LOANS - Loan Summary (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Loans receivable, net [Abstract]
Gross loans, including deferred origination costs	$ 7,415,952	$ 6,890,543
Allowance for credit losses	(60,869)	(49,911)	$ (49,284)
Net loans	7,355,083	6,840,632
Net deferred loan origination costs	25,500	32,300
Loans receivable, related parties [Roll Forward]
Balance at beginning of year	17,486	20,661
New loans	4,194	5,720
Payments	(6,131)	(8,895)
Balance at end of year	15,549	17,486
Business Lending [Member]
Loans receivable, net [Abstract]
Gross loans, including deferred origination costs		2,775,876
Consumer Mortgage [Member]
Loans receivable, net [Abstract]
Allowance for credit losses		(10,269)
Residential Portfolio Segment [Member] \| Consumer Mortgage [Member]
Loans receivable, net [Abstract]
Gross loans, including deferred origination costs	2,401,499	2,430,902
Allowance for credit losses		(10,269)	(10,124)
Commercial Portfolio Segment [Member] \| Business Lending [Member]
Loans receivable, net [Abstract]
Gross loans, including deferred origination costs	3,440,077	2,775,876
Allowance for credit losses		(19,426)	(18,522)
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member]
Loans receivable, net [Abstract]
Gross loans, including deferred origination costs	1,021,885	1,113,062
Allowance for credit losses		(13,712)	(14,366)
Consumer Portfolio Segment [Member] \| Consumer Direct [Member]
Loans receivable, net [Abstract]
Gross loans, including deferred origination costs	152,657	184,378
Allowance for credit losses		(3,255)	(3,095)
Consumer Portfolio Segment [Member] \| Home Equity [Member]
Loans receivable, net [Abstract]
Gross loans, including deferred origination costs	$ 399,834	386,325
Allowance for credit losses		$ (2,129)	$ (2,144)

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

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For an unclassified balance sheet, reflects the carrying amount of unpaid loan amounts due from related parties at the balance sheet date.

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Reflects the amount of new advances made to related parties in the form of loan and lease receivables.

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Amount of cash inflow from related parties which reduce the amount of loans and leases receivable due from related parties.

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Amount of deferred fees paid by borrowers and unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of loans that are being amortized into income as an adjustment to yield. Excludes amounts for loans and leases covered under loss sharing agreements.

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