STOCK-BASED COMPENSATION - Equity incentive plan (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Aug. 25, 2022	Mar. 31, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Granted (in shares)			0	48,133	0
Expected future expense of non-vested options				$ 6.7
Shares granted (in shares)				377,695
Performance based shares, forfeited				55,523
Restricted Stock
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expected period of recognition (in years)				4 years 10 months 24 days
Expected future expense of non-vested RSUs				$ 8.0
Option
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expected period of recognition (in years)				4 years 8 months 12 days
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)		184,625
Aggregate grant date fair value		$ 1.8
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)		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		$ 1.8
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%