BANK LOANS, NET, Past Due, Nonaccrual, and Modified Loans and Collateral Dependent Loans (Details) - USD ($) $ in Millions	Dec. 31, 2025	Sep. 30, 2025
Financing Receivable, Past Due [Line Items]
Performing nonaccrual loans	$ 122	$ 109
SBL
Financing Receivable, Past Due [Line Items]
Total loans held for investment	21,667	19,775
C&I loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	10,801	10,777
CRE loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	7,753	7,840
REIT loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	1,779	1,690
Residential mortgage loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	10,567	10,295
Tax-exempt loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	1,148	1,226
Total loans held for investment
Financing Receivable, Past Due [Line Items]
Total loans held for investment	53,715	51,603
Nonaccrual with allowance	181	159
Nonaccrual with no allowance	26	27
Total loans held for investment | SBL
Financing Receivable, Past Due [Line Items]
Total loans held for investment	21,667	19,775
Nonaccrual with allowance	0	0
Nonaccrual with no allowance	0	0
Total loans held for investment | C&I loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	10,801	10,777
Nonaccrual with allowance	52	39
Nonaccrual with no allowance	5	5
Total loans held for investment | CRE loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	7,753	7,840
Nonaccrual with allowance	111	101
Nonaccrual with no allowance	8	9
Total loans held for investment | REIT loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	1,779	1,690
Nonaccrual with allowance	18	19
Nonaccrual with no allowance	0	0
Total loans held for investment | Residential mortgage loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	10,567	10,295
Nonaccrual with allowance	0	0
Nonaccrual with no allowance	13	13
Total loans held for investment | Tax-exempt loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	1,148	1,226
Nonaccrual with allowance	0	0
Nonaccrual with no allowance	0	0
Total loans held for investment | Total past due and accruing
Financing Receivable, Past Due [Line Items]
Total loans held for investment	10	7
Total loans held for investment | Total past due and accruing | SBL
Financing Receivable, Past Due [Line Items]
Total loans held for investment	3	1
Total loans held for investment | Total past due and accruing | C&I loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	2	1
Total loans held for investment | Total past due and accruing | CRE loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	0	0
Total loans held for investment | Total past due and accruing | REIT loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	0	0
Total loans held for investment | Total past due and accruing | Residential mortgage loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	5	5
Total loans held for investment | Total past due and accruing | Tax-exempt loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	0	0
Total loans held for investment | 30-89 days and accruing
Financing Receivable, Past Due [Line Items]
Total loans held for investment	10	7
Total loans held for investment | 30-89 days and accruing | SBL
Financing Receivable, Past Due [Line Items]
Total loans held for investment	3	1
Total loans held for investment | 30-89 days and accruing | C&I loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	2	1
Total loans held for investment | 30-89 days and accruing | CRE loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	0	0
Total loans held for investment | 30-89 days and accruing | REIT loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	0	0
Total loans held for investment | 30-89 days and accruing | Residential mortgage loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	5	5
Total loans held for investment | 30-89 days and accruing | Tax-exempt loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	0	0
Total loans held for investment | 90 days or more and accruing
Financing Receivable, Past Due [Line Items]
Total loans held for investment	0	0
Total loans held for investment | 90 days or more and accruing | SBL
Financing Receivable, Past Due [Line Items]
Total loans held for investment	0	0
Total loans held for investment | 90 days or more and accruing | C&I loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	0	0
Total loans held for investment | 90 days or more and accruing | CRE loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	0	0
Total loans held for investment | 90 days or more and accruing | REIT loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	0	0
Total loans held for investment | 90 days or more and accruing | Residential mortgage loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	0	0
Total loans held for investment | 90 days or more and accruing | Tax-exempt loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	0	0
Total loans held for investment | Current and accruing
Financing Receivable, Past Due [Line Items]
Total loans held for investment	53,498	51,410
Total loans held for investment | Current and accruing | SBL
Financing Receivable, Past Due [Line Items]
Total loans held for investment	21,664	19,774
Total loans held for investment | Current and accruing | C&I loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	10,742	10,732
Total loans held for investment | Current and accruing | CRE loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	7,634	7,730
Total loans held for investment | Current and accruing | REIT loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	1,761	1,671
Total loans held for investment | Current and accruing | Residential mortgage loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	10,549	10,277
Total loans held for investment | Current and accruing | Tax-exempt loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	$ 1,148	$ 1,226