LOANS, Credit Quality By Past Due Status (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2020	Dec. 31, 2019
Aged analysis of the company's loans
90 + days past due and still accruing		$ 3,487
Nonaccrual		28,756
Total past due		58,793
Current		7,399,850
Total loans		7,458,643	$ 6,890,543
Interest income on nonaccrual loans		0
Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due		26,550
Legacy Loans [Member]
Aged analysis of the company's loans
90 + days past due and still accruing			2,707
Nonaccrual			15,415
Total past due			48,684
Current			5,401,908
Total loans			5,450,592
Legacy Loans [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			30,562
Acquired Loans [Member]
Aged analysis of the company's loans
90 + days past due and still accruing			2,719
Nonaccrual			3,420
Total past due			16,429
Impaired	[1]		11,797
Current			1,411,725
Total loans			1,439,951
Acquired Loans [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			10,290
Business Lending [Member]
Aged analysis of the company's loans
Total loans			2,775,876
Business Lending [Member] | Legacy Loans [Member]
Aged analysis of the company's loans
Total loans			1,856,585
Business Lending [Member] | Acquired Loans [Member]
Aged analysis of the company's loans
Total loans			919,291
Consumer Mortgage [Member] | Legacy Loans [Member]
Aged analysis of the company's loans
Total loans			1,996,716
Consumer Mortgage [Member] | Acquired Loans [Member]
Aged analysis of the company's loans
Total loans			434,186
Consumer Indirect [Member] | Legacy Loans [Member]
Aged analysis of the company's loans
Total loans			1,107,308
Consumer Indirect [Member] | Acquired Loans [Member]
Aged analysis of the company's loans
Total loans			5,754
Consumer Direct [Member] | Legacy Loans [Member]
Aged analysis of the company's loans
Total loans			175,976
Consumer Direct [Member] | Acquired Loans [Member]
Aged analysis of the company's loans
Total loans			8,402
Home Equity [Member] | Legacy Loans [Member]
Aged analysis of the company's loans
Total loans			314,007
Home Equity [Member] | Acquired Loans [Member]
Aged analysis of the company's loans
Total loans			72,318
Commercial Portfolio Segment [Member] | Business Lending [Member]
Aged analysis of the company's loans
90 + days past due and still accruing		39
Nonaccrual		11,750
Total past due		14,940
Current		3,418,625
Total loans		3,433,565	2,775,876
Commercial Portfolio Segment [Member] | Business Lending [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due		3,151
Commercial Portfolio Segment [Member] | Business Lending [Member] | Legacy Loans [Member]
Aged analysis of the company's loans
90 + days past due and still accruing			126
Nonaccrual			3,840
Total past due			7,902
Current			1,848,683
Total loans			1,856,585
Commercial Portfolio Segment [Member] | Business Lending [Member] | Legacy Loans [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			3,936
Commercial Portfolio Segment [Member] | Business Lending [Member] | Acquired Loans [Member]
Aged analysis of the company's loans
90 + days past due and still accruing			2,173
Nonaccrual			570
Total past due			11,261
Impaired	[1]		11,797
Current			896,233
Total loans			919,291
Commercial Portfolio Segment [Member] | Business Lending [Member] | Acquired Loans [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			8,518
Residential Portfolio Segment [Member] | Consumer Mortgage [Member]
Aged analysis of the company's loans
90 + days past due and still accruing		3,182
Nonaccrual		14,821
Total past due		29,452
Current		2,380,797
Total loans		2,410,249	2,430,902
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due		11,449
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Legacy Loans [Member]
Aged analysis of the company's loans
90 + days past due and still accruing			2,052
Nonaccrual			10,131
Total past due			23,173
Current			1,973,543
Total loans			1,996,716
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Legacy Loans [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			10,990
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Acquired Loans [Member]
Aged analysis of the company's loans
90 + days past due and still accruing			277
Nonaccrual			2,386
Total past due			3,553
Impaired	[1]		0
Current			430,633
Total loans			434,186
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Acquired Loans [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			890
Consumer Portfolio Segment [Member] | Consumer Indirect [Member]
Aged analysis of the company's loans
90 + days past due and still accruing		12
Nonaccrual		1
Total past due		8,724
Current		1,031,201
Total loans		1,039,925	1,113,062
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due		8,711
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Legacy Loans [Member]
Aged analysis of the company's loans
90 + days past due and still accruing			125
Nonaccrual			0
Total past due			12,798
Current			1,094,510
Total loans			1,107,308
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Legacy Loans [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			12,673
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Acquired Loans [Member]
Aged analysis of the company's loans
90 + days past due and still accruing			31
Nonaccrual			0
Total past due			110
Impaired	[1]		0
Current			5,644
Total loans			5,754
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Acquired Loans [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			79
Consumer Portfolio Segment [Member] | Consumer Direct [Member]
Aged analysis of the company's loans
90 + days past due and still accruing		34
Nonaccrual		3
Total past due		1,097
Current		160,542
Total loans		161,639	184,378
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due		1,060
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Legacy Loans [Member]
Aged analysis of the company's loans
90 + days past due and still accruing			76
Nonaccrual			0
Total past due			1,531
Current			174,445
Total loans			175,976
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Legacy Loans [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			1,455
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Acquired Loans [Member]
Aged analysis of the company's loans
90 + days past due and still accruing			0
Nonaccrual			52
Total past due			111
Impaired	[1]		0
Current			8,291
Total loans			8,402
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Acquired Loans [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			59
Consumer Portfolio Segment [Member] | Home Equity [Member]
Aged analysis of the company's loans
90 + days past due and still accruing		220
Nonaccrual		2,181
Total past due		4,580
Current		408,685
Total loans		413,265	386,325
Consumer Portfolio Segment [Member] | Home Equity [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due		$ 2,179
Consumer Portfolio Segment [Member] | Home Equity [Member] | Legacy Loans [Member]
Aged analysis of the company's loans
90 + days past due and still accruing			328
Nonaccrual			1,444
Total past due			3,280
Current			310,727
Total loans			314,007
Consumer Portfolio Segment [Member] | Home Equity [Member] | Legacy Loans [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			1,508
Consumer Portfolio Segment [Member] | Home Equity [Member] | Acquired Loans [Member]
Aged analysis of the company's loans
90 + days past due and still accruing			238
Nonaccrual			412
Total past due			1,394
Impaired	[1]		0
Current			70,924
Total loans			72,318
Consumer Portfolio Segment [Member] | Home Equity [Member] | Acquired Loans [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			$ 744

[1] Acquired impaired loans were not classified as nonperforming assets as the loans are considered to be performing under ASC 310-30. As a result interest income, through the accretion of the difference between the carrying amount of the loans and the expected cashflows, is being recognized on all acquired impaired loans.