LOANS AND ALLOWANCE FOR CREDIT LOSSES - Credit Quality By Past Due Status (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2022	Dec. 31, 2021
Aged analysis of the company's loans
Total loans	$ 7,422,253	$ 7,373,639
Interest income on nonaccrual loans	0
Legacy Loan [Member]
Aged analysis of the company's loans
Total past due	62,050	73,495
Nonaccrual	32,213	41,665
Total loans	7,422,253	7,373,639
Legacy Loan [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	26,021
Total loans		28,022
Legacy Loan [Member] \| Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due	3,816	3,808
Legacy Loan [Member] \| Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due	7,360,203	7,300,144
Business Lending
Aged analysis of the company's loans
Total past due	3,102,533	3,075,904
Consumer Mortgage
Aged analysis of the company's loans
Total past due	2,592,586	2,556,114
Consumer Indirect
Aged analysis of the company's loans
Total past due	1,176,373	1,189,749
Consumer Direct
Aged analysis of the company's loans
Total past due	152,445	153,811
Home Equity
Aged analysis of the company's loans
Total past due	398,316	398,061
Commercial Portfolio Segment \| Business Lending
Aged analysis of the company's loans
Total loans	3,102,533	3,075,904
Commercial Portfolio Segment \| Business Lending \| Legacy Loan [Member]
Aged analysis of the company's loans
Total past due	20,755	29,744
Nonaccrual	13,759	24,105
Total loans	3,102,533	3,075,904
Commercial Portfolio Segment \| Business Lending \| Legacy Loan [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	6,957	5,540
Commercial Portfolio Segment \| Business Lending \| Legacy Loan [Member] \| Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due	39	99
Commercial Portfolio Segment \| Business Lending \| Legacy Loan [Member] \| Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due	3,081,778	3,046,160
Residential Portfolio Segment \| Consumer Mortgage \| Legacy Loan [Member]
Aged analysis of the company's loans
Total past due	28,387	28,652
Nonaccrual	16,097	15,027
Total loans	2,592,586	2,556,114
Residential Portfolio Segment \| Consumer Mortgage \| Legacy Loan [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	8,768	10,297
Residential Portfolio Segment \| Consumer Mortgage \| Legacy Loan [Member] \| Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due	3,522	3,328
Residential Portfolio Segment \| Consumer Mortgage \| Legacy Loan [Member] \| Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due	2,564,199	2,527,462
Consumer Portfolio Segment \| Consumer Mortgage
Aged analysis of the company's loans
Total loans	2,592,586	2,556,114
Consumer Portfolio Segment \| Consumer Indirect
Aged analysis of the company's loans
Total loans	1,176,373	1,189,749
Consumer Portfolio Segment \| Consumer Indirect \| Legacy Loan [Member]
Aged analysis of the company's loans
Total past due	7,969	9,698
Nonaccrual	0	0
Total loans	1,176,373	1,189,749
Consumer Portfolio Segment \| Consumer Indirect \| Legacy Loan [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	7,936	9,611
Consumer Portfolio Segment \| Consumer Indirect \| Legacy Loan [Member] \| Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due	33	87
Consumer Portfolio Segment \| Consumer Indirect \| Legacy Loan [Member] \| Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due	1,168,404	1,180,051
Consumer Portfolio Segment \| Consumer Direct
Aged analysis of the company's loans
Total loans	152,445	153,811
Consumer Portfolio Segment \| Consumer Direct \| Legacy Loan [Member]
Aged analysis of the company's loans
Total past due	751	819
Nonaccrual	1	1
Total loans	152,445	153,811
Consumer Portfolio Segment \| Consumer Direct \| Legacy Loan [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	750	796
Consumer Portfolio Segment \| Consumer Direct \| Legacy Loan [Member] \| Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due	0	22
Consumer Portfolio Segment \| Consumer Direct \| Legacy Loan [Member] \| Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due	151,694	152,992
Consumer Portfolio Segment \| Home Equity
Aged analysis of the company's loans
Total loans	398,316	398,061
Consumer Portfolio Segment \| Home Equity \| Legacy Loan [Member]
Aged analysis of the company's loans
Total past due	4,188	4,582
Nonaccrual	2,356	2,532
Total loans	398,316	398,061
Consumer Portfolio Segment \| Home Equity \| Legacy Loan [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	1,610	1,778
Consumer Portfolio Segment \| Home Equity \| Legacy Loan [Member] \| Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due	222	272
Consumer Portfolio Segment \| Home Equity \| Legacy Loan [Member] \| Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due	$ 394,128	$ 393,479

Amount of interest income on financing receivable on nonaccrual status.

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