LOANS RECEIVABLE, NET - Summary of changes in allowance for loan losses by class of loans (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Allowance for loan losses
Beginning balance	$ 15,593	$ 17,251	$ 15,593	$ 17,251	$ 16,150	$ 16,959	$ 16,000	$ 14,501
Charge-offs	(4)	(1)	(5)	(3)
Recoveries	0	0	0	0
(Recovery of) provision for loan losses	(553)	1,252	(1,361)	2,753
Total ending allowance balance	15,593	17,251	15,593	17,251
Residential Portfolio Segment \| Residential one-to-four family
Allowance for loan losses
Beginning balance	2,918	3,691	2,918	3,691	3,342	3,579	3,581	3,446
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
(Recovery of) provision for loan losses	(424)	110	(661)	245
Total ending allowance balance	2,918	3,691	2,918	3,691
Residential Portfolio Segment \| Multifamily
Allowance for loan losses
Beginning balance	5,350	4,776	5,350	4,776	5,748	5,460	4,706	4,256
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
(Recovery of) provision for loan losses	(398)	70	(110)	520
Total ending allowance balance	5,350	4,776	5,350	4,776
Nonresidential Portfolio Segment \| Non-residential
Allowance for loan losses
Beginning balance	3,243	2,742	3,243	2,742	3,145	3,244	2,663	2,548
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
(Recovery of) provision for loan losses	98	79	(1)	194
Total ending allowance balance	3,243	2,742	3,243	2,742
Construction and Land Portfolio Segment \| Construction and land
Allowance for loan losses
Beginning balance	3,185	4,823	3,185	4,823	2,928	3,655	3,833	3,028
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
(Recovery of) provision for loan losses	257	990	(470)	1,795
Total ending allowance balance	3,185	4,823	3,185	4,823
Junior Lien Portfolio Segment \| Junior liens
Allowance for loan losses
Beginning balance	758	1,067	758	1,067	813	916	1,060	1,002
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
(Recovery of) provision for loan losses	(55)	7	(158)	65
Total ending allowance balance	758	1,067	758	1,067
Commercial Portfolio Segment \| Commercial and industrial (PPP)
Allowance for loan losses
Beginning balance	4	0	4	0	7	2	0	0
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
(Recovery of) provision for loan losses	(3)		2
Total ending allowance balance	4	0	4	0
Consumer and Other Portfolio Segment \| Consumer and other
Allowance for loan losses
Beginning balance	42	53	42	53	44	48	55	57
Charge-offs	(4)	(1)	(5)	(3)
Recoveries	0	0	0	0
(Recovery of) provision for loan losses	2	(1)	(1)	(1)
Total ending allowance balance	42	53	42	53
Unallocated Financing Receivables
Allowance for loan losses
Beginning balance	93	99	93	99	$ 123	$ 55	$ 102	$ 164
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
(Recovery of) provision for loan losses	(30)	(3)	38	(65)
Total ending allowance balance	$ 93	$ 99	$ 93	$ 99

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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