SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended	12 Months Ended
	Dec. 01, 2021	Jul. 15, 2021	Dec. 31, 2021	Dec. 31, 2021	Dec. 31, 2020
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	10 years
Goodwill impairment				$ 0	$ 15,460
Increase in deferred tax allowance			$ 16,800	$ 16,800
Termination expense on multiemployer plan withdrawal	$ 11,200
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	1 year
Maximum
Lessee, Lease, Description [Line Items]
Remaining lease terms			15 years	15 years
Blue Foundry Bancorp - NJ
Lessee, Lease, Description [Line Items]
Ownership interest percentage			100.00%	100.00%