LOANS, Credit Quality By Past Due Status (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Dec. 31, 2019
Aged analysis of the company's loans [Abstract]
90 + days past due and still accruing		$ 12,736
Nonaccrual		19,046
Total past due		76,044
Current		6,780,267
Total loans		6,866,092	$ 6,890,543
Interest income on nonaccrual loans		0
Purchased Credit Deteriorated ("PCD") [Member]
Aged analysis of the company's loans [Abstract]
Total loans	[1]	9,781
Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans [Abstract]
Total past due		44,262
Legacy Loans [Member]
Aged analysis of the company's loans [Abstract]
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 [Abstract]
Total past due			30,562
Acquired Loans [Member]
Aged analysis of the company's loans [Abstract]
90 + days past due and still accruing			2,719
Nonaccrual			3,420
Total past due			16,429
Impaired	[2]		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 [Abstract]
Total past due			10,290
Business Lending [Member]
Aged analysis of the company's loans [Abstract]
Total loans			2,775,876
Business Lending [Member] | Legacy Loans [Member]
Aged analysis of the company's loans [Abstract]
Total loans			1,856,585
Business Lending [Member] | Acquired Loans [Member]
Aged analysis of the company's loans [Abstract]
Total loans			919,291
Consumer Mortgage [Member] | Legacy Loans [Member]
Aged analysis of the company's loans [Abstract]
Total loans			1,996,716
Consumer Mortgage [Member] | Acquired Loans [Member]
Aged analysis of the company's loans [Abstract]
Total loans			434,186
Consumer Indirect [Member] | Legacy Loans [Member]
Aged analysis of the company's loans [Abstract]
Total loans			1,107,308
Consumer Indirect [Member] | Acquired Loans [Member]
Aged analysis of the company's loans [Abstract]
Total loans			5,754
Consumer Direct [Member] | Legacy Loans [Member]
Aged analysis of the company's loans [Abstract]
Total loans			175,976
Consumer Direct [Member] | Acquired Loans [Member]
Aged analysis of the company's loans [Abstract]
Total loans			8,402
Home Equity [Member] | Legacy Loans [Member]
Aged analysis of the company's loans [Abstract]
Total loans			314,007
Home Equity [Member] | Acquired Loans [Member]
Aged analysis of the company's loans [Abstract]
Total loans			72,318
Commercial Portfolio Segment [Member] | Business Lending [Member]
Aged analysis of the company's loans [Abstract]
90 + days past due and still accruing		10,111
Nonaccrual		4,394
Total past due		27,662
Current		2,751,687
Total loans		2,789,130	2,775,876
Commercial Portfolio Segment [Member] | Business Lending [Member] | Purchased Credit Deteriorated ("PCD") [Member]
Aged analysis of the company's loans [Abstract]
Total loans	[1]	9,781
Commercial Portfolio Segment [Member] | Business Lending [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans [Abstract]
Total past due		13,157
Commercial Portfolio Segment [Member] | Business Lending [Member] | Legacy Loans [Member]
Aged analysis of the company's loans [Abstract]
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 [Abstract]
Total past due			3,936
Commercial Portfolio Segment [Member] | Business Lending [Member] | Acquired Loans [Member]
Aged analysis of the company's loans [Abstract]
90 + days past due and still accruing			2,173
Nonaccrual			570
Total past due			11,261
Impaired	[2]		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 [Abstract]
Total past due			8,518
Residential Portfolio Segment [Member] | Consumer Mortgage [Member]
Aged analysis of the company's loans [Abstract]
90 + days past due and still accruing		1,821
Nonaccrual		12,674
Total past due		30,109
Current		2,394,547
Total loans		2,424,656	2,430,902
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Purchased Credit Deteriorated ("PCD") [Member]
Aged analysis of the company's loans [Abstract]
Total loans	[1]	0
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans [Abstract]
Total past due		15,614
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Legacy Loans [Member]
Aged analysis of the company's loans [Abstract]
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 [Abstract]
Total past due			10,990
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Acquired Loans [Member]
Aged analysis of the company's loans [Abstract]
90 + days past due and still accruing			277
Nonaccrual			2,386
Total past due			3,553
Impaired	[2]		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 [Abstract]
Total past due			890
Consumer Portfolio Segment [Member] | Consumer Indirect [Member]
Aged analysis of the company's loans [Abstract]
90 + days past due and still accruing		418
Nonaccrual		0
Total past due		11,761
Current		1,076,118
Total loans		1,087,879	1,113,062
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Purchased Credit Deteriorated ("PCD") [Member]
Aged analysis of the company's loans [Abstract]
Total loans	[1]	0
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans [Abstract]
Total past due		11,343
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Legacy Loans [Member]
Aged analysis of the company's loans [Abstract]
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 [Abstract]
Total past due			12,673
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Acquired Loans [Member]
Aged analysis of the company's loans [Abstract]
90 + days past due and still accruing			31
Nonaccrual			0
Total past due			110
Impaired	[2]		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 [Abstract]
Total past due			79
Consumer Portfolio Segment [Member] | Consumer Direct [Member]
Aged analysis of the company's loans [Abstract]
90 + days past due and still accruing		62
Nonaccrual		52
Total past due		1,311
Current		176,533
Total loans		177,844	184,378
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Purchased Credit Deteriorated ("PCD") [Member]
Aged analysis of the company's loans [Abstract]
Total loans	[1]	0
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans [Abstract]
Total past due		1,197
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Legacy Loans [Member]
Aged analysis of the company's loans [Abstract]
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 [Abstract]
Total past due			1,455
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Acquired Loans [Member]
Aged analysis of the company's loans [Abstract]
90 + days past due and still accruing			0
Nonaccrual			52
Total past due			111
Impaired	[2]		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 [Abstract]
Total past due			59
Consumer Portfolio Segment [Member] | Home Equity [Member]
Aged analysis of the company's loans [Abstract]
90 + days past due and still accruing		324
Nonaccrual		1,926
Total past due		5,201
Current		381,382
Total loans		386,583	386,325
Consumer Portfolio Segment [Member] | Home Equity [Member] | Purchased Credit Deteriorated ("PCD") [Member]
Aged analysis of the company's loans [Abstract]
Total loans	[1]	0
Consumer Portfolio Segment [Member] | Home Equity [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans [Abstract]
Total past due		$ 2,951
Consumer Portfolio Segment [Member] | Home Equity [Member] | Legacy Loans [Member]
Aged analysis of the company's loans [Abstract]
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 [Abstract]
Total past due			1,508
Consumer Portfolio Segment [Member] | Home Equity [Member] | Acquired Loans [Member]
Aged analysis of the company's loans [Abstract]
90 + days past due and still accruing			238
Nonaccrual			412
Total past due			1,394
Impaired	[2]		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 [Abstract]
Total past due			$ 744

[1] PCD loans were not classified as nonperforming assets as the loans are considered to be performing under ASC 326-10. As a result, the noncredit discount, after the adjustment for the allowance for credit losses, is being accreted into interest income on all PCD loans.

[2] 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.