Loans Receivable - Loans Receivable, Analysis of Age of Loans in Past Due Status (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 14,004,870	$ 12,959,272
% based on $, past due	0.19%	0.24%
Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 13,977,902	$ 12,928,463
Delinquency %, current	99.81%	99.76%
Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 26,968	$ 30,809
% based on $, past due	0.19%	0.24%
30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 4,802	$ 4,470
% based on $, past due	0.03%	0.03%
60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 2,595	$ 3,973
% based on $, past due	0.02%	0.03%
90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 19,571	$ 22,366
% based on $, past due	0.14%	0.17%
Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 8,316,591	$ 7,044,426
% based on $, past due	0.05%	0.04%
Commercial loans | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 8,312,095	$ 7,041,423
Commercial loans | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	4,496	3,003
Commercial loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	971	0
Commercial loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	64	778
Commercial loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	3,461	2,225
Commercial loans | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 2,273,689	$ 1,538,240
% based on $, past due	0.02%	0.00%
Commercial loans | Multi-family | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 2,273,214	$ 1,538,240
Commercial loans | Multi-family | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	475	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	475	0
Commercial loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 2,429,332	$ 1,884,688
% based on $, past due	0.05%	0.03%
Commercial loans | Commercial real estate | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 2,428,014	$ 1,884,210
Commercial loans | Commercial real estate | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	1,318	478
Commercial loans | Commercial real estate | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	971	0
Commercial loans | Commercial real estate | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	64	195
Commercial loans | Commercial real estate | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	283	283
Commercial loans | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 2,303,927	$ 2,115,513
% based on $, past due	0.01%	0.04%
Commercial loans | Commercial & industrial | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 2,303,605	$ 2,114,650
Commercial loans | Commercial & industrial | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	322	863
Commercial loans | Commercial & industrial | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	0
Commercial loans | Commercial & industrial | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	583
Commercial loans | Commercial & industrial | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	322	280
Commercial loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 1,117,227	$ 1,352,414
% based on $, past due	0.00%	0.12%
Commercial loans | Construction | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 1,117,186	$ 1,350,752
Commercial loans | Construction | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	41	1,662
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	41	1,662
Commercial loans | Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 192,416	$ 153,571
% based on $, past due	1.22%	0.00%
Commercial loans | Land - acquisition & development | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 190,076	$ 153,571
Commercial loans | Land - acquisition & development | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	2,340	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	2,340	0
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 5,688,279	$ 5,914,846
% based on $, past due	0.40%	0.47%
Consumer loans | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 5,665,807	$ 5,887,040
Consumer loans | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	22,472	27,806
Consumer loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	3,831	4,470
Consumer loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	2,531	3,195
Consumer loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	16,110	20,141
Consumer loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 347,752	$ 295,953
% based on $, past due	0.00%	0.00%
Consumer loans | Construction | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 347,752	$ 295,953
Consumer loans | Construction | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	0
Consumer loans | Construction | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	0
Consumer loans | Construction | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	0
Consumer loans | Construction | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	0
Consumer loans | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 4,937,064	$ 5,293,962
% based on $, past due	0.43%	0.50%
Consumer loans | Single-family residential | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 4,915,749	$ 5,267,608
Consumer loans | Single-family residential | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	21,315	26,354
Consumer loans | Single-family residential | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	3,627	3,922
Consumer loans | Single-family residential | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	2,165	3,108
Consumer loans | Single-family residential | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	15,523	19,324
Consumer loans | Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 148,534	$ 101,394
% based on $, past due	0.39%	0.36%
Consumer loans | Land - consumer lot loans | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 147,952	$ 101,029
Consumer loans | Land - consumer lot loans | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	582	365
Consumer loans | Land - consumer lot loans | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	5	152
Consumer loans | Land - consumer lot loans | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	307	0
Consumer loans | Land - consumer lot loans | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	270	213
Consumer loans | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 166,940	$ 140,222
% based on $, past due	0.19%	0.52%
Consumer loans | HELOC | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 166,627	$ 139,491
Consumer loans | HELOC | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	313	731
Consumer loans | HELOC | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	47	275
Consumer loans | HELOC | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	0	76
Consumer loans | HELOC | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	266	380
Consumer loans | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 87,989	$ 83,315
% based on $, past due	0.30%	0.43%
Consumer loans | Consumer | Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 87,727	$ 82,959
Consumer loans | Consumer | Total Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	262	356
Consumer loans | Consumer | 30 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	152	121
Consumer loans | Consumer | 60 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	59	11
Consumer loans | Consumer | 90 Days Delinquent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net of loans in process	$ 51	$ 224