SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) - USD ($) $ / shares in Units, $ in Thousands	Jul. 15, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Lessee, Lease, Description [Line Items]
Stock price per share	$ 10
Gross offering proceeds	$ 277,700
Charitable contribution in cash	$ 1,500
Common shares purchased for ESOP	2,281,800
Renewal or option to extend term (up to)		10 years
Loans receivable, allowance		$ 14,154	$ 13,400	$ 14,425
Equity		(355,640)	(393,718)	(429,471)
Allowance for credit loss		158	0
Retained Earnings
Lessee, Lease, Description [Line Items]
Equity		$ (164,340)	(171,763)	(169,457)
Cumulative Effect, Period of Adoption, Adjustment
Lessee, Lease, Description [Line Items]
Increase in allowance for credit losses			18
Loans receivable, allowance			668	$ 0
Allowance for credit loss			170
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-13
Lessee, Lease, Description [Line Items]
Loans receivable, allowance			660
Equity			18
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-13 | Retained Earnings
Lessee, Lease, Description [Line Items]
Equity			18
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2022-02
Lessee, Lease, Description [Line Items]
Equity			8
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2022-02 | Retained Earnings
Lessee, Lease, Description [Line Items]
Equity			$ 8
Public stock offering
Lessee, Lease, Description [Line Items]
Shares of common stock sold	27,772,500
Donation of shares to Blue Foundry Charitable Foundation
Lessee, Lease, Description [Line Items]
Shares of common stock sold	750,000
Minimum
Lessee, Lease, Description [Line Items]
Remaining lease terms		1 year
Maximum
Lessee, Lease, Description [Line Items]
Remaining lease terms		15 years
Blue Foundry Bancorp - NJ
Lessee, Lease, Description [Line Items]
Ownership interest percentage		100.00%