LOANS - Credit Quality By Past Due Status (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2021	Dec. 31, 2020
Aged analysis of the company's loans
Total loans	$ 7,244,147	$ 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	1,722
Nonaccrual	68,476
Total past due	88,509
Current	7,155,638
Total loans	7,244,147
Acquired Loans [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	18,311
Commercial Portfolio Segment [Member] \| Business Lending [Member]
Aged analysis of the company's loans
90 + days past due and still accruing	228
Nonaccrual	49,877
Total past due	51,077
Current	3,135,410
Total loans	3,186,487	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	972
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	1,259
Nonaccrual	16,025
Total past due	25,929
Current	2,382,570
Total loans	2,408,499
Residential Portfolio Segment [Member] \| Consumer Mortgage [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	8,645
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	2,408,499	2,401,499
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member]
Aged analysis of the company's loans
90 + days past due and still accruing	73
Nonaccrual	0
Total past due	7,183
Current	1,102,321
Total loans	1,109,504	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	7,110
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	15
Nonaccrual	2
Total past due	524
Current	148,016
Total loans	148,540	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	507
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	147
Nonaccrual	2,572
Total past due	3,796
Current	387,321
Total loans	391,117	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	$ 1,077
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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