Loans Receivable - Loans Based on Credit Quality Indicators (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Sep. 30, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 13,638,781	$ 12,959,272
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	13,058,441	12,303,135
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	114,141	247,543
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	466,199	408,594
Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	1,865,283
2020	1,769,008
2019	956,599
2018	722,888
2017	640,056
Prior to 2017	973,907
Revolving Loans	978,872
Revolving to Term Loans	12,307
Total Loans	7,918,920	7,044,426
Commercial loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	1,865,283
2020	1,696,158
2019	862,155
2018	658,301
2017	549,858
Prior to 2017	872,967
Revolving Loans	848,115
Revolving to Term Loans	12,039
Total Loans	7,364,876	6,412,415
Commercial loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	11,130
2019	10,969
2018	36,134
2017	592
Prior to 2017	24,230
Revolving Loans	30,897
Revolving to Term Loans	0
Total Loans	113,952	247,351
Commercial loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	61,720
2019	83,475
2018	28,453
2017	89,606
Prior to 2017	76,710
Revolving Loans	99,860
Revolving to Term Loans	268
Total Loans	440,092	384,660
Commercial loans | Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	423,580
2020	486,155
2019	226,102
2018	281,734
2017	235,140
Prior to 2017	313,698
Revolving Loans	46,161
Revolving to Term Loans	0
Total Loans	2,012,570	1,538,240
Commercial loans | Multi-family | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	423,580
2020	484,385
2019	218,555
2018	277,958
2017	230,822
Prior to 2017	307,216
Revolving Loans	46,161
Revolving to Term Loans	0
Total Loans	1,988,677	1,506,692
Commercial loans | Multi-family | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	1,770
2019	0
2018	3,125
2017	592
Prior to 2017	1,939
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	7,426	13,721
Commercial loans | Multi-family | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	7,547
2018	651
2017	3,726
Prior to 2017	4,543
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	16,467	17,827
Commercial loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	421,039
2020	455,919
2019	350,651
2018	334,434
2017	274,902
Prior to 2017	464,600
Revolving Loans	2,190
Revolving to Term Loans	248
Total Loans	2,303,983	1,884,688
Commercial loans | Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	421,039
2020	447,503
2019	276,598
2018	278,685
2017	228,012
Prior to 2017	389,739
Revolving Loans	2,190
Revolving to Term Loans	248
Total Loans	2,044,014	1,681,230
Commercial loans | Commercial real estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	4,433
2018	33,009
2017	0
Prior to 2017	22,291
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	59,733	92,184
Commercial loans | Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	8,416
2019	69,620
2018	22,740
2017	46,890
Prior to 2017	52,570
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	200,236	111,274
Commercial loans | Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	717,712
2020	429,773
2019	63,432
2018	48,394
2017	76,925
Prior to 2017	178,485
Revolving Loans	846,755
Revolving to Term Loans	12,059
Total Loans	2,373,535	2,115,513
Commercial loans | Commercial & industrial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	717,712
2020	372,722
2019	53,031
2018	43,332
2017	76,880
Prior to 2017	158,888
Revolving Loans	715,998
Revolving to Term Loans	11,791
Total Loans	2,150,354	1,898,709
Commercial loans | Commercial & industrial | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	9,360
2019	6,536
2018	0
2017	0
Prior to 2017	0
Revolving Loans	30,897
Revolving to Term Loans	0
Total Loans	46,793	64,695
Commercial loans | Commercial & industrial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	47,691
2019	3,865
2018	5,062
2017	45
Prior to 2017	19,597
Revolving Loans	99,860
Revolving to Term Loans	268
Total Loans	176,388	152,109
Commercial loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	248,999
2020	355,803
2019	298,135
2018	42,024
2017	36,605
Prior to 2017	0
Revolving Loans	81,167
Revolving to Term Loans	0
Total Loans	1,062,733	1,352,414
Commercial loans | Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	248,999
2020	350,190
2019	295,692
2018	42,024
2017	0
Prior to 2017	0
Revolving Loans	81,167
Revolving to Term Loans	0
Total Loans	1,018,072	1,187,786
Commercial loans | Construction | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	61,178
Commercial loans | Construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	5,613
2019	2,443
2018	0
2017	36,605
Prior to 2017	0
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	44,661	103,450
Commercial loans | Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	53,953
2020	41,358
2019	18,279
2018	16,302
2017	16,484
Prior to 2017	17,124
Revolving Loans	2,599
Revolving to Term Loans	0
Total Loans	166,099	153,571
Commercial loans | Land - acquisition & development | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	53,953
2020	41,358
2019	18,279
2018	16,302
2017	14,144
Prior to 2017	17,124
Revolving Loans	2,599
Revolving to Term Loans	0
Total Loans	163,759	137,998
Commercial loans | Land - acquisition & development | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	0	15,573
Commercial loans | Land - acquisition & development | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	0
2018	0
2017	2,340
Prior to 2017	0
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	2,340	0
Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	1,260,369
2020	1,071,525
2019	475,362
2018	448,236
2017	454,948
Prior to 2017	1,827,711
Revolving Loans	180,353
Revolving to Term Loans	1,357
Total Loans	5,719,861	5,914,846
Consumer loans | Current
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	1,260,369
2020	1,071,525
2019	474,958
2018	448,078
2017	453,105
Prior to 2017	1,808,286
Revolving Loans	180,042
Revolving to Term Loans	1,357
Total Loans	5,697,720
Consumer loans | 30 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	11
2018	43
2017	743
