SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) - USD ($) $ in Thousands	Jul. 15, 2021	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Right-of-use assets		$ 26,101	$ 24,878
Recognition of operating lease liabilities		$ 27,293	$ 25,535
Charitable contribution in cash	$ 1,500
Blue Foundry Bancorp - NJ
Lessee, Lease, Description [Line Items]
Ownership interest percentage		100.00%
Cumulative Effect, Period of Adoption, Adjustment
Lessee, Lease, Description [Line Items]
Right-of-use assets				$ 6,000
Recognition of operating lease liabilities				$ 6,000