LOANS, Credit Quality By Past Due Status (Details) - USD ($) $ in Thousands		3 Months Ended	6 Months Ended
		Jun. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Aged analysis of the company's loans [Abstract]
90 + days past due and still accruing		$ 6,063	$ 6,063
Nonaccrual		20,697	20,697
Total past due		54,347	54,347
Current		7,473,692	7,473,692
Total loans		7,528,039	7,528,039	$ 6,890,543
Interest income on nonaccrual loans		0	0
Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans [Abstract]
Total past due		27,587	27,587
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	[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 [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		1,186	1,186
Nonaccrual		5,001	5,001
Total past due		8,682	8,682
Current		3,446,661	3,446,661
Total loans		3,455,343	3,455,343	2,775,876
Commercial Portfolio Segment [Member] | Business Lending [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans [Abstract]
Total past due		2,495	2,495
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	[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 [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		3,651	3,651
Nonaccrual		13,544	13,544
Total past due		30,734	30,734
Current		2,397,326	2,397,326
Total loans		2,428,060	2,428,060	2,430,902
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans [Abstract]
Total past due		13,539	13,539
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	[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 [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		744	744
Nonaccrual		2	2
Total past due		8,862	8,862
Current		1,048,003	1,048,003
Total loans		1,056,865	1,056,865	1,113,062
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans [Abstract]
Total past due		8,116	8,116
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	[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 [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		72	72
Nonaccrual		54	54
Total past due		1,001	1,001
Current		168,227	168,227
Total loans		169,228	169,228	184,378
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans [Abstract]
Total past due		875	875
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	[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 [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		410	410
Nonaccrual		2,096	2,096
Total past due		5,068	5,068
Current		413,475	413,475
Total loans		418,543	418,543	386,325
Consumer Portfolio Segment [Member] | Home Equity [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans [Abstract]
Total past due		$ 2,562	$ 2,562
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	[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 [Abstract]
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.