Loans and Allowance for Loan Losses - Risk category of our loan portfolio (Details) $ in Thousands	Mar. 31, 2021 USD ($)
2021	$ 333,817
2020	864,422
2019	968,954
2018	1,085,096
2017	786,229
Prior	2,439,744
Revolving Loans Amortized Cost Basis	247,599
Lines of credit converted to term loans	19,455
Total	6,745,316
One-To-Four Family - Residential Portfolio Segment [Member]
2021	841
2020	32,558
2019	37,680
2018	35,560
2017	24,517
Prior	84,549
Revolving Loans Amortized Cost Basis	11,845
Lines of credit converted to term loans	19,455
Total	247,005
One-To-Four Family - Residential Portfolio Segment [Member] \| Pass [Member]
2021	841
2020	32,075
2019	36,958
2018	33,729
2017	20,948
Prior	77,700
Revolving Loans Amortized Cost Basis	10,421
Lines of credit converted to term loans	15,298
Total	227,970
One-To-Four Family - Residential Portfolio Segment [Member] \| Watch [Member]
2020	483
2019	722
2017	3,569
Prior	1,575
Revolving Loans Amortized Cost Basis	1,249
Lines of credit converted to term loans	2,369
Total	9,967
One-To-Four Family - Residential Portfolio Segment [Member] \| Special Mention [Member]
Prior	989
Lines of credit converted to term loans	371
Total	1,360
One-To-Four Family - Residential Portfolio Segment [Member] \| Substandard [Member]
2018	1,831
Prior	4,285
Revolving Loans Amortized Cost Basis	175
Lines of credit converted to term loans	1,417
Total	7,708
One-To-Four Family - Mixed-Use Property Portfolio Segment [Member]
2021	9,680
2020	36,229
2019	72,302
2018	80,178
2017	62,648
Prior	337,982
Total	599,019
One-To-Four Family - Mixed-Use Property Portfolio Segment [Member] \| Pass [Member]
2021	9,680
2020	36,229
2019	72,302
2018	75,961
2017	55,411
Prior	322,422
Total	572,005
One-To-Four Family - Mixed-Use Property Portfolio Segment [Member] \| Watch [Member]
2018	3,689
2017	7,237
Prior	10,262
Total	21,188
One-To-Four Family - Mixed-Use Property Portfolio Segment [Member] \| Special Mention [Member]
2018	528
Prior	2,284
Total	2,812
One-To-Four Family - Mixed-Use Property Portfolio Segment [Member] \| Substandard [Member]
Prior	3,014
Total	3,014
Commercial Real Estate
2021	21,339
2020	173,254
2019	268,324
2018	280,727
2017	196,297
Prior	783,536
Total	1,723,477
Commercial Real Estate \| Pass [Member]
2021	19,502
2020	172,318
2019	259,301
2018	274,998
2017	191,249
Prior	694,789
Total	1,612,157
Commercial Real Estate \| Watch [Member]
2021	1,837
2020	936
2019	1,439
2018	5,729
2017	5,048
Prior	87,590
Total	102,579
Commercial Real Estate \| Special Mention [Member]
Prior	1,122
Total	1,122
Commercial Real Estate \| Substandard [Member]
2019	7,584
Prior	35
Total	7,619
Construction Portfolio Segment [Member]
2021	2,354
2020	20,510
2019	16,667
2018	13,362
2017	8,540
Total	61,433
Construction Portfolio Segment [Member] \| Pass [Member]
2021	2,354
2020	20,510
2019	15,552
2018	2,823
Total	41,239
Construction Portfolio Segment [Member] \| Watch [Member]
2019	1,115
2018	7,299
2017	5,956
Total	14,370
Construction Portfolio Segment [Member] \| Special Mention [Member]
2018	859
2017	2,584
Total	3,443
Construction Portfolio Segment [Member] \| Substandard [Member]
2018	2,381
Total	2,381
Multi-family Residential Portfolio Segment [Member]
2021	91,859
2020	245,381
2019	346,706
2018	478,959
2017	367,585
Prior	990,739
Revolving Loans Amortized Cost Basis	5,887
Total	2,527,116
