LOANS RECEIVABLE, NET - Summary of past due loans in non-accrual and past 90 days still on accrual (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Nonaccrual	$ 12,464	$ 12,856
Loans Past Due 90 Days and Still Accruing	0	0
Residential Portfolio Segment | Residential one-to-four family
Financing Receivable, Past Due [Line Items]
Nonaccrual	10,969	11,813
Loans Past Due 90 Days and Still Accruing	0	0
Residential Portfolio Segment | Multifamily
Financing Receivable, Past Due [Line Items]
Nonaccrual	145	156
Loans Past Due 90 Days and Still Accruing	0	0
Nonresidential Portfolio Segment | Non-residential
Financing Receivable, Past Due [Line Items]
Nonaccrual	1,236	805
Loans Past Due 90 Days and Still Accruing	0	0
Construction and Land Portfolio Segment | Construction and land
Financing Receivable, Past Due [Line Items]
Nonaccrual	0	0
Loans Past Due 90 Days and Still Accruing	0	0
Commercial Portfolio Segment | Commercial and industrial (including PPP)
Financing Receivable, Past Due [Line Items]
Nonaccrual	0	0
Loans Past Due 90 Days and Still Accruing	0	0
Junior Lien Portfolio Segment | Junior liens
Financing Receivable, Past Due [Line Items]
Nonaccrual	114	82
Loans Past Due 90 Days and Still Accruing	$ 0	$ 0