BANK LOANS, NET, Analysis of Payment Status of Loans Held for Investment (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	$ 22,002	$ 20,979
Nonaccrual	24	43
Performing nonaccrual loans	9	32
Other assets
Analysis of payment status of loans held for investment [Abstract]
Other real estate owned	3	3
30-89 days and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	1	2
90 days or more and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
Total past due and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	1	2
Current and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	21,977	20,934
C&I loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	8,316	8,098
Nonaccrual	4	19
C&I loans | 30-89 days and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
C&I loans | 90 days or more and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
C&I loans | Total past due and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
C&I loans | Current and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	8,312	8,079
CRE construction loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	180	185
Nonaccrual	0	0
CRE construction loans | 30-89 days and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
CRE construction loans | 90 days or more and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
CRE construction loans | Total past due and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
CRE construction loans | Current and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	180	185
CRE loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	3,830	3,652
Nonaccrual	6	8
CRE loans | 30-89 days and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
CRE loans | 90 days or more and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
CRE loans | Total past due and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
CRE loans | Current and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	3,824	3,644
Tax-exempt loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	1,266	1,241
Nonaccrual	0	0
Tax-exempt loans | 30-89 days and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
Tax-exempt loans | 90 days or more and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
Tax-exempt loans | Total past due and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
Tax-exempt loans | Current and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	1,266	1,241
Residential - first mortgage loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	4,839	4,427
Nonaccrual	14	16
Residential - first mortgage loans | 30-89 days and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	1	2
Residential - first mortgage loans | 90 days or more and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
Residential - first mortgage loans | Total past due and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	1	2
Residential - first mortgage loans | Current and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	4,824	4,409
Residential mortgage - home equity loans/lines
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	25	27
Nonaccrual	0	0
Residential mortgage - home equity loans/lines | 30-89 days and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
Residential mortgage - home equity loans/lines | 90 days or more and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
Residential mortgage - home equity loans/lines | Total past due and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
Residential mortgage - home equity loans/lines | Current and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	25	27
SBL and other
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	3,546	3,349
Nonaccrual	0	0
SBL and other | 30-89 days and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
SBL and other | 90 days or more and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
SBL and other | Total past due and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
SBL and other | Current and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	3,546	3,349
Residential mortgage loans | One-to-four family residential mortgage loans
Analysis of payment status of loans held for investment [Abstract]
Financing receivable past due	$ 7	$ 7