Prior to 2017	2,097
Revolving Loans	35
Revolving to Term Loans	0
Total Loans	2,929
Consumer loans | 60 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	359
2018	0
2017	265
Prior to 2017	1,721
Revolving Loans	25
Revolving to Term Loans	0
Total Loans	2,370
Consumer loans | 90+ days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	34
2018	115
2017	835
Prior to 2017	15,607
Revolving Loans	251
Revolving to Term Loans	0
Total Loans	16,842
Consumer loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	5,693,565	5,890,720
Consumer loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	189	192
Consumer loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	26,107	23,934
Consumer loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	115,870
2020	210,457
2019	7,617
2018	1,109
2017	0
Prior to 2017	0
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	335,053	295,953
Consumer loans | Construction | Current
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	115,870
2020	210,457
2019	7,617
2018	1,109
2017	0
Prior to 2017	0
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	335,053
Consumer loans | Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	335,053	295,953
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 | Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	1,067,608
2020	820,533
2019	455,373
2018	401,364
2017	448,809
Prior to 2017	1,792,573
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	4,986,260	5,293,962
Consumer loans | Single-family residential | Current
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	1,067,608
2020	820,533
2019	455,021
2018	401,206
2017	447,227
Prior to 2017	1,773,626
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	4,965,221
Consumer loans | Single-family residential | 30 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	0
2018	43
2017	592
Prior to 2017	2,007
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	2,642
Consumer loans | Single-family residential | 60 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	352
2018	0
2017	155
Prior to 2017	1,563
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	2,070
Consumer loans | Single-family residential | 90+ days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	0
2018	115
2017	835
Prior to 2017	15,377
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	16,327
Consumer loans | Single-family residential | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	4,962,172	5,270,666
Consumer loans | Single-family residential | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	189	192
Consumer loans | Single-family residential | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	23,899	23,104
Consumer loans | Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	65,377
2020	32,321
2019	11,527
2018	4,976
2017	5,994
Prior to 2017	17,507
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	137,702	101,394
Consumer loans | Land - consumer lot loans | Current
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	65,377
2020	32,321
2019	11,527
2018	4,976
2017	5,878
Prior to 2017	17,258
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	137,337
Consumer loans | Land - consumer lot loans | 30 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	0
2018	0
2017	116
Prior to 2017	0
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	116
Consumer loans | Land - consumer lot loans | 60 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	0
2018	0
2017	0
Prior to 2017	54
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	54
Consumer loans | Land - consumer lot loans | 90+ days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	0
2018	0
2017	0
Prior to 2017	195
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	195
Consumer loans | Land - consumer lot loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	137,412	101,151
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	290	243
Consumer loans | HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	0
2018	0
2017	0
Prior to 2017	5,542
Revolving Loans	147,437
Revolving to Term Loans	1,357
Total Loans	154,336	140,222
Consumer loans | HELOC | Current
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	0
2018	0
2017	0
Prior to 2017	5,431
Revolving Loans	147,126
Revolving to Term Loans	1,357
Total Loans	153,914
Consumer loans | HELOC | 30 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	0
2018	0
2017	0
Prior to 2017	81
Revolving Loans	35
Revolving to Term Loans	0
Total Loans	116
Consumer loans | HELOC | 60 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	0
2018	0
2017	0
Prior to 2017	0
Revolving Loans	25
Revolving to Term Loans	0
Total Loans	25
Consumer loans | HELOC | 90+ days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	0
2018	0
2017	0
Prior to 2017	30
Revolving Loans	251
Revolving to Term Loans	0
Total Loans	281
Consumer loans | HELOC | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	152,439	139,646
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	1,897	576
Consumer loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	11,514
2020	8,214
2019	845
2018	40,787
2017	145
Prior to 2017	12,089
Revolving Loans	32,916
Revolving to Term Loans	0
Total Loans	106,510	83,315
Consumer loans | Consumer | Current
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	11,514
2020	8,214
2019	793
2018	40,787
2017	0
Prior to 2017	11,971
Revolving Loans	32,916
Revolving to Term Loans	0
Total Loans	106,195
Consumer loans | Consumer | 30 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	11
2018	0
2017	35
Prior to 2017	9
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	55
Consumer loans | Consumer | 60 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	7
2018	0
2017	110
Prior to 2017	104
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	221
Consumer loans | Consumer | 90+ days past due
Financing Receivable, Credit Quality Indicator [Line Items]
YTD 2021	0
2020	0
2019	34
2018	0
2017	0
Prior to 2017	5
Revolving Loans	0
Revolving to Term Loans	0
Total Loans	39
Consumer loans | Consumer | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	106,489	83,304
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	$ 21	$ 11