FAIR VALUE, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions	Dec. 31, 2021	Sep. 30, 2021
Assets at fair value on a recurring basis:
Available-for-sale securities	$ 8,547	$ 8,315
Derivative assets, gross	286	342
Amount of derivative assets offset	(72)	(87)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	214	255
Liabilities at fair value on a recurring basis:
Derivative liabilities	306	316
Total amounts offset	(74)	(88)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	232	228
Recurring
Assets at fair value on a recurring basis:
Assets segregated for regulatory purposes	9,599	2,100
Total debt securities	352	568
Equity securities	11	12
Brokered certificates of deposit	5	16
Other	2	14
Total trading assets	370	610
Available-for-sale securities	8,547	8,315
Amount of derivative assets offset	(72)	(87)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	214	255
Other	554	188
Total assets at fair value on a recurring basis	19,441	11,637
Netting adjustments	(72)	(87)
Liabilities at fair value on a recurring basis:
Total debt securities	139	145
Equity securities	32	31
Total trading instruments sold but not yet purchased	171	176
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	232	228
Netting adjustments	(74)	(88)
Total liabilities at fair value on a recurring basis	403	404
Recurring | Interest rate
Assets at fair value on a recurring basis:
Interest rate - matched book	171	193
Amount of derivative assets offset	(72)	(87)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	42	57
Liabilities at fair value on a recurring basis:
Interest rate - matched book	171	193
Total amounts offset	(74)	(88)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	29	34
Recurring | Foreign exchange
Assets at fair value on a recurring basis:
Interest rate - matched book		5
Liabilities at fair value on a recurring basis:
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	32
Recurring | Other
Assets at fair value on a recurring basis:
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	1
Liabilities at fair value on a recurring basis:
Net amounts presented on the Condensed Consolidated Statements of Financial Condition		1
Recurring | Municipal and provincial obligations
Liabilities at fair value on a recurring basis:
Total debt securities		2
Recurring | Corporate obligations
Liabilities at fair value on a recurring basis:
Total debt securities	11	6
Recurring | Government and agency obligations
Liabilities at fair value on a recurring basis:
Total debt securities	128	137
Recurring | Municipal and provincial obligations
Assets at fair value on a recurring basis:
Total debt securities	91	155
Recurring | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	51	79
Recurring | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	94	109
Other	436	86
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	89	211
Recurring | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	27	14
Recurring | Other
Assets at fair value on a recurring basis:
Other	118	102
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	19,359	11,543
Recurring | Private equity investments
Assets at fair value on a recurring basis:
Private equity investments not measured at NAV	75	75
Private equity investments measured at NAV	82	94
Recurring | Level 1
Assets at fair value on a recurring basis:
Assets segregated for regulatory purposes	9,599	2,100
Total debt securities	34	31
Equity securities	10	8
Brokered certificates of deposit	0	0
Other	0	0
Total trading assets	44	39
Available-for-sale securities	15	15
Derivative assets, gross	3	16
Other	529	163
Total assets at fair value on a recurring basis	10,190	2,333
Liabilities at fair value on a recurring basis:
Total debt securities	128	139
Equity securities	32	28
Total trading instruments sold but not yet purchased	160	167
Derivative contracts liability, gross	2	16
Total liabilities at fair value on a recurring basis	162	183
Recurring | Level 1 | Interest rate
Assets at fair value on a recurring basis:
Interest rate - matched book	0	0
Derivative assets, gross	3	16
Liabilities at fair value on a recurring basis:
Interest rate - matched book	0	0
Derivative liabilities	2	16
Recurring | Level 1 | Foreign exchange
Assets at fair value on a recurring basis:
Derivative assets, gross		0
Liabilities at fair value on a recurring basis:
Derivative contracts liability, gross	0
Recurring | Level 1 | Other
Assets at fair value on a recurring basis:
Derivative assets, gross	0
Liabilities at fair value on a recurring basis:
Derivative liabilities		0
Recurring | Level 1 | Municipal and provincial obligations
Liabilities at fair value on a recurring basis:
Total debt securities		2
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	128	137
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	11	16
Recurring | Level 1 | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	23	15
Other	436	86
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	77
Recurring | Level 1 | Private equity investments
Assets at fair value on a recurring basis:
Private equity investments not measured at NAV	0	0
Recurring | Level 2
Assets at fair value on a recurring basis:
Assets segregated for regulatory purposes	0	0
Total debt securities	318	537
Equity securities	1	4
Brokered certificates of deposit	5	16
Other	0	0
Total trading assets	324	557
Available-for-sale securities	8,532	8,300
Derivative assets, gross	282	326
Other	2	2
Total assets at fair value on a recurring basis	9,140	9,185
Liabilities at fair value on a recurring basis:
Total debt securities	11	6
Equity securities	0	3
Total trading instruments sold but not yet purchased	11	9
Derivative contracts liability, gross	304	299
Total liabilities at fair value on a recurring basis	315	308
Recurring | Level 2 | Interest rate
Assets at fair value on a recurring basis:
Interest rate - matched book	171	193
Derivative assets, gross	111	128
Liabilities at fair value on a recurring basis:
Interest rate - matched book	171	193
Derivative liabilities	101	106
Recurring | Level 2 | Foreign exchange
Assets at fair value on a recurring basis:
Derivative assets, gross		5
Liabilities at fair value on a recurring basis:
Derivative liabilities	32
Recurring | Level 2 | Other
Assets at fair value on a recurring basis:
Derivative assets, gross	0
Liabilities at fair value on a recurring basis:
Derivative liabilities		0
Recurring | Level 2 | Municipal and provincial obligations
Liabilities at fair value on a recurring basis:
Total debt securities		0
Recurring | Level 2 | Corporate obligations
Liabilities at fair value on a recurring basis:
Total debt securities	11	6
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	91	155
Recurring | Level 2 | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	40	63
Recurring | Level 2 | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	71	94
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	89	211
Recurring | Level 2 | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	27	14
Recurring | Level 2 | Other
Assets at fair value on a recurring basis:
Other	2	2
Recurring | Level 2 | Private equity investments
Assets at fair value on a recurring basis:
Private equity investments not measured at NAV	0	0
Recurring | Level 3
Assets at fair value on a recurring basis:
Assets segregated for regulatory purposes	0	0
Total debt securities	0	0
Equity securities	0	0
Brokered certificates of deposit	0	0
Other	2	14
Total trading assets	2	14
Available-for-sale securities	0	0
Derivative assets, gross	1	0
Other	23	23
Total assets at fair value on a recurring basis	101	112
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	0	1
Total liabilities at fair value on a recurring basis	0	1
Recurring | Level 3 | Interest rate
Assets at fair value on a recurring basis:
Interest rate - matched book	0	0
Derivative assets, gross	0	0
Liabilities at fair value on a recurring basis:
Interest rate - matched book	0	0
Derivative liabilities	0	0
Recurring | Level 3 | Foreign exchange
Assets at fair value on a recurring basis:
Derivative assets, gross		0
Liabilities at fair value on a recurring basis:
Derivative contracts liability, gross	0
Recurring | Level 3 | Other
Assets at fair value on a recurring basis:
Derivative assets, gross	1
Liabilities at fair value on a recurring basis:
Derivative liabilities		1
Recurring | Level 3 | Municipal and provincial obligations
Liabilities at fair value on a recurring basis:
Total debt securities		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	23	23
Recurring | Level 3 | Private equity investments
Assets at fair value on a recurring basis:
Private equity investments not measured at NAV	$ 75	$ 75