BANK LOANS, NET, Credit Quality of Held for Investment Loan Portfolio (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	$ 21,452	$ 20,979	$ 19,578	$ 17,158	$ 15,235
C&I loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	7,450	8,098	7,786	7,386	7,470
CRE construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	177	185	151	113	123
CRE loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	3,534	3,652	3,624	3,106	2,554
Tax-exempt
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	1,259	1,241	1,227	1,018	741
Residential mortgage - first mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	4,925	4,427
Residential mortgage - Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	22	27
SBL and other
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	4,085	3,349	$ 3,033	$ 2,386	$ 1,905
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	20,519	20,694
Pass | C&I loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	6,966	7,870
Pass | CRE construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	177	185
Pass | CRE loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	3,113	3,630
Pass | Tax-exempt
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	1,259	1,241
Pass | Residential mortgage - first mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	4,897	4,392
Pass | Residential mortgage - Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	22	27
Pass | SBL and other
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	4,085	3,349
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	498	162
Special mention | C&I loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	236	152
Special mention | CRE construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	0	0
Special mention | CRE loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	256	0
Special mention | Tax-exempt
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	0	0
Special mention | Residential mortgage - first mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	6	10
Special mention | Residential mortgage - Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	0	0
Special mention | SBL and other
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	0	0
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	435	123
Substandard | C&I loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	248	76
Substandard | CRE construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	0	0
Substandard | CRE loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	165	22
Substandard | Tax-exempt
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	0	0
Substandard | Residential mortgage - first mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	22	25
Substandard | Residential mortgage - Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	0	0
Substandard | SBL and other
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	0	0
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	0	0
Doubtful | C&I loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	0	0
Doubtful | CRE construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	0	0
Doubtful | CRE loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	0	0
Doubtful | Tax-exempt
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	0	0
Doubtful | Residential mortgage - first mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	0	0
Doubtful | Residential mortgage - Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	0	0
Doubtful | SBL and other
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans held for investment	$ 0	$ 0