Loans Receivable - Loan Delinquencies by Loan Portfolio and Class (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Sep. 30, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 15,736,144	$ 14,004,870
% based on $, past due	0.26%	0.19%
Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 15,695,446	$ 13,977,902
% based on $, current	99.74%	99.81%
Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 40,698	$ 26,968
% based on $, past due	0.26%	0.19%
30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 5,419	$ 4,802
% based on $, past due	0.03%	0.03%
60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,971	$ 2,595
% based on $, past due	0.01%	0.02%
90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 33,308	$ 19,571
% based on $, past due	0.21%	0.14%
90 Days Delinquent | SBA Payroll Protection Program, CARES Act
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 10,000
Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 9,330,158	$ 8,316,591
% based on $, past due	0.18%	0.05%
Commercial loans | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 9,313,386	$ 8,312,095
Commercial loans | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	16,772	4,496
Commercial loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	369	971
Commercial loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	359	64
Commercial loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	16,044	3,461
Commercial loans | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,478,932	$ 2,273,689
% based on $, past due	0.00%	0.02%
Commercial loans | Multi-family | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,478,932	$ 2,273,214
Commercial loans | Multi-family | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	475
Commercial loans | Multi-family | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	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	475
Commercial loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,876,995	$ 2,429,332
% based on $, past due	0.01%	0.05%
Commercial loans | Commercial real estate | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,876,644	$ 2,428,014
Commercial loans | Commercial real estate | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	351	1,318
Commercial loans | Commercial real estate | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	971
Commercial loans | Commercial real estate | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	64
Commercial loans | Commercial real estate | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	351	283
Commercial loans | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,344,046	$ 2,303,927
% based on $, past due	0.70%	0.01%
Commercial loans | Commercial & industrial | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,327,625	$ 2,303,605
Commercial loans | Commercial & industrial | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	16,421	322
Commercial loans | Commercial & industrial | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	369	0
Commercial loans | Commercial & industrial | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	359	0
Commercial loans | Commercial & industrial | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	15,693	322
Commercial loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,407,211	$ 1,117,227
% based on $, past due	0.00%	0.00%
Commercial loans | Construction | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,407,211	$ 1,117,186
Commercial loans | Construction | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	41
Commercial loans | Construction | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	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	41
Commercial loans | Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 222,974	$ 192,416
% based on $, past due	0.00%	1.22%
Commercial loans | Land - acquisition & development | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 222,974	$ 190,076
Commercial loans | Land - acquisition & development | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	2,340
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	2,340
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 6,405,986	$ 5,688,279
% based on $, past due	0.37%	0.40%
Consumer loans | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 6,382,060	$ 5,665,807
Consumer loans | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	23,926	22,472
Consumer loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	5,050	3,831
Consumer loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,612	2,531
Consumer loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	17,264	16,110
Consumer loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 368,839	$ 347,752
% based on $, past due	0.24%	0.00%
Consumer loans | Construction | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 367,939	$ 347,752
Consumer loans | Construction | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	900	0
Consumer loans | Construction | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	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	900	0
Consumer loans | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 5,619,448	$ 4,937,064
% based on $, past due	0.39%	0.43%
Consumer loans | Single-family residential | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 5,597,259	$ 4,915,749
Consumer loans | Single-family residential | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	22,189	21,315
Consumer loans | Single-family residential | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	4,611	3,627
Consumer loans | Single-family residential | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,533	2,165
Consumer loans | Single-family residential | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	16,045	15,523
Consumer loans | Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 157,076	$ 148,534
% based on $, past due	0.24%	0.39%
Consumer loans | Land - consumer lot loans | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 156,693	$ 147,952
Consumer loans | Land - consumer lot loans | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	383	582
Consumer loans | Land - consumer lot loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	206	5
Consumer loans | Land - consumer lot loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	307
Consumer loans | Land - consumer lot loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	177	270
Consumer loans | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 187,469	$ 166,940
% based on $, past due	0.11%	0.19%
Consumer loans | HELOC | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 187,267	$ 166,627
Consumer loans | HELOC | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	202	313
Consumer loans | HELOC | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	103	47
Consumer loans | HELOC | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	16	0
Consumer loans | HELOC | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	83	266
Consumer loans | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 73,154	$ 87,989
% based on $, past due	0.34%	0.30%
Consumer loans | Consumer | Financial Asset, Not Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 72,902	$ 87,727
Consumer loans | Consumer | Financial Asset, Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	252	262
Consumer loans | Consumer | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	130	152
Consumer loans | Consumer | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	63	59
Consumer loans | Consumer | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 59	$ 51