Loans Receivable - Loan Delinquencies by Loan Portfolio and Class (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Sep. 30, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 21,077,743	$ 17,653,757
% based on $, past due	0.22%	0.36%
Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 21,031,022	$ 17,590,442
% based on $, current	99.78%	99.64%
Total Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 46,721	$ 63,315
% based on $, past due	0.22%	0.36%
30 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 13,432	$ 9,715
% based on $, past due	0.06%	0.06%
60 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 9,036	$ 11,682
% based on $, past due	0.04%	0.07%
90 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 24,253	$ 41,918
% based on $, past due	0.12%	0.24%
Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 12,246,080	$ 10,507,610
% based on $, past due	0.09%	0.31%
Commercial loans | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 12,235,547	$ 10,474,752
Commercial loans | Total Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	10,533	32,858
Commercial loans | 30 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	878
Commercial loans | 60 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	3,600	2,331
Commercial loans | 90 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	6,933	29,649
Commercial loans | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 4,513,323	$ 2,886,594
% based on $, past due	0.15%	0.00%
Commercial loans | Multi-family | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 4,506,711	$ 2,886,462
Commercial loans | Multi-family | Total Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	6,612	132
Commercial loans | Multi-family | 30 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	0
Commercial loans | Multi-family | 60 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	2,651	0
Commercial loans | Multi-family | 90 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	3,961	132
Commercial loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 3,746,945	$ 3,310,101
% based on $, past due	0.02%	0.74%
Commercial loans | Commercial real estate | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 3,746,356	$ 3,285,673
Commercial loans | Commercial real estate | Total Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	589	24,428
Commercial loans | Commercial real estate | 30 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	848
Commercial loans | Commercial real estate | 60 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	589	145
Commercial loans | Commercial real estate | 90 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	23,435
Commercial loans | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,389,236	$ 2,315,318
% based on $, past due	0.09%	0.36%
Commercial loans | Commercial & industrial | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,387,098	$ 2,307,020
Commercial loans | Commercial & industrial | Total Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	2,138	8,298
Commercial loans | Commercial & industrial | 30 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	30
Commercial loans | Commercial & industrial | 60 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	2,186
Commercial loans | Commercial & industrial | 90 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	2,138	6,082
Commercial loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,440,152	$ 1,838,936
% based on $, past due	0.08%	0.00%
Commercial loans | Construction | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,439,032	$ 1,838,936
Commercial loans | Construction | Total Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,120	0
Commercial loans | Construction | 30 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	0
Commercial loans | Construction | 60 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	360	0
Commercial loans | Construction | 90 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	760	0
Commercial loans | Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 156,424	$ 156,661
% based on $, past due	0.05%	0.00%
Commercial loans | Land - acquisition & development | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 156,350	$ 156,661
Commercial loans | Land - acquisition & development | Total Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	74	0
Commercial loans | Land - acquisition & development | 30 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	0
Commercial loans | Land - acquisition & development | 60 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	0
Commercial loans | Land - acquisition & development | 90 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	74	0
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 8,831,663	$ 7,146,147
% based on $, past due	0.41%	0.43%
Consumer loans | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 8,795,475	$ 7,115,690
Consumer loans | Total Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	36,188	30,457
Consumer loans | 30 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	13,432	8,837
Consumer loans | 60 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	5,436	9,351
Consumer loans | 90 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	17,320	12,269
Consumer loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 190,433	$ 324,451
% based on $, past due	0.45%	1.07%
Consumer loans | Construction | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 189,585	$ 320,987
Consumer loans | Construction | Total Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	848	3,464
Consumer loans | Construction | 30 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	760	760
Consumer loans | Construction | 60 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	2,617
Consumer loans | Construction | 90 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	88	87
Consumer loans | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 8,186,361	$ 6,388,990
% based on $, past due	0.39%	0.37%
Consumer loans | Single-family residential | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 8,154,250	$ 6,365,065
Consumer loans | Single-family residential | Total Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	32,111	23,925
Consumer loans | Single-family residential | 30 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	10,925	6,441
Consumer loans | Single-family residential | 60 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	4,937	6,068
Consumer loans | Single-family residential | 90 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	16,249	11,416
Consumer loans | Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 111,574	$ 124,842
% based on $, past due	0.63%	0.49%
Consumer loans | Land - consumer lot loans | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 110,871	$ 124,231
Consumer loans | Land - consumer lot loans | Total Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	703	611
Consumer loans | Land - consumer lot loans | 30 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	254	358
Consumer loans | Land - consumer lot loans | 60 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	213	245
Consumer loans | Land - consumer lot loans | 90 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	236	8
Consumer loans | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 258,833	$ 237,754
% based on $, past due	0.87%	0.86%
Consumer loans | HELOC | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 256,582	$ 235,708
Consumer loans | HELOC | Total Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	2,251	2,046
Consumer loans | HELOC | 30 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,366	1,050
Consumer loans | HELOC | 60 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	178	314
Consumer loans | HELOC | 90 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	707	682
Consumer loans | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 84,462	$ 70,110
% based on $, past due	0.33%	0.59%
Consumer loans | Consumer | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 84,187	$ 69,699
Consumer loans | Consumer | Total Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	275	411
Consumer loans | Consumer | 30 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	127	228
Consumer loans | Consumer | 60 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	108	107
Consumer loans | Consumer | 90 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 40	$ 76