Loans Receivable - Loans Based on Credit Quality Indicators (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 12,898,775	$ 13,312,388
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	12,319,108	13,208,853
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	298,341	20,879
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	280,344	82,656
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	982	0
Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	1,723,715
2019	1,027,999
2018	1,021,884
2017	855,455
2016	601,033
Prior to 2016	791,773
Revolving Loans	793,354
Revolving to Term Loans	20,836
Total Loans	6,836,049	6,564,698
Commercial loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	1,663,569
2019	961,506
2018	946,771
2017	768,835
2016	520,115
Prior to 2016	694,249
Revolving Loans	716,129
Revolving to Term Loans	14,031
Total Loans	6,285,205	6,489,697
Commercial loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	34,119
2019	28,978
2018	59,362
2017	45,693
2016	49,628
Prior to 2016	62,468
Revolving Loans	11,626
Revolving to Term Loans	6,467
Total Loans	298,341	20,879
Commercial loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	26,027
2019	37,515
2018	15,751
2017	40,927
2016	31,290
Prior to 2016	35,056
Revolving Loans	64,617
Revolving to Term Loans	338
Total Loans	251,521	54,122
Commercial loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	0
2017	0
2016	0
Prior to 2016	0
Revolving Loans	982
Revolving to Term Loans	0
Total Loans	982	0
Commercial loans | Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	328,020
2019	173,076
2018	252,214
2017	258,202
2016	199,458
Prior to 2016	284,626
Revolving Loans	14,241
Revolving to Term Loans	0
Total Loans	1,509,837	1,422,674
Commercial loans | Multi-family | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	327,371
2019	162,686
2018	247,680
2017	249,863
2016	194,498
Prior to 2016	282,212
Revolving Loans	14,241
Revolving to Term Loans	0
Total Loans	1,478,551	1,418,837
Commercial loans | Multi-family | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	649
2019	2,815
2018	907
2017	4,526
2016	2,662
Prior to 2016	2,190
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	13,749	0
Commercial loans | Multi-family | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	7,575
2018	3,627
2017	3,813
2016	2,298
Prior to 2016	224
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	17,537	3,837
Commercial loans | Multi-family | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Commercial loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	220,108
2019	286,896
2018	283,183
2017	308,526
2016	193,093
Prior to 2016	406,177
Revolving Loans	2,576
Revolving to Term Loans	9
Total Loans	1,700,568	1,631,170
Commercial loans | Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	210,996
2019	249,584
2018	276,743
2017	259,663
2016	182,134
Prior to 2016	331,689
Revolving Loans	2,576
Revolving to Term Loans	9
Total Loans	1,513,394	1,602,634
Commercial loans | Commercial real estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	8,366
2019	19,503
2018	3,723
2017	36,905
2016	4,551
Prior to 2016	44,705
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	117,753	2,754
Commercial loans | Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	746
2019	17,809
2018	2,717
2017	11,958
2016	6,408
Prior to 2016	29,783
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	69,421	25,782
Commercial loans | Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Commercial loans | Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	902,037
2019	67,840
2018	123,929
2017	95,244
2016	149,885
Prior to 2016	83,188
Revolving Loans	695,512
Revolving to Term Loans	20,827
Total Loans	2,138,462	1,268,695
Commercial loans | Commercial & industrial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	851,652
2019	50,997
2018	110,901
2017	90,081
2016	127,232
Prior to 2016	78,217
Revolving Loans	618,287
Revolving to Term Loans	14,022
Total Loans	1,941,389	1,229,891
Commercial loans | Commercial & industrial | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	25,104
2019	6,660
2018	5,559
2017	4,262
2016	69
Prior to 2016	0
Revolving Loans	11,626
Revolving to Term Loans	6,467
Total Loans	59,747	18,125
Commercial loans | Commercial & industrial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	25,281
2019	10,183
2018	7,469
2017	901
2016	22,584
Prior to 2016	4,971
Revolving Loans	64,617
Revolving to Term Loans	338
Total Loans	136,344	20,679
Commercial loans | Commercial & industrial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	0
2017	0
2016	0
Prior to 2016	0
Revolving Loans	982
Revolving to Term Loans	0
Total Loans	982	0
Commercial loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	236,202
2019	450,517
2018	343,460
2017	174,700
2016	54,357
Prior to 2016	0
Revolving Loans	75,712
Revolving to Term Loans	0
Total Loans	1,334,948	2,038,052
Commercial loans | Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	236,202
2019	448,569
2018	292,349
2017	152,785
2016	12,011
Prior to 2016	0
Revolving Loans	75,712
Revolving to Term Loans	0
Total Loans	1,217,628	2,038,052
Commercial loans | Construction | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	49,173
2017	0
2016	42,346
Prior to 2016	0
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	91,519	0
Commercial loans | Construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	1,948
2018	1,938
2017	21,915
2016	0
Prior to 2016	0
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	25,801	0
Commercial loans | Construction | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Commercial loans | Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	37,348
2019	49,670
2018	19,098
2017	18,783
2016	4,240
Prior to 2016	17,782
Revolving Loans	5,313
Revolving to Term Loans	0
Total Loans	152,234	204,107
Commercial loans | Land - acquisition & development | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	37,348
2019	49,670
2018	19,098
2017	16,443
2016	4,240
Prior to 2016	2,131
Revolving Loans	5,313
Revolving to Term Loans	0
Total Loans	134,243	200,283
Commercial loans | Land - acquisition & development | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	0
2017	0
2016	0
Prior to 2016	15,573
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	15,573	0
Commercial loans | Land - acquisition & development | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	0
2017	2,340
2016	0
Prior to 2016	78
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	2,418	3,824
Commercial loans | Land - acquisition & development | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	720,318
2019	737,902
2018	747,387
2017	813,809
2016	586,022
Prior to 2016	2,320,588
Revolving Loans	133,644
Revolving to Term Loans	3,056
Total Loans	6,062,726	6,747,690
Consumer loans | Current
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	720,318
2019	737,204
2018	746,882
2017	812,734
2016	585,382
Prior to 2016	2,293,256
Revolving Loans	132,963
Revolving to Term Loans	2,961
Total Loans	6,031,700
Consumer loans | 30 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	464
2017	277
2016	0
