SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended
	Dec. 01, 2021	Jul. 15, 2021	Dec. 31, 2021	Dec. 31, 2022
Lessee, Lease, Description [Line Items]
Stock price per share		$ 10
Gross offering proceeds		$ 277.7
Charitable contribution in cash		$ 1.5
Common shares purchased for ESOP		2,281,800
Renewal or option to extend term (up to)				10 years
Increase in deferred tax allowance			$ 16.8
Termination expense on multiemployer plan withdrawal	$ 11.2
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%