BANK LOANS, NET, Credit Quality of Held for Investment Loan Portfolio (Details) - USD ($) $ in Millions	Jun. 30, 2020	Sep. 30, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	$ 21,483	$ 20,979
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	20,750	20,694
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	552	162
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	181	123
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	0	0
C&I loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	7,731	8,098
C&I loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	7,384	7,870
C&I loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	281	152
C&I loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	66	76
C&I loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	0	0
CRE construction loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	219	185
CRE construction loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	219	185
CRE construction loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	0	0
CRE construction loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	0	0
CRE construction loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	0	0
CRE loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	3,695	3,652
CRE loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	3,341	3,630
CRE loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	262	0
CRE loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	92	22
CRE loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	0	0
Tax-exempt loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	1,290	1,241
Tax-exempt loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	1,290	1,241
Tax-exempt loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	0	0
Tax-exempt loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	0	0
Tax-exempt loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	0	0
Residential – first mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	4,893	4,427
Residential – first mortgage loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	4,861	4,392
Residential – first mortgage loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	9	10
Residential – first mortgage loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	23	25
Residential – first mortgage loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	0	0
Home equity loans/lines
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	24	27
Home equity loans/lines | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	24	27
Home equity loans/lines | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	0	0
Home equity loans/lines | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	0	0
Home equity loans/lines | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	0	0
SBL and other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	3,631	3,349
SBL and other | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	3,631	3,349
SBL and other | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	0	0
SBL and other | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	0	0
SBL and other | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment:	$ 0	$ 0