LOANS AND ALLOWANCE FOR CREDIT LOSSES - Credit Quality By Past Due Status (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2023	Dec. 31, 2022
Aged analysis of the company's loans
Total Loans	$ 9,450,066	$ 9,450,066	$ 8,809,394
Interest income on nonaccrual loans	0	0
Legacy Loan
Aged analysis of the company's loans
Total Loans	9,450,066	9,450,066	8,809,394
Nonaccrual	33,122	33,122	29,245
Legacy Loan | Past Due 30 - 89 Days
Aged analysis of the company's loans
Total Loans	48,646	48,646	45,244
Legacy Loan | 90+ Days Past Due and Still Accruing
Aged analysis of the company's loans
Total Loans	3,731	3,731	4,119
Legacy Loan | Financial Asset, Past Due
Aged analysis of the company's loans
Total Loans	85,499	85,499	78,608
Legacy Loan | Financial Asset, Not Past Due
Aged analysis of the company's loans
Total Loans	9,364,567	9,364,567	8,730,786
Business lending
Aged analysis of the company's loans
Total Loans	3,914,935	3,914,935	3,645,665
Consumer mortgage
Aged analysis of the company's loans
Total Loans	3,196,764	3,196,764	3,012,475
Consumer indirect
Aged analysis of the company's loans
Total Loans	1,708,302	1,708,302	1,539,653
Consumer direct
Aged analysis of the company's loans
Total Loans	185,301	185,301	177,605
Home equity
Aged analysis of the company's loans
Total Loans	444,764	444,764	433,996
Commercial Portfolio Segment | Business lending
Aged analysis of the company's loans
Total Loans	3,914,935	3,914,935	3,645,665
Commercial Portfolio Segment | Business lending | Legacy Loan
Aged analysis of the company's loans
Total Loans	3,914,935	3,914,935	3,645,665
Nonaccrual	5,638	5,638	4,689
Commercial Portfolio Segment | Business lending | Legacy Loan | Past Due 30 - 89 Days
Aged analysis of the company's loans
Total Loans	13,545	13,545	9,818
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	0
Commercial Portfolio Segment | Business lending | Legacy Loan | Financial Asset, Past Due
Aged analysis of the company's loans
Total Loans	19,183	19,183	14,507
Commercial Portfolio Segment | Business lending | Legacy Loan | Financial Asset, Not Past Due
Aged analysis of the company's loans
Total Loans	3,895,752	3,895,752	3,631,158
Residential Portfolio Segment | Consumer mortgage
Aged analysis of the company's loans
Total Loans	3,196,764	3,196,764	3,012,475
Residential Portfolio Segment | Consumer mortgage | Legacy Loan
Aged analysis of the company's loans
Total Loans	3,196,764	3,196,764	3,012,475
Nonaccrual	25,205	25,205	22,583
Residential Portfolio Segment | Consumer mortgage | Legacy Loan | Past Due 30 - 89 Days
Aged analysis of the company's loans
Total Loans	15,282	15,282	13,757
Residential Portfolio Segment | Consumer mortgage | Legacy Loan | 90+ Days Past Due and Still Accruing
Aged analysis of the company's loans
Total Loans	2,908	2,908	3,510
Residential Portfolio Segment | Consumer mortgage | Legacy Loan | Financial Asset, Past Due
Aged analysis of the company's loans
Total Loans	43,395	43,395	39,850
Residential Portfolio Segment | Consumer mortgage | Legacy Loan | Financial Asset, Not Past Due
Aged analysis of the company's loans
Total Loans	3,153,369	3,153,369	2,972,625
Residential Portfolio Segment | Home equity
Aged analysis of the company's loans
Total Loans	444,764	444,764	433,996
Residential Portfolio Segment | Home equity | Legacy Loan
Aged analysis of the company's loans
Total Loans	444,764	444,764	433,996
Nonaccrual	2,255	2,255	1,945
Residential Portfolio Segment | Home equity | Legacy Loan | Past Due 30 - 89 Days
Aged analysis of the company's loans
Total Loans	3,331	3,331	3,595
Residential Portfolio Segment | Home equity | Legacy Loan | 90+ Days Past Due and Still Accruing
Aged analysis of the company's loans
Total Loans	504	504	299
Residential Portfolio Segment | Home equity | Legacy Loan | Financial Asset, Past Due
Aged analysis of the company's loans
Total Loans	6,090	6,090	5,839
Residential Portfolio Segment | Home equity | Legacy Loan | Financial Asset, Not Past Due
Aged analysis of the company's loans
Total Loans	438,674	438,674	428,157
Consumer Portfolio Segment | Consumer indirect
Aged analysis of the company's loans
Total Loans	1,708,302	1,708,302	1,539,653
Consumer Portfolio Segment | Consumer indirect | Legacy Loan
Aged analysis of the company's loans
Total Loans	1,708,302	1,708,302	1,539,653
Nonaccrual	0	0	0
Consumer Portfolio Segment | Consumer indirect | Legacy Loan | Past Due 30 - 89 Days
Aged analysis of the company's loans
Total Loans	15,175	15,175	16,767
Consumer Portfolio Segment | Consumer indirect | Legacy Loan | 90+ Days Past Due and Still Accruing
Aged analysis of the company's loans
Total Loans	290	290	178
Consumer Portfolio Segment | Consumer indirect | Legacy Loan | Financial Asset, Past Due
Aged analysis of the company's loans
Total Loans	15,465	15,465	16,945
Consumer Portfolio Segment | Consumer indirect | Legacy Loan | Financial Asset, Not Past Due
Aged analysis of the company's loans
Total Loans	1,692,837	1,692,837	1,522,708
Consumer Portfolio Segment | Consumer direct
Aged analysis of the company's loans
Total Loans	185,301	185,301	177,605
Consumer Portfolio Segment | Consumer direct | Legacy Loan
Aged analysis of the company's loans
Total Loans	185,301	185,301	177,605
Nonaccrual	24	24	28
Consumer Portfolio Segment | Consumer direct | Legacy Loan | Past Due 30 - 89 Days
Aged analysis of the company's loans
Total Loans	1,313	1,313	1,307
Consumer Portfolio Segment | Consumer direct | Legacy Loan | 90+ Days Past Due and Still Accruing
Aged analysis of the company's loans
Total Loans	29	29	132
Consumer Portfolio Segment | Consumer direct | Legacy Loan | Financial Asset, Past Due
Aged analysis of the company's loans
Total Loans	1,366	1,366	1,467
Consumer Portfolio Segment | Consumer direct | Legacy Loan | Financial Asset, Not Past Due
Aged analysis of the company's loans
Total Loans	$ 183,935	$ 183,935	$ 176,138