BANK LOANS, NET, Analysis of Payment Status of Loans Held for Investment and Collateral Dependent Loans (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Past Due [Line Items]
Total loans held for investment	$ 25,314	$ 21,549	$ 21,109	$ 19,721	$ 17,197
Performing nonaccrual loans	61	15
C&I loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	8,440	7,421
CRE loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	2,872	2,489
TDRs	12	6
Impaired loans with allowance for loan losses, gross recorded investment	20
REIT loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	1,112	1,210
Tax-exempt loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	1,321	1,259
Residential mortgage loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	5,318	4,973
Impaired loans with allowance for loan losses, gross recorded investment	5
Residential mortgage - first mortgage loans
Financing Receivable, Past Due [Line Items]
TDRs	13	15
SBL and other
Financing Receivable, Past Due [Line Items]
Total loans held for investment	6,106	4,087
Total loans held for investment
Financing Receivable, Past Due [Line Items]
Total loans held for investment	25,169	21,439	20,967	19,557	17,127
Nonaccrual with allowance	41	5
Nonaccrual with no allowance	33	25
Total loans held for investment | C&I loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	8,440	7,421	8,056	7,741	7,339
Nonaccrual with allowance	39	2
Nonaccrual with no allowance	0	0
Total loans held for investment | CRE loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	2,872	2,489	2,507	2,309	1,859
Nonaccrual with allowance	0	0
Nonaccrual with no allowance	20	14
Total loans held for investment | REIT loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	1,112	1,210	1,333	1,470	1,361
Nonaccrual with allowance	0	0
Nonaccrual with no allowance	0	0
Total loans held for investment | Tax-exempt loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	1,321	1,259	1,241	1,227	1,018
Nonaccrual with allowance	0	0
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	5,318	4,973	4,479	3,775	3,162
Nonaccrual with allowance	2	3
Nonaccrual with no allowance	13	11
Total loans held for investment | SBL and other
Financing Receivable, Past Due [Line Items]
Total loans held for investment	6,106	4,087	$ 3,351	$ 3,035	$ 2,388
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	2	0
Total loans held for investment | Total past due and accruing | C&I loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	0	0
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 | Tax-exempt 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	2	0
Total loans held for investment | Total past due and accruing | SBL and other
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	2	0
Total loans held for investment | 30-89 days and accruing | C&I loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	0	0
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 | 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 | Residential mortgage loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	2	0
Total loans held for investment | 30-89 days and accruing | SBL and other
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 | 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 | 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 | 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 | SBL and other
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	25,093	21,409
Total loans held for investment | Current and accruing | C&I loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	8,401	7,419
Total loans held for investment | Current and accruing | CRE loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	2,852	2,475
Total loans held for investment | Current and accruing | REIT loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	1,112	1,210
Total loans held for investment | Current and accruing | Tax-exempt loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	1,321	1,259
Total loans held for investment | Current and accruing | Residential mortgage loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	5,301	4,959
Total loans held for investment | Current and accruing | SBL and other
Financing Receivable, Past Due [Line Items]
Total loans held for investment	6,106	4,087
One-to-four family residential mortgage loans | Total past due and accruing | Residential mortgage loans
Financing Receivable, Past Due [Line Items]
Total loans held for investment	$ 4	$ 6