STOCK-BASED COMPENSATION - Share-based compensation expense (Details) - Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Share-Based Payment Arrangement, Expense	$ 811	$ 693	$ 2,348	$ 2,137
Employee Stock Option
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Share-Based Payment Arrangement, Expense	373	403	1,087	1,199
Restricted stock expense
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Share-Based Payment Arrangement, Expense	$ 438	$ 290	$ 1,261	$ 938