Loans Receivable - Schedule of Collateral-Dependent Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Sep. 30, 2025
Financing Receivable, Past Due [Line Items]
Net loans	$ 20,017,876	$ 20,088,618
Residential Real Estate
Financing Receivable, Past Due [Line Items]
Net loans	8,010
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	8,010
Residential Real Estate \| Consumer loans \| Single-family residential
Financing Receivable, Past Due [Line Items]
Net loans	7,732
Residential Real Estate \| Consumer loans \| Construction
Financing Receivable, Past Due [Line Items]
Net loans	132
Residential Real Estate \| Consumer loans \| Land - consumer lot loans
Financing Receivable, Past Due [Line Items]
Net loans	60
Residential Real Estate \| Consumer loans \| HELOC
Financing Receivable, Past Due [Line Items]
Net loans	86
Commercial real estate
Financing Receivable, Past Due [Line Items]
Net loans	38,108
Commercial real estate \| Commercial loans
Financing Receivable, Past Due [Line Items]
Net loans	38,108
Commercial real estate \| Commercial loans \| Multi-family
Financing Receivable, Past Due [Line Items]
Net loans	34,248
Commercial real estate \| Commercial loans \| Commercial real estate
Financing Receivable, Past Due [Line Items]
Net loans	3,114
Commercial real estate \| Commercial loans \| Commercial & industrial
Financing Receivable, Past Due [Line Items]
Net loans	746
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
General Business Assets
Financing Receivable, Past Due [Line Items]
Net loans	63,255
General Business Assets \| Commercial loans
Financing Receivable, Past Due [Line Items]
Net loans	63,255
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	63,255
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