FAIR VALUE, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions	Jun. 30, 2023	Sep. 30, 2022
Assets at fair value on a recurring basis:
Available-for-sale securities	$ 9,566	$ 9,885
Derivative assets, gross	428	536
Amount of derivative assets offset	(199)	(348)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	229	188
Liabilities at fair value on a recurring basis:
Derivative liabilities	473	595
Total amounts offset	(80)	(65)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	393	530
Recurring
Assets at fair value on a recurring basis:
Total debt securities	1,107	1,219
Equity securities	12	20
Brokered certificates of deposit	28	30
Other	9	1
Total trading assets	1,156	1,270
Available-for-sale securities	9,566	9,885
Amount of derivative assets offset	(199)	(348)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	229	188
Other	206	202
Other assets - client-owned fractional shares	100	78
Total assets at fair value on a recurring basis	11,357	11,713
Netting adjustments	(199)	(348)
Liabilities at fair value on a recurring basis:
Total debt securities	702	809
Equity securities	66	27
Total trading instruments sold but not yet purchased	768	836
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	393	530
Netting adjustments	(80)	(65)
Other payables - repurchase liabilities related to client-owned fractional shares	100	78
Total liabilities at fair value on a recurring basis	1,261	1,444
Recurring | Interest rate
Assets at fair value on a recurring basis:
Amount of derivative assets offset	(199)	(348)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	224	178
Liabilities at fair value on a recurring basis:
Total amounts offset	(80)	(65)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	389	522
Recurring | Foreign exchange
Assets at fair value on a recurring basis:
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	5	10
Liabilities at fair value on a recurring basis:
Net amounts presented on the Condensed Consolidated Statements of Financial Condition		5
Recurring | Other
Liabilities at fair value on a recurring basis:
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	4	3
Recurring | Municipal and provincial obligations
Liabilities at fair value on a recurring basis:
Total debt securities	12	5
Recurring | Corporate obligations
Liabilities at fair value on a recurring basis:
Total debt securities	561	555
Recurring | Government and agency obligations
Liabilities at fair value on a recurring basis:
Total debt securities	129	249
Recurring | Municipal and provincial obligations
Assets at fair value on a recurring basis:
Total debt securities	257	269
Recurring | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	622	595
Recurring | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	149	171
Other	82	79
Recurring | Agency mortgage-backed securities (“MBS”), collateralized mortgage obligations (“CMOs”) and asset-backed securities (“ABS”)
Assets at fair value on a recurring basis:
Total debt securities	19	123
Recurring | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	60	61
Recurring | Other
Assets at fair value on a recurring basis:
Other	124	123
Recurring | Assets at Fair Value on a Recurring Basis, Before Investments Measured at NAV
Assets at fair value on a recurring basis:
Total assets at fair value on a recurring basis	11,257	11,623
Recurring | Private equity investments
Assets at fair value on a recurring basis:
Private equity investments measured at NAV	100	90
Recurring | Level 1
Assets at fair value on a recurring basis:
Total debt securities	68	102
Equity securities	10	20
Brokered certificates of deposit	0	0
Other	0	0
Total trading assets	78	122
Available-for-sale securities	1,238	986
Derivative assets, gross	9	42
Other	175	171
Other assets - client-owned fractional shares	100	78
Total assets at fair value on a recurring basis	1,600	1,399
Liabilities at fair value on a recurring basis:
Total debt securities	139	254
Equity securities	66	27
Total trading instruments sold but not yet purchased	205	281
Derivative contracts liability, gross	8	40
Other payables - repurchase liabilities related to client-owned fractional shares	100	78
Total liabilities at fair value on a recurring basis	313	399
Recurring | Level 1 | Interest rate
Assets at fair value on a recurring basis:
Derivative assets, gross	9	42
Liabilities at fair value on a recurring basis:
Derivative liabilities	8	40
Recurring | Level 1 | Foreign exchange
Assets at fair value on a recurring basis:
Derivative assets, gross	0	0
Liabilities at fair value on a recurring basis:
Derivative contracts liability, gross		0
Recurring | Level 1 | Other
Liabilities at fair value on a recurring basis:
Derivative liabilities	0	0
Recurring | Level 1 | Municipal and provincial obligations
Liabilities at fair value on a recurring basis:
Total debt securities	10	5
Recurring | Level 1 | Corporate obligations
Liabilities at fair value on a recurring basis:
Total debt securities	0	0
Recurring | Level 1 | Government and agency obligations
Liabilities at fair value on a recurring basis:
Total debt securities	129	249
Recurring | Level 1 | Municipal and provincial obligations
Assets at fair value on a recurring basis:
