BANK LOANS, NET, Analysis of Payment Status of Loans Held for Investment (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	$ 21,452	$ 20,979	$ 19,578	$ 17,158	$ 15,235
Nonaccrual	30	43
Performing nonaccrual loans	15	32
C&I loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	7,450	8,098	7,786	7,386	7,470
Nonaccrual	2	19
CRE construction loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	177	185	151	113	123
Nonaccrual	0	0
CRE loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	3,534	3,652	3,624	3,106	2,554
Nonaccrual	14	8
Tax-exempt
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	1,259	1,241	1,227	1,018	741
Nonaccrual	0	0
Residential mortgage - first mortgage loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	4,925	4,427
Nonaccrual	14	16
Residential mortgage - Home equity loans/lines
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	22	27
Nonaccrual	0	0
SBL and other
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	4,085	3,349	$ 3,033	$ 2,386	$ 1,905
Nonaccrual	0	0
30-89 days and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	2
30-89 days and accruing | C&I loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
30-89 days and accruing | CRE construction loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
30-89 days and accruing | CRE loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
30-89 days and accruing | Tax-exempt
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
30-89 days and accruing | Residential mortgage - first mortgage loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	2
30-89 days and accruing | Residential mortgage - Home equity loans/lines
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
30-89 days and accruing | SBL and other
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
90 days or more and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
90 days or more and accruing | C&I loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
90 days or more and accruing | CRE construction loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
90 days or more and accruing | CRE loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
90 days or more and accruing | Tax-exempt
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
90 days or more and accruing | Residential mortgage - first mortgage loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
90 days or more and accruing | Residential mortgage - Home equity loans/lines
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
90 days or more and accruing | SBL and other
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	0	2
Total past due and accruing | C&I loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
Total past due and accruing | CRE construction loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
Total past due and accruing | CRE loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
Total past due and accruing | Tax-exempt
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
Total past due and accruing | Residential mortgage - first mortgage loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	2
Total past due and accruing | Residential mortgage - Home equity loans/lines
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
Total past due and accruing | SBL and other
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	0	0
Current and accruing
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	21,422	20,934
Current and accruing | C&I loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	7,448	8,079
Current and accruing | CRE construction loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	177	185
Current and accruing | CRE loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	3,520	3,644
Current and accruing | Tax-exempt
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	1,259	1,241
Current and accruing | Residential mortgage - first mortgage loans
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	4,911	4,409
Current and accruing | Residential mortgage - Home equity loans/lines
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	22	27
Current and accruing | SBL and other
Analysis of payment status of loans held for investment [Abstract]
Total loans held for investment	4,085	3,349
One-to-Four Family Residential Mortgage Loans | Residential mortgage loans
Analysis of payment status of loans held for investment [Abstract]
Total past due and accruing	6	7
Other assets
Analysis of payment status of loans held for investment [Abstract]
Other real estate owned	$ 2	$ 3