LOANS, Credit Quality By Past Due Status (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Dec. 31, 2020
Aged analysis of the company's loans
Total loans	$ 7,368,327	$ 7,415,952
Interest income on nonaccrual loans	0
Legacy Loan [Member]
Aged analysis of the company's loans
90 + days past due and still accruing		3,922
Nonaccrual		72,929
Total past due		111,610
Current		7,304,342
Total loans		7,415,952
Legacy Loan [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due		34,759
Acquired Loans [Member]
Aged analysis of the company's loans
90 + days past due and still accruing	2,152
Nonaccrual	73,310
Total past due	95,113
Current	7,273,214
Total loans	7,368,327
Acquired Loans [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	19,651
Commercial Portfolio Segment [Member] \| Business Lending [Member]
Aged analysis of the company's loans
90 + days past due and still accruing	88
Nonaccrual	54,930
Total past due	58,699
Current	3,333,087
Total loans	3,391,786	3,440,077
Commercial Portfolio Segment [Member] \| Business Lending [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	3,681
Commercial Portfolio Segment [Member] \| Business Lending [Member] \| Legacy Loan [Member]
Aged analysis of the company's loans
90 + days past due and still accruing		59
Nonaccrual		55,709
Total past due		60,664
Current		3,379,413
Total loans		3,440,077
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
Residential Portfolio Segment [Member] \| Consumer Mortgage [Member]
Aged analysis of the company's loans
90 + days past due and still accruing	1,688
Nonaccrual	15,836
Total past due	25,546
Current	2,383,827
Total loans	2,409,373
Residential Portfolio Segment [Member] \| Consumer Mortgage [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	8,022
Residential Portfolio Segment [Member] \| Consumer Mortgage [Member] \| Legacy Loan [Member]
Aged analysis of the company's loans
90 + days past due and still accruing		3,051
Nonaccrual		14,970
Total past due		31,257
Current		2,370,242
Total loans		2,401,499
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
Consumer Portfolio Segment [Member] \| Consumer Mortgage [Member]
Aged analysis of the company's loans
Total loans	2,409,373	2,401,499
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member]
Aged analysis of the company's loans
90 + days past due and still accruing	115
Nonaccrual	0
Total past due	6,546
Current	1,022,789
Total loans	1,029,335	1,021,885
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	6,431
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member] \| Legacy Loan [Member]
Aged analysis of the company's loans
90 + days past due and still accruing		219
Nonaccrual		1
Total past due		13,381
Current		1,008,504
Total loans		1,021,885
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 Direct [Member]
Aged analysis of the company's loans
90 + days past due and still accruing	16
Nonaccrual	2
Total past due	577
Current	141,848
Total loans	142,425	152,657
Consumer Portfolio Segment [Member] \| Consumer Direct [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	559
Consumer Portfolio Segment [Member] \| Consumer Direct [Member] \| Legacy Loan [Member]
Aged analysis of the company's loans
90 + days past due and still accruing		28
Nonaccrual		3
Total past due		1,201
Current		151,456
Total loans		152,657
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] \| Home Equity [Member]
Aged analysis of the company's loans
90 + days past due and still accruing	245
Nonaccrual	2,542
Total past due	3,745
Current	391,663
Total loans	395,408	399,834
Consumer Portfolio Segment [Member] \| Home Equity [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	$ 958
Consumer Portfolio Segment [Member] \| Home Equity [Member] \| Legacy Loan [Member]
Aged analysis of the company's loans
90 + days past due and still accruing		565
Nonaccrual		2,246
Total past due		5,107
Current		394,727
Total loans		399,834
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

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