Total debt securities	0	0
Recurring | Level 1 | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	24	16
Recurring | Level 1 | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	44	86
Other	82	79
Recurring | Level 1 | Agency mortgage-backed securities (“MBS”), collateralized mortgage obligations (“CMOs”) and asset-backed securities (“ABS”)
Assets at fair value on a recurring basis:
Total debt securities	0	0
Recurring | Level 1 | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	0	0
Recurring | Level 1 | Other
Assets at fair value on a recurring basis:
Other	93	92
Recurring | Level 2
Assets at fair value on a recurring basis:
Total debt securities	1,039	1,117
Equity securities	2	0
Brokered certificates of deposit	28	30
Other	0	0
Total trading assets	1,069	1,147
Available-for-sale securities	8,328	8,899
Derivative assets, gross	419	494
Other	2	2
Other assets - client-owned fractional shares	0	0
Total assets at fair value on a recurring basis	9,818	10,542
Liabilities at fair value on a recurring basis:
Total debt securities	563	555
Equity securities	0	0
Total trading instruments sold but not yet purchased	563	555
Derivative contracts liability, gross	461	552
Other payables - repurchase liabilities related to client-owned fractional shares	0	0
Total liabilities at fair value on a recurring basis	1,024	1,107
Recurring | Level 2 | Interest rate
Assets at fair value on a recurring basis:
Derivative assets, gross	414	484
Liabilities at fair value on a recurring basis:
Derivative liabilities	461	547
Recurring | Level 2 | Foreign exchange
Assets at fair value on a recurring basis:
Derivative assets, gross	5	10
Liabilities at fair value on a recurring basis:
Derivative liabilities		5
Recurring | Level 2 | Other
Liabilities at fair value on a recurring basis:
Derivative liabilities	0	0
Recurring | Level 2 | Municipal and provincial obligations
Liabilities at fair value on a recurring basis:
Total debt securities	2	0
Recurring | Level 2 | Corporate obligations
Liabilities at fair value on a recurring basis:
Total debt securities	561	555
Recurring | Level 2 | Government and agency obligations
Liabilities at fair value on a recurring basis:
Total debt securities	0	0
Recurring | Level 2 | Municipal and provincial obligations
Assets at fair value on a recurring basis:
Total debt securities	257	269
Recurring | Level 2 | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	598	579
Recurring | Level 2 | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	105	85
Other	0	0
Recurring | Level 2 | Agency mortgage-backed securities (“MBS”), collateralized mortgage obligations (“CMOs”) and asset-backed securities (“ABS”)
Assets at fair value on a recurring basis:
Total debt securities	19	123
Recurring | Level 2 | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	60	61
Recurring | Level 2 | Other
Assets at fair value on a recurring basis:
Other	2	2
Recurring | Level 3
Assets at fair value on a recurring basis:
Total debt securities	0	0
Equity securities	0	0
Brokered certificates of deposit	0	0
Other	9	1
Total trading assets	9	1
Available-for-sale securities	0	0
Derivative assets, gross	0	0
Other	29	29
Other assets - client-owned fractional shares	0	0
Total assets at fair value on a recurring basis	38	30
Liabilities at fair value on a recurring basis:
Total debt securities	0	0
Equity securities	0	0
Total trading instruments sold but not yet purchased	0	0
Derivative contracts liability, gross	4	3
Other payables - repurchase liabilities related to client-owned fractional shares	0	0
Total liabilities at fair value on a recurring basis	4	3
Recurring | Level 3 | Interest rate
Assets at fair value on a recurring basis:
Derivative assets, gross	0	0
Liabilities at fair value on a recurring basis:
Derivative liabilities	0	0
Recurring | Level 3 | Foreign exchange
Assets at fair value on a recurring basis:
Derivative assets, gross	0	0
Liabilities at fair value on a recurring basis:
Derivative contracts liability, gross		0
Recurring | Level 3 | Other
Liabilities at fair value on a recurring basis:
Derivative liabilities	4	3
Recurring | Level 3 | Municipal and provincial obligations
Liabilities at fair value on a recurring basis:
Total debt securities	0	0
Recurring | Level 3 | Corporate obligations
Liabilities at fair value on a recurring basis:
Total debt securities	0	0
Recurring | Level 3 | Government and agency obligations
Liabilities at fair value on a recurring basis:
Total debt securities	0	0
Recurring | Level 3 | Municipal and provincial obligations
Assets at fair value on a recurring basis:
Total debt securities	0	0
Recurring | Level 3 | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	0	0
Recurring | Level 3 | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	0	0
Other	0	0
Recurring | Level 3 | Agency mortgage-backed securities (“MBS”), collateralized mortgage obligations (“CMOs”) and asset-backed securities (“ABS”)
Assets at fair value on a recurring basis:
Total debt securities	0	0
Recurring | Level 3 | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	0	0
Recurring | Level 3 | Other
Assets at fair value on a recurring basis:
Other	$ 29	$ 29