Multi-family Residential Portfolio Segment [Member] \| Pass [Member]
2021	91,859
2020	243,458
2019	341,802
2018	467,841
2017	364,625
Prior	973,720
Revolving Loans Amortized Cost Basis	5,105
Total	2,488,410
Multi-family Residential Portfolio Segment [Member] \| Watch [Member]
2020	1,923
2019	4,203
2018	9,121
2017	1,152
Prior	16,054
Revolving Loans Amortized Cost Basis	398
Total	32,851
Multi-family Residential Portfolio Segment [Member] \| Special Mention [Member]
2017	806
Prior	230
Revolving Loans Amortized Cost Basis	183
Total	1,219
Multi-family Residential Portfolio Segment [Member] \| Substandard [Member]
2019	701
2018	1,997
2017	1,002
Prior	735
Revolving Loans Amortized Cost Basis	201
Total	4,636
Commercial Business Secured By Portfolio Segment [member]
2021	71,965
2020	131,882
2019	90,670
2018	71,075
2017	47,669
Prior	155,014
Total	568,275
Commercial Business Secured By Portfolio Segment [member] \| Pass [Member]
2021	71,965
2020	108,227
2019	38,984
2018	52,512
2017	34,540
Prior	103,314
Total	409,542
Commercial Business Secured By Portfolio Segment [member] \| Watch [Member]
2020	23,655
2019	51,079
2018	18,563
2017	13,129
Prior	47,474
Total	153,900
Commercial Business Secured By Portfolio Segment [member] \| Special Mention [Member]
2019	607
Total	607
Commercial Business Secured By Portfolio Segment [member] \| Substandard [Member]
Prior	4,226
Total	4,226
Commercial Business Portfolio Segment [Member]
2021	15,647
2020	96,510
2019	135,229
2018	121,069
2017	75,052
Prior	83,955
Revolving Loans Amortized Cost Basis	229,784
Total	757,246
Commercial Business Portfolio Segment [Member] \| Pass [Member]
2021	15,647
2020	91,117
2019	111,749
2018	93,011
2017	48,402
Prior	75,988
Revolving Loans Amortized Cost Basis	201,571
Total	637,485
Commercial Business Portfolio Segment [Member] \| Watch [Member]
2020	500
2019	22,509
2018	25,308
2017	20,383
Prior	4,171
Revolving Loans Amortized Cost Basis	25,733
Total	98,604
Commercial Business Portfolio Segment [Member] \| Special Mention [Member]
2019	285
2018	2,742
2017	74
Prior	2
Revolving Loans Amortized Cost Basis	236
Total	3,339
Commercial Business Portfolio Segment [Member] \| Substandard [Member]
2020	4,893
2019	686
2018	8
2017	6,193
Prior	3,794
Revolving Loans Amortized Cost Basis	1,008
Total	16,582
Commercial Business Portfolio Segment [Member] \| Doubtful [Member]
Revolving Loans Amortized Cost Basis	1,236
Total	1,236
Small Business Administration Portfolio Segment [Member]
2021	120,132
2020	128,098
2019	1,376
2018	4,166
2017	3,921
Prior	3,946
Total	261,639
Small Business Administration Portfolio Segment [Member] \| Pass [Member]
2021	120,132
2020	127,810
2019	1,376
2018	1,574
2017	806
Prior	2,649
Total	254,347
Small Business Administration Portfolio Segment [Member] \| Watch [Member]
2020	288
2018	2,592
2017	1,947
Prior	1,242
Total	6,069
Small Business Administration Portfolio Segment [Member] \| Special Mention [Member]
Prior	49
Total	49
Small Business Administration Portfolio Segment [Member] \| Substandard [Member]
2017	1,168
Prior	6
Total	1,174
Other Portfolio Segment [Member]
Prior	23
Revolving Loans Amortized Cost Basis	83
Total	106
Other Portfolio Segment [Member] \| Pass [Member]
Prior	23
Revolving Loans Amortized Cost Basis	83
Total	$ 106

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