LOANS AND ALLOWANCE FOR CREDIT LOSSES - Credit Quality By Past Due Status (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2022	Dec. 31, 2021
Aged analysis of the company's loans
Total past due	$ 8,144,696	$ 7,373,639
Interest income on nonaccrual loans	0
Legacy Loan [Member]
Aged analysis of the company's loans
Total past due	61,098	73,495
Nonaccrual	31,686	41,665
Total Loans	8,144,696	7,373,639
Legacy Loan [Member] \| Past Due 30 - 89 Days [Member]
Aged analysis of the company's loans
Total past due	23,973	28,022
Legacy Loan [Member] \| Financial Asset, Past Due [Member]
Aged analysis of the company's loans
Total past due	5,439	3,808
Legacy Loan [Member] \| Financial Asset, Not Past Due [Member]
Aged analysis of the company's loans
Total past due	8,083,598	7,300,144
Business Lending
Aged analysis of the company's loans
Total past due	3,331,998	3,075,904
Consumer Mortgage
Aged analysis of the company's loans
Total past due	2,903,822	2,556,114
Consumer Indirect
Aged analysis of the company's loans
Total past due	1,309,753	1,189,749
Consumer Direct
Aged analysis of the company's loans
Total past due	173,686	153,811
Home Equity
Aged analysis of the company's loans
Total past due	425,437	398,061
Commercial Portfolio Segment \| Business Lending
Aged analysis of the company's loans
Total past due	3,331,998	3,075,904
Commercial Portfolio Segment \| Business Lending \| Legacy Loan [Member]
Aged analysis of the company's loans
Total past due	13,338	29,744
Nonaccrual	10,160	24,105
Total Loans	3,331,998	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	2,567	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	611	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	3,318,660	3,046,160
Residential Portfolio Segment \| Consumer Mortgage \| Legacy Loan [Member]
Aged analysis of the company's loans
Total past due	32,441	28,652
Nonaccrual	18,557	15,027
Total Loans	2,903,822	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	9,391	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	4,493	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	2,871,381	2,527,462
Consumer Portfolio Segment \| Consumer Mortgage
Aged analysis of the company's loans
Total past due	2,903,822	2,556,114
Consumer Portfolio Segment \| Consumer Indirect
Aged analysis of the company's loans
Total past due	1,309,753	1,189,749
Consumer Portfolio Segment \| Consumer Indirect \| Legacy Loan [Member]
Aged analysis of the company's loans
Total past due	9,665	9,698
Nonaccrual	7	0
Total Loans	1,309,753	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	9,534	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	124	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	1,300,088	1,180,051
Consumer Portfolio Segment \| Consumer Direct
Aged analysis of the company's loans
Total past due	173,686	153,811
Consumer Portfolio Segment \| Consumer Direct \| Legacy Loan [Member]
Aged analysis of the company's loans
Total past due	1,061	819
Nonaccrual	32	1
Total Loans	173,686	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	1,029	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	172,625	152,992
Consumer Portfolio Segment \| Home Equity
Aged analysis of the company's loans
Total past due	425,437	398,061
Consumer Portfolio Segment \| Home Equity \| Legacy Loan [Member]
Aged analysis of the company's loans
Total past due	4,593	4,582
Nonaccrual	2,930	2,532
Total Loans	425,437	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	1,452	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	211	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	$ 420,844	$ 393,479

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Amount of interest income on financing receivable on nonaccrual status.

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