Allowance for Losses on Loans - Schedule of Gross Loans by Payment Activity (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Sep. 30, 2025
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 20,168,933	$ 20,288,338
Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	12,241,938	11,851,657
Commercial loans \| Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	4,672,599	4,631,321
Commercial loans \| Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	3,652,683	3,588,950
Commercial loans \| Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	2,791,133	2,386,363
Commercial loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	956,957	1,105,101
Commercial loans \| Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	168,566	139,922
Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	7,926,995	8,436,681
Consumer loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	38,867	78,243
Consumer loans \| Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	7,501,331	7,936,931
Consumer loans \| Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	76,763	88,696
Consumer loans \| HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	255,352	271,286
Consumer loans \| Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	54,682	61,525
Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 20,045,074	$ 20,159,710
% of Total Loans	99.40%	99.40%
Performing Loans \| Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 12,148,143	$ 11,748,117
% of Total Loans	99.20%	99.10%
Performing Loans \| Commercial loans \| Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 4,638,214	$ 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	$ 3,648,001	$ 3,518,978
% of Total Loans	99.90%	98.10%
Performing Loans \| Commercial loans \| Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 2,736,405	$ 2,375,316
% of Total Loans	98.00%	99.50%
Performing Loans \| Commercial loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 956,957	$ 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	$ 168,566	$ 139,922
% of Total Loans	100.00%	100.00%
Performing Loans \| Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 7,896,931	$ 8,411,593
% of Total Loans	99.60%	99.70%
Performing Loans \| Consumer loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 38,107	$ 77,483
% of Total Loans	98.00%	99.00%
Performing Loans \| Consumer loans \| Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 7,473,143	$ 7,913,190
% of Total Loans	99.60%	99.70%
Performing Loans \| Consumer loans \| Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 76,438	$ 88,673
% of Total Loans	99.60%	100.00%
Performing Loans \| Consumer loans \| HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 254,759	$ 270,874
% of Total Loans	99.80%	99.80%
Performing Loans \| Consumer loans \| Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 54,484	$ 61,373
% of Total Loans	99.60%	99.80%
Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 123,859	$ 128,628
% of Total Loans	0.60%	0.60%
Non-Performing Loans \| Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 93,795	$ 103,540
% of Total Loans	0.80%	0.90%
Non-Performing Loans \| Commercial loans \| Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 34,385	$ 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	$ 4,682	$ 69,972
% of Total Loans	0.10%	1.90%
Non-Performing Loans \| Commercial loans \| Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 54,728	$ 11,047
% of Total Loans	2.00%	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	$ 30,064	$ 25,088
% of Total Loans	0.40%	0.30%
Non-Performing Loans \| Consumer loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 760	$ 760
% of Total Loans	2.00%	1.00%
Non-Performing Loans \| Consumer loans \| Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 28,188	$ 23,741
% of Total Loans	0.40%	0.30%
Non-Performing Loans \| Consumer loans \| Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 325	$ 23
% of Total Loans	0.40%	0.00%
Non-Performing Loans \| Consumer loans \| HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 593	$ 412
% of Total Loans	0.20%	0.20%
Non-Performing Loans \| Consumer loans \| Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 198	$ 152
% of Total Loans	0.40%	0.20%