FAIR VALUE, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions	Jun. 30, 2022	Sep. 30, 2021
Assets at fair value on a recurring basis:
Available-for-sale securities	$ 10,464	$ 8,315
Derivative assets, gross	387	342
Amount of derivative assets offset	(235)	(87)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	152	255
Liabilities at fair value on a recurring basis:
Derivative liabilities	437	316
Total amounts offset	(58)	(88)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	379	228
Assets segregated for regulatory purposes and restricted cash	16,251	11,348
U.S. Treasuries
Liabilities at fair value on a recurring basis:
Assets segregated for regulatory purposes and restricted cash	6,190	3,550
Recurring
Assets at fair value on a recurring basis:
Assets segregated for regulatory purposes	2,000	2,100
Total debt securities	431	568
Equity securities	7	12
Brokered certificates of deposit	20	16
Other		14
Total trading assets	458	610
Available-for-sale securities	10,464	8,315
Amount of derivative assets offset	(235)	(87)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	152	255
Other	243	188
Total assets at fair value on a recurring basis	13,439	11,637
Netting adjustments	(235)	(87)
Liabilities at fair value on a recurring basis:
Total debt securities	112	145
Equity securities	46	31
Total trading instruments sold but not yet purchased	158	176
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	379	228
Netting adjustments	(58)	(88)
Total liabilities at fair value on a recurring basis	537	404
Recurring | Interest rate
Assets at fair value on a recurring basis:
Interest rate - matched book	84	193
Amount of derivative assets offset	(235)	(87)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	62	57
Liabilities at fair value on a recurring basis:
Interest rate - matched book	84	193
Total amounts offset	(58)	(88)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	294	34
Recurring | Foreign exchange
Assets at fair value on a recurring basis:
Interest rate - matched book		5
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	4
Recurring | Other
Assets at fair value on a recurring basis:
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	2
Liabilities at fair value on a recurring basis:
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	1	1
Recurring | Municipal and provincial obligations
Liabilities at fair value on a recurring basis:
Total debt securities	8	2
Recurring | Corporate obligations
Liabilities at fair value on a recurring basis:
Total debt securities	3	6
Recurring | Government and agency obligations
Liabilities at fair value on a recurring basis:
Total debt securities	101	137
Recurring | Municipal and provincial obligations
Assets at fair value on a recurring basis:
Total debt securities	105	155
Recurring | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	66	79
Recurring | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	57	109
Other	120	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	96	211
Recurring | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	107	14
Recurring | Other
Assets at fair value on a recurring basis:
Other	123	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	13,344	11,543
Recurring | Private equity investments
Assets at fair value on a recurring basis:
Private equity investments not measured at NAV	27	75
Private equity investments measured at NAV	95	94
Recurring | Level 1
Assets at fair value on a recurring basis:
Assets segregated for regulatory purposes	2,000	2,100
Total debt securities	35	31
Equity securities	7	8
Brokered certificates of deposit	0	0
Other		0
Total trading assets	42	39
Available-for-sale securities	912	15
Derivative assets, gross	8	16
Other	217	163
Total assets at fair value on a recurring basis	3,179	2,333
Liabilities at fair value on a recurring basis:
Total debt securities	103	139
Equity securities	46	28
Total trading instruments sold but not yet purchased	149	167
Derivative contracts liability, gross	7	16
Total liabilities at fair value on a recurring basis	156	183
Recurring | Level 1 | Interest rate
Assets at fair value on a recurring basis:
Interest rate - matched book	0	0
Derivative assets, gross	8	16
Liabilities at fair value on a recurring basis:
Interest rate - matched book	0	0
Derivative liabilities	7	16
Recurring | Level 1 | Foreign exchange
Assets at fair value on a recurring basis:
Derivative assets, gross	0	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	0
Recurring | Level 1 | Municipal and provincial obligations
Liabilities at fair value on a recurring basis:
Total debt securities	2	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	101	137
Recurring | Level 1 | Municipal and provincial obligations
Assets at fair value on a recurring basis:
Total debt securities	1	0
Recurring | Level 1 | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	10	16
Recurring | Level 1 | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	24	15
Other	120	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	97	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	396	537
Equity securities	0	4
Brokered certificates of deposit	20	16
Other		0
Total trading assets	416	557
Available-for-sale securities	9,552	8,300
Derivative assets, gross	377	326
Other	2	2
Total assets at fair value on a recurring basis	10,347	9,185
Liabilities at fair value on a recurring basis:
Total debt securities	9	6
Equity securities	0	3
Total trading instruments sold but not yet purchased	9	9
Derivative contracts liability, gross	429	299
Total liabilities at fair value on a recurring basis	438	308
Recurring | Level 2 | Interest rate
Assets at fair value on a recurring basis:
Interest rate - matched book	84	193
Derivative assets, gross	289	128
Liabilities at fair value on a recurring basis:
Interest rate - matched book	84	193
Derivative liabilities	345	106
Recurring | Level 2 | Foreign exchange
Assets at fair value on a recurring basis:
Derivative assets, gross	4	5
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	0
Recurring | Level 2 | Municipal and provincial obligations
Liabilities at fair value on a recurring basis:
Total debt securities	6	0
Recurring | Level 2 | Corporate obligations
Liabilities at fair value on a recurring basis:
Total debt securities	3	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	104	155
Recurring | Level 2 | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	56	63
Recurring | Level 2 | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	33	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	96	211
Recurring | Level 2 | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	107	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		14
Total trading assets	0	14
Available-for-sale securities	0	0
Derivative assets, gross	2	0
Other	24	23
Total assets at fair value on a recurring basis	53	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	1	1
Total liabilities at fair value on a recurring basis	1	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	0
Recurring | Level 3 | Other
Assets at fair value on a recurring basis:
Derivative assets, gross	2
Liabilities at fair value on a recurring basis:
Derivative liabilities	1	1
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	24	23
Recurring | Level 3 | Private equity investments
Assets at fair value on a recurring basis:
Private equity investments not measured at NAV	$ 27	$ 75