LOANS - Credit Quality By Past Due Status (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Dec. 31, 2020
Aged analysis of the company's loans
Total loans	$ 7,282,582	$ 7,282,582	$ 7,415,952
Interest income on nonaccrual loans	0	0
Legacy Loan [Member]
Aged analysis of the company's loans
Nonaccrual			72,929
Total loans			7,415,952
Total past due			111,610
Legacy Loan [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total loans			34,759
Legacy Loan [Member] | Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due			3,922
Legacy Loan [Member] | Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due			7,304,342
Acquired Loans [Member]
Aged analysis of the company's loans
Nonaccrual	65,967	65,967
Total loans	7,282,582	7,282,582
Total past due	93,056	93,056
Acquired Loans [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	25,215	25,215
Acquired Loans [Member] | Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due	1,874	1,874
Acquired Loans [Member] | Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due	7,189,526	7,189,526
Business Lending [Member]
Aged analysis of the company's loans
Total past due	3,092,177	3,092,177	3,440,077
Consumer Mortgage [Member]
Aged analysis of the company's loans
Total past due	2,470,974	2,470,974	2,401,499
Consumer Indirect [Member]
Aged analysis of the company's loans
Total past due	1,168,378	1,168,378	1,021,885
Consumer Direct [Member]
Aged analysis of the company's loans
Total past due	155,602	155,602	152,657
Home Equity [Member]
Aged analysis of the company's loans
Total past due	395,451	395,451	399,834
Commercial Portfolio Segment [Member] | Business Lending [Member]
Aged analysis of the company's loans
Nonaccrual	47,820	47,820
Total loans	3,092,177	3,092,177	3,440,077
Total past due	53,077	53,077
Commercial Portfolio Segment [Member] | Business Lending [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	5,109	5,109
Commercial Portfolio Segment [Member] | Business Lending [Member] | Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due	148	148
Commercial Portfolio Segment [Member] | Business Lending [Member] | Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due	3,039,100	3,039,100
Commercial Portfolio Segment [Member] | Business Lending [Member] | Legacy Loan [Member]
Aged analysis of the company's loans
Nonaccrual			55,709
Total loans			3,440,077
Total past due			60,664
Commercial Portfolio Segment [Member] | Business Lending [Member] | Legacy Loan [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			4,896
Commercial Portfolio Segment [Member] | Business Lending [Member] | Legacy Loan [Member] | Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due			59
Commercial Portfolio Segment [Member] | Business Lending [Member] | Legacy Loan [Member] | Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due			3,379,413
Residential Portfolio Segment [Member] | Consumer Mortgage [Member]
Aged analysis of the company's loans
Nonaccrual	15,605	15,605
Total loans	2,470,974	2,470,974
Total past due	25,762	25,762
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	8,869	8,869
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due	1,288	1,288
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due	2,445,212	2,445,212
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Legacy Loan [Member]
Aged analysis of the company's loans
Nonaccrual			14,970
Total loans			2,401,499
Total past due			31,257
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Legacy Loan [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			13,236
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Legacy Loan [Member] | Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due			3,051
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] | Legacy Loan [Member] | Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due			2,370,242
Consumer Portfolio Segment [Member] | Consumer Mortgage [Member]
Aged analysis of the company's loans
Total loans	2,470,974	2,470,974	2,401,499
Consumer Portfolio Segment [Member] | Consumer Indirect [Member]
Aged analysis of the company's loans
Nonaccrual	0	0
Total loans	1,168,378	1,168,378	1,021,885
Total past due	8,769	8,769
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	8,638	8,638
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due	131	131
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due	1,159,609	1,159,609
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Legacy Loan [Member]
Aged analysis of the company's loans
Nonaccrual			1
Total loans			1,021,885
Total past due			13,381
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Legacy Loan [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			13,161
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Legacy Loan [Member] | Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due			219
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | Legacy Loan [Member] | Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due			1,008,504
Consumer Portfolio Segment [Member] | Consumer Direct [Member]
Aged analysis of the company's loans
Nonaccrual	1	1
Total loans	155,602	155,602	152,657
Total past due	628	628
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	608	608
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due	19	19
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due	154,974	154,974
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Legacy Loan [Member]
Aged analysis of the company's loans
Nonaccrual			3
Total loans			152,657
Total past due			1,201
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Legacy Loan [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			1,170
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Legacy Loan [Member] | Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due			28
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | Legacy Loan [Member] | Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due			151,456
Consumer Portfolio Segment [Member] | Home Equity [Member]
Aged analysis of the company's loans
Nonaccrual	2,541	2,541
Total loans	395,451	395,451	399,834
Total past due	4,820	4,820
Consumer Portfolio Segment [Member] | Home Equity [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	1,991	1,991
Consumer Portfolio Segment [Member] | Home Equity [Member] | Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due	288	288
Consumer Portfolio Segment [Member] | Home Equity [Member] | Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due	$ 390,631	$ 390,631
Consumer Portfolio Segment [Member] | Home Equity [Member] | Legacy Loan [Member]
Aged analysis of the company's loans
Nonaccrual			2,246
Total loans			399,834
Total past due			5,107
Consumer Portfolio Segment [Member] | Home Equity [Member] | Legacy Loan [Member] | Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due			2,296
Consumer Portfolio Segment [Member] | Home Equity [Member] | Legacy Loan [Member] | Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due			565
Consumer Portfolio Segment [Member] | Home Equity [Member] | Legacy Loan [Member] | Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due			$ 394,727