LOANS AND ALLOWANCE FOR CREDIT LOSSES - Credit Quality By Past Due Status (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2022	Dec. 31, 2021
Aged analysis of the company's loans
Total Loans	$ 8,543,607	$ 8,543,607	$ 7,373,639
Interest income on nonaccrual loans	0	0
Legacy Loan
Aged analysis of the company's loans
Total Loans	8,543,607	8,543,607	7,373,639
Nonaccrual	28,076	28,076	41,665
Legacy Loan | Financial Asset, Past Due
Aged analysis of the company's loans
Total Loans	60,324	60,324	73,495
Legacy Loan | Past Due 30 - 89 Days
Aged analysis of the company's loans
Total Loans	27,832	27,832	28,022
Legacy Loan | 90+ Days Past Due and Still Accruing
Aged analysis of the company's loans
Total Loans	4,416	4,416	3,808
Legacy Loan | Financial Asset, Not Past Due
Aged analysis of the company's loans
Total Loans	8,483,283	8,483,283	7,300,144
Business lending
Aged analysis of the company's loans
Total Loans	3,494,425	3,494,425	3,075,904
Consumer mortgage
Aged analysis of the company's loans
Total Loans	2,975,521	2,975,521	2,556,114
Consumer indirect
Aged analysis of the company's loans
Total Loans	1,461,235	1,461,235	1,189,749
Consumer direct
Aged analysis of the company's loans
Total Loans	179,399	179,399	153,811
Home equity
Aged analysis of the company's loans
Total Loans	433,027	433,027	398,061
Commercial Portfolio Segment | Business lending
Aged analysis of the company's loans
Total Loans	3,494,425	3,494,425	3,075,904
Commercial Portfolio Segment | Business lending | Legacy Loan
Aged analysis of the company's loans
Total Loans	3,494,425	3,494,425	3,075,904
Nonaccrual	4,848	4,848	24,105
Commercial Portfolio Segment | Business lending | Legacy Loan | Financial Asset, Past Due
Aged analysis of the company's loans
Total Loans	5,887	5,887	29,744
Commercial Portfolio Segment | Business lending | Legacy Loan | Past Due 30 - 89 Days
Aged analysis of the company's loans
Total Loans	1,039	1,039	5,540
Commercial Portfolio Segment | Business lending | Legacy Loan | 90+ Days Past Due and Still Accruing
Aged analysis of the company's loans
Total Loans	0	0	99
Commercial Portfolio Segment | Business lending | Legacy Loan | Financial Asset, Not Past Due
Aged analysis of the company's loans
Total Loans	3,488,538	3,488,538	3,046,160
Residential Portfolio Segment | Consumer mortgage
Aged analysis of the company's loans
Total Loans	2,975,521	2,975,521	2,556,114
Residential Portfolio Segment | Consumer mortgage | Legacy Loan
Aged analysis of the company's loans
Total Loans	2,975,521	2,975,521	2,556,114
Nonaccrual	20,890	20,890	15,027
Residential Portfolio Segment | Consumer mortgage | Legacy Loan | Financial Asset, Past Due
Aged analysis of the company's loans
Total Loans	37,025	37,025	28,652
Residential Portfolio Segment | Consumer mortgage | Legacy Loan | Past Due 30 - 89 Days
Aged analysis of the company's loans
Total Loans	12,138	12,138	10,297
Residential Portfolio Segment | Consumer mortgage | Legacy Loan | 90+ Days Past Due and Still Accruing
Aged analysis of the company's loans
Total Loans	3,997	3,997	3,328
Residential Portfolio Segment | Consumer mortgage | Legacy Loan | Financial Asset, Not Past Due
Aged analysis of the company's loans
Total Loans	2,938,496	2,938,496	2,527,462
Residential Portfolio Segment | Home equity
Aged analysis of the company's loans
Total Loans	433,027	433,027	398,061
Residential Portfolio Segment | Home equity | Legacy Loan
Aged analysis of the company's loans
Total Loans	433,027	433,027	398,061
Nonaccrual	2,309	2,309	2,532
Residential Portfolio Segment | Home equity | Legacy Loan | Financial Asset, Past Due
Aged analysis of the company's loans
Total Loans	4,236	4,236	4,582
Residential Portfolio Segment | Home equity | Legacy Loan | Past Due 30 - 89 Days
Aged analysis of the company's loans
Total Loans	1,761	1,761	1,778
Residential Portfolio Segment | Home equity | Legacy Loan | 90+ Days Past Due and Still Accruing
Aged analysis of the company's loans
Total Loans	166	166	272
Residential Portfolio Segment | Home equity | Legacy Loan | Financial Asset, Not Past Due
Aged analysis of the company's loans
Total Loans	428,791	428,791	393,479
Consumer Portfolio Segment | Consumer indirect
Aged analysis of the company's loans
Total Loans	1,461,235	1,461,235	1,189,749
Consumer Portfolio Segment | Consumer indirect | Legacy Loan
Aged analysis of the company's loans
Total Loans	1,461,235	1,461,235	1,189,749
Nonaccrual	0	0	0
Consumer Portfolio Segment | Consumer indirect | Legacy Loan | Financial Asset, Past Due
Aged analysis of the company's loans
Total Loans	12,153	12,153	9,698
Consumer Portfolio Segment | Consumer indirect | Legacy Loan | Past Due 30 - 89 Days
Aged analysis of the company's loans
Total Loans	11,938	11,938	9,611
Consumer Portfolio Segment | Consumer indirect | Legacy Loan | 90+ Days Past Due and Still Accruing
Aged analysis of the company's loans
Total Loans	215	215	87
Consumer Portfolio Segment | Consumer indirect | Legacy Loan | Financial Asset, Not Past Due
Aged analysis of the company's loans
Total Loans	1,449,082	1,449,082	1,180,051
Consumer Portfolio Segment | Consumer direct
Aged analysis of the company's loans
Total Loans	179,399	179,399	153,811
Consumer Portfolio Segment | Consumer direct | Legacy Loan
Aged analysis of the company's loans
Total Loans	179,399	179,399	153,811
Nonaccrual	29	29	1
Consumer Portfolio Segment | Consumer direct | Legacy Loan | Financial Asset, Past Due
Aged analysis of the company's loans
Total Loans	1,023	1,023	819
Consumer Portfolio Segment | Consumer direct | Legacy Loan | Past Due 30 - 89 Days
Aged analysis of the company's loans
Total Loans	956	956	796
Consumer Portfolio Segment | Consumer direct | Legacy Loan | 90+ Days Past Due and Still Accruing
Aged analysis of the company's loans
Total Loans	38	38	22
Consumer Portfolio Segment | Consumer direct | Legacy Loan | Financial Asset, Not Past Due
Aged analysis of the company's loans
Total Loans	$ 178,376	$ 178,376	$ 152,992