Loans Receivable - Loans Receivable, Analysis of Age of Loans in Past Due Status (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 12,898,775	$ 12,111,047
Days delinquent, current	12,857,170	12,075,677
Days delinquent, past due	$ 41,605	$ 35,370
% based on $, current	99.68%	99.71%
% based on $, past due	0.32%	0.29%
30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 4,773	$ 6,273
% based on $, past due	0.04%	0.05%
60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 10,776	$ 4,415
% based on $, past due	0.08%	0.04%
90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 26,056	$ 24,682
% based on $, past due	0.20%	0.20%
Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 6,836,049	$ 5,648,601
Days delinquent, current	6,825,470	5,642,072
Days delinquent, past due	$ 10,579	$ 6,529
% based on $, past due	0.15%	0.12%
Commercial loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 177	$ 1,614
Commercial loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	5,196	285
Commercial loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	5,206	4,630
Commercial loans | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	1,509,837	1,422,652
Days delinquent, current	1,509,613	1,422,652
Days delinquent, past due	$ 224	$ 0
% based on $, past due	0.01%	0.00%
Commercial loans | Multi-family | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 0	$ 0
Commercial loans | Multi-family | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	0	0
Commercial loans | Multi-family | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	224	0
Commercial loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	1,700,568	1,631,171
Days delinquent, current	1,697,302	1,625,509
Days delinquent, past due	$ 3,266	$ 5,662
% based on $, past due	0.19%	0.35%
Commercial loans | Commercial real estate | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 177	$ 1,614
Commercial loans | Commercial real estate | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	1,938	285
Commercial loans | Commercial real estate | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	1,151	3,763
Commercial loans | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	2,138,462	1,268,695
Days delinquent, current	2,134,726	1,267,828
Days delinquent, past due	$ 3,736	$ 867
% based on $, past due	0.17%	0.07%
Commercial loans | Commercial & industrial | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 0	$ 0
Commercial loans | Commercial & industrial | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	3,258	0
Commercial loans | Commercial & industrial | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	478	867
Commercial loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	1,334,948	1,164,889
Days delinquent, current	1,331,595	1,164,889
Days delinquent, past due	$ 3,353	$ 0
% based on $, past due	0.25%	0.00%
Commercial loans | Construction | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 0	$ 0
Commercial loans | Construction | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	0	0
Commercial loans | Construction | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	3,353	0
Commercial loans | Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	152,234	161,194
Days delinquent, current	152,234	161,194
Days delinquent, past due	$ 0	$ 0
% based on $, past due	0.00%	0.00%
Commercial loans | Land - acquisition & development | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 0	$ 0
Commercial loans | Land - acquisition & development | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	0	0
Commercial loans | Land - acquisition & development | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	0	0
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	6,062,726	6,462,446
Days delinquent, current	6,031,700	6,433,605
Days delinquent, past due	$ 31,026	$ 28,841
% based on $, past due	0.51%	0.45%
Consumer loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 4,596	$ 4,659
Consumer loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	5,580	4,130
Consumer loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	20,850	20,052
Consumer loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	278,182	255,505
Days delinquent, current	278,182	255,505
Days delinquent, past due	$ 0	$ 0
% based on $, past due	0.00%	0.00%
Consumer loans | Construction | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 0	$ 0
Consumer loans | Construction | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	0	0
Consumer loans | Construction | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	0	0
Consumer loans | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	5,452,424	5,835,186
Days delinquent, current	5,423,302	5,809,239
Days delinquent, past due	$ 29,122	$ 25,947
% based on $, past due	0.53%	0.44%
Consumer loans | Single-family residential | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 3,971	$ 3,672
Consumer loans | Single-family residential | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	5,418	3,211
Consumer loans | Single-family residential | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	19,733	19,064
Consumer loans | Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	99,255	99,694
Days delinquent, current	98,773	98,916
Days delinquent, past due	$ 482	$ 778
% based on $, past due	0.49%	0.78%
Consumer loans | Land - consumer lot loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 136	$ 112
Consumer loans | Land - consumer lot loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	103	619
Consumer loans | Land - consumer lot loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	243	47
Consumer loans | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	141,165	142,178
Days delinquent, current	140,445	140,718
Days delinquent, past due	$ 720	$ 1,460
% based on $, past due	0.51%	1.03%
Consumer loans | HELOC | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 14	$ 580
Consumer loans | HELOC | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	17	183
Consumer loans | HELOC | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	689	697
Consumer loans | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	91,700	129,883
Days delinquent, current	90,998	129,227
Days delinquent, past due	$ 702	$ 656
% based on $, past due	0.77%	0.51%
Consumer loans | Consumer | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 475	$ 295
Consumer loans | Consumer | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	42	117
Consumer loans | Consumer | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 185	$ 244