Loans Receivable - Schedule of Collateral-Dependent Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Sep. 30, 2025
Financing Receivable, Past Due [Line Items]
Net loans	$ 19,966,983	$ 20,088,618
Residential Real Estate
Financing Receivable, Past Due [Line Items]
Net loans	5,247
Residential Real Estate \| Commercial loans
Financing Receivable, Past Due [Line Items]
Net loans	0
Residential Real Estate \| Commercial loans \| Multi-family
Financing Receivable, Past Due [Line Items]
Net loans	0
Residential Real Estate \| Commercial loans \| Commercial real estate
Financing Receivable, Past Due [Line Items]
Net loans	0
Residential Real Estate \| Commercial loans \| Commercial & industrial
Financing Receivable, Past Due [Line Items]
Net loans	0
Residential Real Estate \| Consumer loans
Financing Receivable, Past Due [Line Items]
Net loans	5,247
Residential Real Estate \| Consumer loans \| Single-family residential
Financing Receivable, Past Due [Line Items]
Net loans	4,133
Residential Real Estate \| Consumer loans \| Construction
Financing Receivable, Past Due [Line Items]
Net loans	760
Residential Real Estate \| Consumer loans \| Land - consumer lot loans
Financing Receivable, Past Due [Line Items]
Net loans	16
Residential Real Estate \| Consumer loans \| HELOC
Financing Receivable, Past Due [Line Items]
Net loans	338
Residential Real Estate \| Consumer loans \| Consumer
Financing Receivable, Past Due [Line Items]
Net loans	0
Commercial real estate
Financing Receivable, Past Due [Line Items]
Net loans	39,267
Commercial real estate \| Commercial loans
Financing Receivable, Past Due [Line Items]
Net loans	39,267
Commercial real estate \| Commercial loans \| Multi-family
Financing Receivable, Past Due [Line Items]
Net loans	34,385
Commercial real estate \| Commercial loans \| Commercial real estate
Financing Receivable, Past Due [Line Items]
Net loans	4,682
Commercial real estate \| Commercial loans \| Commercial & industrial
Financing Receivable, Past Due [Line Items]
Net loans	200
Commercial real estate \| Consumer loans
Financing Receivable, Past Due [Line Items]
Net loans	0
Commercial real estate \| Consumer loans \| Single-family residential
Financing Receivable, Past Due [Line Items]
Net loans	0
Commercial real estate \| Consumer loans \| Construction
Financing Receivable, Past Due [Line Items]
Net loans	0
Commercial real estate \| Consumer loans \| Land - consumer lot loans
Financing Receivable, Past Due [Line Items]
Net loans	0
Commercial real estate \| Consumer loans \| HELOC
Financing Receivable, Past Due [Line Items]
Net loans	0
Commercial real estate \| Consumer loans \| Consumer
Financing Receivable, Past Due [Line Items]
Net loans	0
General Business Assets
Financing Receivable, Past Due [Line Items]
Net loans	54,369
General Business Assets \| Commercial loans
Financing Receivable, Past Due [Line Items]
Net loans	54,369
General Business Assets \| Commercial loans \| Multi-family
Financing Receivable, Past Due [Line Items]
Net loans	0
General Business Assets \| Commercial loans \| Commercial real estate
Financing Receivable, Past Due [Line Items]
Net loans	0
General Business Assets \| Commercial loans \| Commercial & industrial
Financing Receivable, Past Due [Line Items]
Net loans	54,369
General Business Assets \| Consumer loans
Financing Receivable, Past Due [Line Items]
Net loans	0
General Business Assets \| Consumer loans \| Single-family residential
Financing Receivable, Past Due [Line Items]
Net loans	0
General Business Assets \| Consumer loans \| Construction
Financing Receivable, Past Due [Line Items]
Net loans	0
General Business Assets \| Consumer loans \| Land - consumer lot loans
Financing Receivable, Past Due [Line Items]
Net loans	0
General Business Assets \| Consumer loans \| HELOC
Financing Receivable, Past Due [Line Items]
Net loans	0
General Business Assets \| Consumer loans \| Consumer
Financing Receivable, Past Due [Line Items]
Net loans	$ 0