Allowance for Losses on Loans - Schedule of Gross Loans by Payment Activity (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Sep. 30, 2025
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 20,232,707	$ 20,288,338
Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	12,574,277	11,851,657
Commercial loans \| Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	4,645,902	4,631,321
Commercial loans \| Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	3,626,043	3,588,950
Commercial loans \| Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	3,101,192	2,386,363
Commercial loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	1,015,908	1,105,101
Commercial loans \| Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	185,232	139,922
Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	7,658,430	8,436,681
Consumer loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	16,417	78,243
Consumer loans \| Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	7,268,163	7,936,931
Consumer loans \| Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	71,982	88,696
Consumer loans \| HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	247,584	271,286
Consumer loans \| Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	54,284	61,525
Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 20,105,138	$ 20,159,710
% of Total Loans	99.40%	99.40%
Performing Loans \| Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 12,471,173	$ 11,748,117
% of Total Loans	99.20%	99.10%
Performing Loans \| Commercial loans \| Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 4,611,653	$ 4,612,200
% of Total Loans	99.30%	99.60%
Performing Loans \| Commercial loans \| Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 3,622,929	$ 3,518,978
% of Total Loans	99.90%	98.10%
Performing Loans \| Commercial loans \| Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 3,035,451	$ 2,375,316
% of Total Loans	97.90%	99.50%
Performing Loans \| Commercial loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 1,015,908	$ 1,101,701
% of Total Loans	100.00%	99.70%
Performing Loans \| Commercial loans \| Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 185,232	$ 139,922
% of Total Loans	100.00%	100.00%
Performing Loans \| Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 7,633,965	$ 8,411,593
% of Total Loans	99.70%	99.70%
Performing Loans \| Consumer loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 15,525	$ 77,483
% of Total Loans	94.60%	99.00%
Performing Loans \| Consumer loans \| Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 7,245,718	$ 7,913,190
% of Total Loans	99.70%	99.70%
Performing Loans \| Consumer loans \| Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 71,743	$ 88,673
% of Total Loans	99.70%	100.00%
Performing Loans \| Consumer loans \| HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 246,957	$ 270,874
% of Total Loans	99.70%	99.80%
Performing Loans \| Consumer loans \| Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 54,022	$ 61,373
% of Total Loans	99.50%	99.80%
Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 127,569	$ 128,628
% of Total Loans	0.60%	0.60%
Non-Performing Loans \| Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 103,104	$ 103,540
% of Total Loans	0.80%	0.90%
Non-Performing Loans \| Commercial loans \| Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 34,249	$ 19,121
% of Total Loans	0.70%	0.40%
Non-Performing Loans \| Commercial loans \| Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 3,114	$ 69,972
% of Total Loans	0.10%	1.90%
Non-Performing Loans \| Commercial loans \| Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 65,741	$ 11,047
% of Total Loans	2.10%	0.50%
Non-Performing Loans \| Commercial loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 0	$ 3,400
% of Total Loans	0.00%	0.30%
Non-Performing Loans \| Commercial loans \| Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 0	$ 0
% of Total Loans	0.00%	0.00%
Non-Performing Loans \| Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 24,465	$ 25,088
% of Total Loans	0.30%	0.30%
Non-Performing Loans \| Consumer loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 892	$ 760
% of Total Loans	5.40%	1.00%
Non-Performing Loans \| Consumer loans \| Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 22,445	$ 23,741
% of Total Loans	0.30%	0.30%
Non-Performing Loans \| Consumer loans \| Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 239	$ 23
% of Total Loans	0.30%	0.00%
Non-Performing Loans \| Consumer loans \| HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 627	$ 412
% of Total Loans	0.30%	0.20%
Non-Performing Loans \| Consumer loans \| Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 262	$ 152
% of Total Loans	0.50%	0.20%