allowance for credit losses - loans [Roll Forward]
Beginning balance				$ 51,268
Charge-offs				(7,861)
Recoveries				4,144
Steuben acquisition				3,662
Provision				13,749
Ending balance		$ 64,962		64,962		$ 51,268
Liabilities for off-balance-sheet credit exposures [Roll Forward]
Beginning balance				1,185
Charge-offs				0
Recoveries				0
Steuben acquisition				67
Provision				363
Ending balance		1,615		1,615		1,185
Total allowance for credit losses [Roll Forward]
Beginning balance				52,453
Charge-offs				(7,861)
Recoveries				4,144
Steuben acquisition				3,729
Provision				14,112
Ending balance		66,577		66,577		52,453
Topic ASC 326 \| Cumulative Effect Adjustment
Allowance for credit losses - loans [Roll Forward]
Beginning balance				1,357
Ending balance						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
Allowance for credit losses - loans [Roll Forward]
Beginning balance			155	163	33	33
Charge-offs			0		0
Recoveries			0		0
Provision			1		123
Ending balance			156		156	163
Acquired Impaired Loans \| Topic ASC 326
Allowance for credit losses - loans [Roll Forward]
Beginning balance				0
Charge-offs				0
Recoveries				0
Steuben acquisition				0
Provision				0
Ending balance		0		0		0
Acquired Impaired Loans \| Topic ASC 326 \| Cumulative Effect Adjustment
Allowance for credit losses - loans [Roll Forward]
Beginning balance				(163)
Ending balance						(163)
Purchased Credit Deteriorated
Allowance for credit losses - loans [Roll Forward]
Beginning balance		3,561		0
Charge-offs		(91)
Recoveries		32
Provision		45
Ending balance		3,547		3,547		0
Purchased Credit Deteriorated \| Topic ASC 326
Allowance for credit losses - loans [Roll Forward]
Beginning balance				3,072
Charge-offs				(91)
Recoveries				80
Steuben acquisition				528
Provision				(42)
Ending balance		3,547		3,547		3,072
Purchased Credit Deteriorated \| Topic ASC 326 \| Cumulative Effect Adjustment
Allowance for credit losses - loans [Roll Forward]
Beginning balance				3,072
Ending balance						3,072
Commercial Portfolio Segment [Member] \| Business Lending [Member]
Allowance for credit losses - loans [Roll Forward]
Beginning balance		24,204	17,769	19,426	18,522	18,522
Charge-offs		(736)	(305)		(1,774)
Recoveries		67	290		593
Provision		3,539	301		714
Ending balance		27,074	18,055	27,074	18,055	19,426
Commercial Portfolio Segment [Member] \| Business Lending [Member] \| Topic ASC 326
Allowance for credit losses - loans [Roll Forward]
Beginning balance				19,714
Charge-offs				(919)
Recoveries				289
Steuben acquisition				2,483
Provision				5,507
Ending balance		27,074		27,074		19,714
Commercial Portfolio Segment [Member] \| Business Lending [Member] \| Topic ASC 326 \| Cumulative Effect Adjustment
Allowance for credit losses - loans [Roll Forward]
Beginning balance				288
Ending balance						288
Residential Portfolio Segment [Member] \| Consumer Mortgage [Member]
Allowance for credit losses - loans [Roll Forward]
Beginning balance		13,088	10,763	10,269	10,124	10,124
Charge-offs		(248)	(200)		(1,040)
Recoveries		23	8		44
Provision		(599)	(94)		1,349
Ending balance		12,264	10,477	12,264	10,477	10,269
Residential Portfolio Segment [Member] \| Consumer Mortgage [Member] \| Topic ASC 326
Allowance for credit losses - loans [Roll Forward]
Beginning balance				9,218
Charge-offs				(668)
Recoveries				67
Steuben acquisition				146
Provision				3,501
Ending balance		12,264		12,264		9,218
Residential Portfolio Segment [Member] \| Consumer Mortgage [Member] \| Topic ASC 326 \| Cumulative Effect Adjustment
Allowance for credit losses - loans [Roll Forward]
Beginning balance				(1,051)
Ending balance						(1,051)
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member]
Allowance for credit losses - loans [Roll Forward]
Beginning balance		15,866	14,345	13,712	14,366	14,366
Charge-offs		(1,281)	(2,224)		(5,529)
Recoveries		1,111	1,095		3,296
Provision		(963)	1,221		2,304
Ending balance		14,733	14,437	14,733	14,437	13,712
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member] \| Topic ASC 326
Allowance for credit losses - loans [Roll Forward]
Beginning balance				12,715
Charge-offs				(4,791)
Recoveries				3,107
Steuben acquisition				183
Provision				3,519
Ending balance		14,733		14,733		12,715
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member] \| Topic ASC 326 \| Cumulative Effect Adjustment
Allowance for credit losses - loans [Roll Forward]
Beginning balance				(997)
Ending balance						(997)
Consumer Portfolio Segment [Member] \| Consumer Direct [Member]
Allowance for credit losses - loans [Roll Forward]
Beginning balance		4,028	3,184	3,255	3,095	3,095
Charge-offs		(340)	(456)		(1,436)
Recoveries		225	167		567
Provision		(120)	383		1,052
Ending balance		3,793	3,278	3,793	3,278	3,255
Consumer Portfolio Segment [Member] \| Consumer Direct [Member] \| Topic ASC 326
Allowance for credit losses - loans [Roll Forward]
Beginning balance				2,612
Charge-offs				(1,214)
Recoveries				578
Steuben acquisition				87
Provision				1,730
Ending balance		3,793		3,793		2,612
Consumer Portfolio Segment [Member] \| Consumer Direct [Member] \| Topic ASC 326 \| Cumulative Effect Adjustment
Allowance for credit losses - loans [Roll Forward]
Beginning balance				(643)
Ending balance						(643)
Consumer Portfolio Segment [Member] \| Home Equity [Member]
Allowance for credit losses - loans [Roll Forward]
Beginning balance		2,690	2,103	2,129	2,144	2,144
Charge-offs		(24)	(45)		(223)
Recoveries		5	32		68
Provision		(124)	(8)		93
Ending balance		2,547	2,082	2,547	2,082	2,129
Consumer Portfolio Segment [Member] \| Home Equity [Member] \| Topic ASC 326
Allowance for credit losses - loans [Roll Forward]
Beginning balance				2,937
Charge-offs				(178)
Recoveries				23
Steuben acquisition				235
Provision				(470)
Ending balance		2,547		2,547		2,937
Consumer Portfolio Segment [Member] \| Home Equity [Member] \| Topic ASC 326 \| Cumulative Effect Adjustment
Allowance for credit losses - loans [Roll Forward]
Beginning balance				808
Ending balance						808
Unallocated Financing Receivables [Member]
Allowance for credit losses - loans [Roll Forward]
Beginning balance		1,000	991	957	1,000	1,000
Charge-offs		0	0		0
Recoveries		0	0		0
Provision		4	(53)		(62)
Ending balance		1,004	$ 938	1,004	$ 938	957
Unallocated Financing Receivables [Member] \| Topic ASC 326
Allowance for credit losses - loans [Roll Forward]
Beginning balance				1,000
Charge-offs				0
Recoveries				0
Steuben acquisition				0
Provision				4
Ending balance		$ 1,004		1,004		1,000
Unallocated Financing Receivables [Member] \| Topic ASC 326 \| Cumulative Effect Adjustment
Allowance for credit losses - loans [Roll Forward]
Beginning balance				$ 43
Ending balance						$ 43