Loans Receivable - Loan Delinquencies by Loan Portfolio and Class (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Sep. 30, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 17,449,326	$ 16,286,372
% based on $, past due	0.16%	0.17%
Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 17,421,492	$ 16,259,445
% based on $, current	99.84%	99.83%
Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 27,834	$ 26,927
% based on $, past due	0.16%	0.17%
30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 9,898	$ 3,651
% based on $, past due	0.06%	0.02%
60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 4,017	$ 2,887
% based on $, past due	0.02%	0.02%
90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 13,919	$ 20,389
% based on $, past due	0.08%	0.13%
Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 10,479,087	$ 9,728,501
% based on $, past due	0.07%	0.08%
Commercial loans | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 10,472,240	$ 9,720,833
Commercial loans | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	6,847	7,668
Commercial loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	3,157	538
Commercial loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	2,960	1,369
Commercial loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	730	5,761
Commercial loans | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,859,994	$ 2,626,479
% based on $, past due	0.00%	0.00%
Commercial loans | Multi-family | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,859,862	$ 2,626,479
Commercial loans | Multi-family | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	132	0
Commercial loans | Multi-family | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	132	0
Commercial loans | Multi-family | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	0
Commercial loans | Multi-family | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	0
Commercial loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 3,258,304	$ 3,111,112
% based on $, past due	0.06%	0.03%
Commercial loans | Commercial real estate | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 3,256,293	$ 3,110,056
Commercial loans | Commercial real estate | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	2,011	1,056
Commercial loans | Commercial real estate | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	2,011	538
Commercial loans | Commercial real estate | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	450
Commercial loans | Commercial real estate | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	68
Commercial loans | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,585,196	$ 2,343,403
% based on $, past due	0.16%	0.28%
Commercial loans | Commercial & industrial | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,580,997	$ 2,336,791
Commercial loans | Commercial & industrial | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	4,199	6,612
Commercial loans | Commercial & industrial | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	509	0
Commercial loans | Commercial & industrial | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	2,960	919
Commercial loans | Commercial & industrial | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	730	5,693
Commercial loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,608,513	$ 1,423,891
% based on $, past due	0.03%	0.00%
Commercial loans | Construction | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,608,008	$ 1,423,891
Commercial loans | Construction | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	505	0
Commercial loans | Construction | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	505	0
Commercial loans | Construction | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	0
Commercial loans | Construction | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0
Commercial loans | Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 167,080	$ 223,616
% based on $, past due	0.00%	0.00%
Commercial loans | Land - acquisition & development | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 167,080	$ 223,616
Commercial loans | Land - acquisition & development | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	0
Commercial loans | Land - acquisition & development | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	0
Commercial loans | Land - acquisition & development | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	0
Commercial loans | Land - acquisition & development | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	0
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 6,970,239	$ 6,557,871
% based on $, past due	0.30%	0.29%
Consumer loans | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 6,949,252	$ 6,538,612
Consumer loans | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	20,987	19,259
Consumer loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	6,741	3,113
Consumer loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,057	1,518
Consumer loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	13,189	14,628
Consumer loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 403,783	$ 397,343
% based on $, past due	0.02%	0.11%
Consumer loans | Construction | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 403,696	$ 396,908
Consumer loans | Construction | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	87	435
Consumer loans | Construction | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	87	0
Consumer loans | Construction | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	0
Consumer loans | Construction | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	435
Consumer loans | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 6,134,021	$ 5,726,979
% based on $, past due	0.31%	0.31%
Consumer loans | Single-family residential | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 6,114,783	$ 5,708,996
Consumer loans | Single-family residential | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	19,238	17,983
Consumer loans | Single-family residential | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	5,894	2,796
Consumer loans | Single-family residential | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	432	1,316
Consumer loans | Single-family residential | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	12,912	13,871
Consumer loans | Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 140,140	$ 151,945
% based on $, past due	0.06%	0.13%
Consumer loans | Land - consumer lot loans | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 140,061	$ 151,746
Consumer loans | Land - consumer lot loans | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	79	199
Consumer loans | Land - consumer lot loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	79	0
Consumer loans | Land - consumer lot loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	139
Consumer loans | Land - consumer lot loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	60
Consumer loans | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 221,159	$ 206,033
% based on $, past due	0.61%	0.21%
Consumer loans | HELOC | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 219,811	$ 205,605
Consumer loans | HELOC | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,348	428
Consumer loans | HELOC | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	600	155
Consumer loans | HELOC | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	624	46
Consumer loans | HELOC | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	124	227
Consumer loans | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 71,136	$ 75,571
% based on $, past due	0.33%	0.28%
Consumer loans | Consumer | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 70,901	$ 75,357
Consumer loans | Consumer | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	235	214
Consumer loans | Consumer | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	81	162
Consumer loans | Consumer | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1	17
Consumer loans | Consumer | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 153	$ 35