Loans Receivable - Loans Receivable, Analysis of Age of Loans in Past Due Status (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 17,653,757	$ 16,286,372
% based on $, past due	0.36%	0.17%
Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 17,590,442	$ 16,259,445
Delinquency %, current	99.64%	99.83%
Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 63,315	$ 26,927
% based on $, past due	0.36%	0.17%
30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 9,715	$ 3,651
% based on $, past due	0.06%	0.02%
60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 11,682	$ 2,887
% based on $, past due	0.07%	0.02%
90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 41,918	$ 20,389
% based on $, past due	0.24%	0.13%
Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 10,507,610	$ 9,728,501
% based on $, past due	0.31%	0.08%
Commercial loans | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 10,474,752	$ 9,720,833
Commercial loans | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	32,858	7,668
Commercial loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	878	538
Commercial loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	2,331	1,369
Commercial loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	29,649	5,761
Commercial loans | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 2,886,594	$ 2,626,479
% based on $, past due	0.00%	0.00%
Commercial loans | Multi-family | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 2,886,462	$ 2,626,479
Commercial loans | Multi-family | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	132	0
Commercial loans | Multi-family | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	0
Commercial loans | Multi-family | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	0
Commercial loans | Multi-family | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	132	0
Commercial loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 3,310,101	$ 3,111,112
% based on $, past due	0.74%	0.03%
Commercial loans | Commercial real estate | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 3,285,673	$ 3,110,056
Commercial loans | Commercial real estate | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	24,428	1,056
Commercial loans | Commercial real estate | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	848	538
Commercial loans | Commercial real estate | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	145	450
Commercial loans | Commercial real estate | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	23,435	68
Commercial loans | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 2,315,318	$ 2,343,403
% based on $, past due	0.36%	0.28%
Commercial loans | Commercial & industrial | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 2,307,020	$ 2,336,791
Commercial loans | Commercial & industrial | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	8,298	6,612
Commercial loans | Commercial & industrial | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	30	0
Commercial loans | Commercial & industrial | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	2,186	919
Commercial loans | Commercial & industrial | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	6,082	5,693
Commercial loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 1,838,936	$ 1,423,891
% based on $, past due	0.00%	0.00%
Commercial loans | Construction | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 1,838,936	$ 1,423,891
Commercial loans | Construction | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	0
Commercial loans | Construction | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	0
Commercial loans | Construction | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	0
Commercial loans | Construction | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0
Commercial loans | Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 156,661	$ 223,616
% based on $, past due	0.00%	0.00%
Commercial loans | Land - acquisition & development | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 156,661	$ 223,616
Commercial loans | Land - acquisition & development | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	0
Commercial loans | Land - acquisition & development | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	0
Commercial loans | Land - acquisition & development | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	0
Commercial loans | Land - acquisition & development | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	0
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 7,146,147	$ 6,557,871
% based on $, past due	0.43%	0.29%
Consumer loans | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 7,115,690	$ 6,538,612
Consumer loans | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	30,457	19,259
Consumer loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	8,837	3,113
Consumer loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	9,351	1,518
Consumer loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	12,269	14,628
Consumer loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 324,451	$ 397,343
% based on $, past due	1.07%	0.11%
Consumer loans | Construction | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 320,987	$ 396,908
Consumer loans | Construction | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	3,464	435
Consumer loans | Construction | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	760	0
Consumer loans | Construction | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	2,617	0
Consumer loans | Construction | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	87	435
Consumer loans | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 6,388,990	$ 5,726,979
% based on $, past due	0.37%	0.31%
Consumer loans | Single-family residential | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 6,365,065	$ 5,708,996
Consumer loans | Single-family residential | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	23,925	17,983
Consumer loans | Single-family residential | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	6,441	2,796
Consumer loans | Single-family residential | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	6,068	1,316
Consumer loans | Single-family residential | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	11,416	13,871
Consumer loans | Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 124,842	$ 151,945
% based on $, past due	0.49%	0.13%
Consumer loans | Land - consumer lot loans | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 124,231	$ 151,746
Consumer loans | Land - consumer lot loans | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	611	199
Consumer loans | Land - consumer lot loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	358	0
Consumer loans | Land - consumer lot loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	245	139
Consumer loans | Land - consumer lot loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	8	60
Consumer loans | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 237,754	$ 206,033
% based on $, past due	0.86%	0.21%
Consumer loans | HELOC | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 235,708	$ 205,605
Consumer loans | HELOC | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	2,046	428
Consumer loans | HELOC | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	1,050	155
Consumer loans | HELOC | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	314	46
Consumer loans | HELOC | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	682	227
Consumer loans | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 70,110	$ 75,571
% based on $, past due	0.59%	0.28%
Consumer loans | Consumer | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 69,699	$ 75,357
Consumer loans | Consumer | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	411	214
Consumer loans | Consumer | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	228	162
Consumer loans | Consumer | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	107	17
Consumer loans | Consumer | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 76	$ 35