Prior to 2016	3,804
Revolving Loans	14
Revolving to Term Loans	37
Total Loans	4,596
Consumer loans | 60 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	41
2017	159
2016	0
Prior to 2016	5,341
Revolving Loans	17
Revolving to Term Loans	22
Total Loans	5,580
Consumer loans | 90+ days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	698
2018	0
2017	639
2016	640
Prior to 2016	18,187
Revolving Loans	650
Revolving to Term Loans	36
Total Loans	20,850
Consumer loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	6,033,903	6,719,156
Consumer loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Consumer loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	28,823	28,534
Consumer loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Consumer loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	113,100
2019	154,502
2018	9,830
2017	750
2016	0
Prior to 2016	0
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	278,182	540,741
Consumer loans | Construction | Current
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	113,100
2019	154,502
2018	9,830
2017	750
2016	0
Prior to 2016	0
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	278,182
Consumer loans | Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	278,182	540,741
Consumer loans | Construction | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Consumer loans | Construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Consumer loans | Construction | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Consumer loans | Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	574,443
2019	560,833
2018	661,425
2017	803,273
2016	582,618
Prior to 2016	2,269,832
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	5,452,424	5,835,194
Consumer loans | Single-family residential | Current
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	574,443
2019	560,153
2018	660,961
2017	802,722
2016	581,978
Prior to 2016	2,243,045
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	5,423,302
Consumer loans | Single-family residential | 30 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	464
2017	0
2016	0
Prior to 2016	3,507
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	3,971
Consumer loans | Single-family residential | 60 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	0
2017	159
2016	0
Prior to 2016	5,259
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	5,418
Consumer loans | Single-family residential | 90+ days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	680
2018	0
2017	392
2016	640
Prior to 2016	18,021
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	19,733
Consumer loans | Single-family residential | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	5,426,365	5,808,444
Consumer loans | Single-family residential | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Consumer loans | Single-family residential | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	26,059	26,750
Consumer loans | Single-family residential | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Consumer loans | Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	31,654
2019	22,303
2018	9,957
2017	9,408
2016	3,064
Prior to 2016	22,869
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	99,255	99,694
Consumer loans | Land - consumer lot loans | Current
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	31,654
2019	22,303
2018	9,916
2017	9,184
2016	3,064
Prior to 2016	22,652
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	98,773
Consumer loans | Land - consumer lot loans | 30 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	0
2017	102
2016	0
Prior to 2016	34
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	136
Consumer loans | Land - consumer lot loans | 60 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	41
2017	0
2016	0
Prior to 2016	62
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	103
Consumer loans | Land - consumer lot loans | 90+ days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	0
2017	122
2016	0
Prior to 2016	121
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	243
Consumer loans | Land - consumer lot loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	98,976	98,828
Consumer loans | Land - consumer lot loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Consumer loans | Land - consumer lot loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	279	866
Consumer loans | Land - consumer lot loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Consumer loans | HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	0
2017	0
2016	0
Prior to 2016	7,005
Revolving Loans	132,837
Revolving to Term Loans	1,323
Total Loans	141,165	142,178
Consumer loans | HELOC | Current
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	0
2017	0
2016	0
Prior to 2016	6,966
Revolving Loans	132,156
Revolving to Term Loans	1,323
Total Loans	140,445
Consumer loans | HELOC | 30 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	0
2017	0
2016	0
Prior to 2016	0
Revolving Loans	14
Revolving to Term Loans	0
Total Loans	14
Consumer loans | HELOC | 60 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	0
2017	0
2016	0
Prior to 2016	0
Revolving Loans	17
Revolving to Term Loans	0
Total Loans	17
Consumer loans | HELOC | 90+ days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	0
2017	0
2016	0
Prior to 2016	39
Revolving Loans	650
Revolving to Term Loans	0
Total Loans	689
Consumer loans | HELOC | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	138,695	141,271
Consumer loans | HELOC | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Consumer loans | HELOC | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	2,470	907
Consumer loans | HELOC | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Consumer loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	1,121
2019	264
2018	66,175
2017	378
2016	340
Prior to 2016	20,882
Revolving Loans	807
Revolving to Term Loans	1,733
Total Loans	91,700	129,883
Consumer loans | Consumer | Current
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	1,121
2019	246
2018	66,175
2017	78
2016	340
Prior to 2016	20,593
Revolving Loans	807
Revolving to Term Loans	1,638
Total Loans	90,998
Consumer loans | Consumer | 30 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	0
2017	175
2016	0
Prior to 2016	263
Revolving Loans	0
Revolving to Term Loans	37
Total Loans	475
Consumer loans | Consumer | 60 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	0
2018	0
2017	0
2016	0
Prior to 2016	20
Revolving Loans	0
Revolving to Term Loans	22
Total Loans	42
Consumer loans | Consumer | 90+ days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD  June 30, 2020	0
2019	18
2018	0
2017	125
2016	0
Prior to 2016	6
Revolving Loans	0
Revolving to Term Loans	36
Total Loans	185
Consumer loans | Consumer | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	91,685	129,872
Consumer loans | Consumer | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	0
Consumer loans | Consumer | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	15	11
Consumer loans | Consumer | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 0	$ 0