BANK LOANS, NET, Analysis of Payment Status of Loans Held for Investment (Details) - USD ($) $ in Millions	Mar. 31, 2022	Sep. 30, 2021
Financing Receivable, Past Due [Line Items]
Financing receivable past due	$ 28,211	$ 25,314
Performing nonaccrual loans	92	61
Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	27,932	25,169
Nonaccrual with allowance	70	41
Nonaccrual with no allowance	34	33
Total past due and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	1	2
30-89 days and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	1	2
90 days or more and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
Current and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	27,827	25,093
C&I loans
Financing Receivable, Past Due [Line Items]
Financing receivable past due	9,067	8,440
C&I loans | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	9,067	8,440
Nonaccrual with allowance	57	39
Nonaccrual with no allowance	0	0
C&I loans | Total past due and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
C&I loans | 30-89 days and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
C&I loans | 90 days or more and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
C&I loans | Current and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	9,010	8,401
CRE loans
Financing Receivable, Past Due [Line Items]
Financing receivable past due	3,321	2,872
Troubled debt restructuring	12	12
CRE loans | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	3,321	2,872
Nonaccrual with allowance	12	0
Nonaccrual with no allowance	19	20
CRE loans | Total past due and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
CRE loans | 30-89 days and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
CRE loans | 90 days or more and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
CRE loans | Current and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	3,290	2,852
REIT loans
Financing Receivable, Past Due [Line Items]
Financing receivable past due	1,408	1,112
REIT loans | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	1,408	1,112
Nonaccrual with allowance	0	0
Nonaccrual with no allowance	0	0
REIT loans | Total past due and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
REIT loans | 30-89 days and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
REIT loans | 90 days or more and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
REIT loans | Current and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	1,408	1,112
Tax-exempt loans
Financing Receivable, Past Due [Line Items]
Financing receivable past due	1,287	1,321
Tax-exempt loans | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	1,287	1,321
Nonaccrual with allowance	0	0
Nonaccrual with no allowance	0	0
Tax-exempt loans | Total past due and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
Tax-exempt loans | 30-89 days and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
Tax-exempt loans | 90 days or more and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
Tax-exempt loans | Current and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	1,287	1,321
Residential mortgage loans
Financing Receivable, Past Due [Line Items]
Financing receivable past due	5,945	5,318
Troubled debt restructuring	12	13
Residential mortgage loans | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	5,945	5,318
Nonaccrual with allowance	1	2
Nonaccrual with no allowance	15	13
Residential mortgage loans | Total past due and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	1	2
Residential mortgage loans | 30-89 days and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	1	2
Residential mortgage loans | 90 days or more and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
Residential mortgage loans | Current and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	5,928	5,301
SBL and other
Financing Receivable, Past Due [Line Items]
Financing receivable past due	6,904	6,106
SBL and other | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	6,904	6,106
Nonaccrual with allowance	0	0
Nonaccrual with no allowance	0	0
SBL and other | Total past due and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
SBL and other | 30-89 days and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
SBL and other | 90 days or more and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	0	0
SBL and other | Current and accruing | Loans held for investment
Financing Receivable, Past Due [Line Items]
Financing receivable past due	$ 6,904	$ 6,106