Loans Receivable - Loans Receivable, Analysis of Age of Loans in Past Due Status (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 12,144,837	$ 12,111,047
Days delinquent, current	12,105,192	12,075,677
Days delinquent, past due	$ 39,645	$ 35,370
% based on $	0.33%	0.29%
% based on $, current	99.67%	99.71%
% based on $, past due	0.33%	0.29%
30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 9,267	$ 6,273
% based on $, past due	0.08%	0.05%
60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 4,664	$ 4,415
% based on $, past due	0.04%	0.04%
90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 25,714	$ 24,682
% based on $, past due	0.21%	0.20%
Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 5,582,243	$ 5,835,186
Days delinquent, current	5,556,051	5,809,239
Days delinquent, past due	$ 26,192	$ 25,947
% based on $	0.47%	0.44%
Single-family residential | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 5,154	$ 3,672
Single-family residential | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	3,857	3,211
Single-family residential | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	17,181	19,064
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	1,235,731	1,164,889
Days delinquent, current	1,232,378	1,164,889
Days delinquent, past due	$ 3,353	$ 0
% based on $	0.27%	0.00%
Construction | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 0	$ 0
Construction | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	0	0
Construction | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	3,353	0
Construction - custom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	270,880	255,505
Days delinquent, current	270,880	255,505
Days delinquent, past due	$ 0	$ 0
% based on $	0.00%	0.00%
Construction - custom | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 0	$ 0
Construction - custom | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	0	0
Construction - custom | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	0	0
Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	152,623	161,194
Days delinquent, current	152,229	161,194
Days delinquent, past due	$ 394	$ 0
% based on $	0.26%	0.00%
Land - acquisition & development | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 394	$ 0
Land - acquisition & development | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	0	0
Land - acquisition & development | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	0	0
Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	96,580	99,694
Days delinquent, current	95,796	98,916
Days delinquent, past due	$ 784	$ 778
% based on $	0.81%	0.78%
Land - consumer lot loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 141	$ 112
Land - consumer lot loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	270	619
Land - consumer lot loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	373	47
Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	1,467,208	1,422,652
Days delinquent, current	1,466,852	1,422,652
Days delinquent, past due	$ 356	$ 0
% based on $	0.02%	0.00%
Multi-family | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 356	$ 0
Multi-family | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	0	0
Multi-family | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	0	0
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	1,717,535	1,631,171
Days delinquent, current	1,712,893	1,625,509
Days delinquent, past due	$ 4,642	$ 5,662
% based on $	0.27%	0.35%
Commercial real estate | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 580	$ 1,614
Commercial real estate | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	414	285
Commercial real estate | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	3,648	3,763
Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	1,371,128	1,268,695
Days delinquent, current	1,368,840	1,267,828
Days delinquent, past due	$ 2,288	$ 867
% based on $	0.17%	0.07%
Commercial & industrial | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 2,215	$ 0
Commercial & industrial | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	0	0
Commercial & industrial | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	73	867
HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	145,761	142,178
Days delinquent, current	144,709	140,718
Days delinquent, past due	$ 1,052	$ 1,460
% based on $	0.72%	1.03%
HELOC | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 151	$ 580
HELOC | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	37	183
HELOC | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	864	697
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	105,148	129,883
Days delinquent, current	104,564	129,227
Days delinquent, past due	$ 584	$ 656
% based on $	0.56%	0.51%
Consumer | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 276	$ 295
Consumer | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	86	117
Consumer | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days delinquent, past due	$ 222	$ 244