STOCK-BASED COMPENSATION - Equity incentive plan (Details) - USD ($) $ in Thousands			3 Months Ended			9 Months Ended
	Sep. 18, 2024	Aug. 25, 2022	Sep. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Granted (in shares)			0		0	48,133	0
Expected future expense of non-vested options			$ 6,300			$ 6,300
Shares granted (in shares)						387,695
Performance based shares, forfeited						442,347
Restricted Stock
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expected period of recognition (in years)						4 years 8 months 12 days
Expected future expense of non-vested RSUs			$ 7,600			$ 7,600
Option
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expected period of recognition (in years)						4 years 6 months
Performance Shares
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Performance based shares, forfeited				347,640
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan)
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares authorized for grant (in shares)		3,993,150
Expected life of stock options (in years)						6 years 6 months
Risk free interest rate						3.94%
Volatility rate						32.26%
Expected dividend yield						0.84%
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Restricted Stock
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares authorized for grant (in shares)		1,140,900
Initial period before vesting begins (in years)		1 year
Shares granted (in shares)	10,000			184,625
Aggregate grant date fair value	$ 109			$ 1,800
Term after grant date vesting starts (in years)				1 year
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Restricted Stock | Share-Based Payment Arrangement, Tranche One
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)	3 years			1 year
Shares granted (in shares)				2,900
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Restricted Stock | Share-Based Payment Arrangement, Tranche Two
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)				5 years
Shares granted (in shares)				19,255
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Restricted Stock | Share-Based Payment Arrangement, Tranche Three
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)				6 years
Shares granted (in shares)				162,470
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Restricted Stock | Minimum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)		5 years
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Restricted Stock | Maximum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)		7 years
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Option
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares authorized for grant (in shares)		2,852,250
Initial period before vesting begins (in years)		1 year
Expiration period of options (in years)		10 years
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Option | Minimum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)		5 years
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Option | Maximum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)		7 years
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Performance Shares
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)				4 years
Shares granted (in shares)				193,070
Aggregate grant date fair value				$ 1,800
Term of measurement period for PSUs (in years)				3 years
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Performance Shares | Minimum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Percentage of target awarded				0.00%
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) | Performance Shares | Maximum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Percentage of target awarded				100.00%