BORROWED FUNDS - Schedule of Borrowed Funds (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Borrowings and junior subordinated debentures
Principal, Short-term borrowings	$ 260,000	$ 0
Principal, Long-term borrowings	246,586	125,509
Total borrowings	$ 506,586	$ 125,509
Weighted average rate on short-term borrowings (as percent)	5.54%	0.00%
Weighted average rate on long-term borrowings (as percent)	5.33%	5.52%
Weighted Average Rate	5.44%	5.52%
Advances from the FHLBB
Borrowings and junior subordinated debentures
Principal, Short-term borrowings	$ 260,000	$ 0
Principal, Long-term borrowings	$ 125,223	$ 4,445
Weighted average rate on short-term borrowings (as percent)	5.54%	0.00%
Weighted average rate on long-term borrowings (as percent)	4.80%	0.71%
Subordinated notes
Borrowings and junior subordinated debentures
Principal, Long-term borrowings	$ 98,335	$ 98,089
Weighted average rate on long-term borrowings (as percent)	5.50%	5.50%
Junior subordinated borrowing - Trust I
Borrowings and junior subordinated debentures
Principal, Long-term borrowings	$ 15,464	$ 15,464
Weighted average rate on long-term borrowings (as percent)	7.49%	6.54%
Junior subordinated borrowing - Trust II
Borrowings and junior subordinated debentures
Principal, Long-term borrowings	$ 7,564	$ 7,511
Weighted average rate on long-term borrowings (as percent)	7.35%	6.47%