LOANS RECEIVABLE, NET - Summary of impaired loans by class of loan (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
With no related allowance recorded:
Unpaid Principal Balance	$ 14,998	$ 13,681	$ 18,257
Recorded Investment	15,157	13,605	16,888
Average Recorded Investment	15,871	3,315	10,667
Interest Income Recognized	201	350	312
Cash Basis Interest Recognized	184	339	291
With an allowance recorded:
Unpaid Principal Balance	1,108	1,921	1,852
Recorded Investment	1,107	1,921	1,850
Allowance for Loan Losses Allocated	71	184	121
Average Recorded Investment	1,339	859	1,874
Interest Income Recognized	40	65	89
Cash Basis Interest Recognized	35	58	82
Total
Unpaid Principal Balance	16,106	15,602	20,109
Recorded Investment	16,264	15,526	18,738
Allowance for Loan Losses Allocated	71	184	121
Average Recorded Investment	17,210	4,174	12,541
Interest Income Recognized	241	415	401
Cash Basis Interest Recognized	219	397	373
Residential Portfolio Segment \| Residential one-to-four family
With no related allowance recorded:
Unpaid Principal Balance	8,757	6,399	11,161
Recorded Investment	9,065	6,508	10,375
Average Recorded Investment	9,666	473	4,143
Interest Income Recognized	9	97	26
Cash Basis Interest Recognized	8	96	25
With an allowance recorded:
Unpaid Principal Balance	1,069	1,519	1,455
Recorded Investment	1,068	1,521	1,454
Allowance for Loan Losses Allocated	32	108	49
Average Recorded Investment	1,297	679	1,472
Interest Income Recognized	38	53	72
Cash Basis Interest Recognized	34	47	66
Total
Allowance for Loan Losses Allocated	32	108	49
Residential Portfolio Segment \| Multifamily
With no related allowance recorded:
Unpaid Principal Balance	1,030	1,216	1,360
Recorded Investment	1,032	1,226	1,371
Average Recorded Investment	1,249	405	1,051
Interest Income Recognized	17	45	40
Cash Basis Interest Recognized	15	41	38
With an allowance recorded:
Unpaid Principal Balance	0	353	351
Recorded Investment	0	352	350
Allowance for Loan Losses Allocated	0	28	26
Average Recorded Investment	0	157	353
Interest Income Recognized	0	11	15
Cash Basis Interest Recognized	0	10	14
Total
Allowance for Loan Losses Allocated	0	28	26
Nonresidential Portfolio Segment \| Non-residential
With no related allowance recorded:
Unpaid Principal Balance	5,155	6,006	5,678
Recorded Investment	5,004	5,812	5,084
Average Recorded Investment	4,899	2,410	5,413
Interest Income Recognized	173	207	243
Cash Basis Interest Recognized	159	201	225
With an allowance recorded:
Unpaid Principal Balance	0	0	0
Recorded Investment	0	0	0
Allowance for Loan Losses Allocated	0	0	0
Average Recorded Investment	0	0	0
Interest Income Recognized	0	0	0
Cash Basis Interest Recognized	0	0	0
Total
Allowance for Loan Losses Allocated	0	0	0
Construction and Land Portfolio Segment \| Construction and land
With no related allowance recorded:
Unpaid Principal Balance	0	0	0
Recorded Investment	0	0	0
Average Recorded Investment	0	0	0
Interest Income Recognized	0	0	0
Cash Basis Interest Recognized	0	0	0
With an allowance recorded:
Unpaid Principal Balance	0	0	0
Recorded Investment	0	0	0
Allowance for Loan Losses Allocated	0	0	0
Average Recorded Investment	0	0	0
Interest Income Recognized	0	0	0
Cash Basis Interest Recognized	0	0	0
Total
Allowance for Loan Losses Allocated	0	0	0
Commercial Portfolio Segment \| Commercial and industrial (including PPP)
With no related allowance recorded:
Unpaid Principal Balance	0	0	0
Recorded Investment	0	0	0
Average Recorded Investment	0	0	0
Interest Income Recognized	0	0	0
Cash Basis Interest Recognized	0	0	0
With an allowance recorded:
Unpaid Principal Balance	0	0	0
Recorded Investment	0	0	0
Allowance for Loan Losses Allocated	0	0	0
Average Recorded Investment	0	0	0
Interest Income Recognized	0	0	0
Cash Basis Interest Recognized	0	0	0
Total
Allowance for Loan Losses Allocated	0	0	0
Junior Lien Portfolio Segment \| Junior liens
With no related allowance recorded:
Unpaid Principal Balance	56	60	58
Recorded Investment	56	59	58
Average Recorded Investment	57	27	60
Interest Income Recognized	2	1	3
Cash Basis Interest Recognized	2	1	3
Consumer and Other Portfolio Segment \| Consumer and other
With an allowance recorded:
Unpaid Principal Balance	39	49	46
Recorded Investment	39	48	46
Allowance for Loan Losses Allocated	39	48	46
Average Recorded Investment	42	23	49
Interest Income Recognized	2	1	2
Cash Basis Interest Recognized	1	1	2
Total
Allowance for Loan Losses Allocated	$ 39	$ 48	$ 46

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