FAIR VALUE, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions	Mar. 31, 2022	Sep. 30, 2021
Assets at fair value on a recurring basis:
Available-for-sale securities	$ 8,815	$ 8,315
Derivative assets, gross	293	342
Amount of derivative assets offset	(110)	(87)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	183	255
Liabilities at fair value on a recurring basis:
Derivative liabilities	334	316
Total amounts offset	(63)	(88)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	271	228
Assets segregated for regulatory purposes and restricted cash	19,531	11,348
U.S. Treasuries
Liabilities at fair value on a recurring basis:
Assets segregated for regulatory purposes and restricted cash	650	3,550
Recurring
Assets at fair value on a recurring basis:
Assets segregated for regulatory purposes	10,394	2,100
Total debt securities	407	568
Equity securities	14	12
Brokered certificates of deposit	41	16
Other	13	14
Total trading assets	475	610
Available-for-sale securities	8,815	8,315
Amount of derivative assets offset	(110)	(87)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	183	255
Other	289	188
Total assets at fair value on a recurring basis	20,274	11,637
Netting adjustments	(110)	(87)
Liabilities at fair value on a recurring basis:
Total debt securities	170	145
Equity securities	15	31
Other	1
Total trading instruments sold but not yet purchased	186	176
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	271	228
Netting adjustments	(63)	(88)
Total liabilities at fair value on a recurring basis	457	404
Recurring | Interest rate
Assets at fair value on a recurring basis:
Interest rate - matched book	124	193
Amount of derivative assets offset	(110)	(87)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	59	57
Liabilities at fair value on a recurring basis:
Interest rate - matched book	124	193
Total amounts offset	(63)	(88)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	134	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	13
Recurring | Other
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	1	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	166	137
Recurring | Municipal and provincial obligations
Assets at fair value on a recurring basis:
Total debt securities	115	155
Recurring | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	57	79
Recurring | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	94	109
Other	136	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	118	211
Recurring | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	23	14
Recurring | Other
Assets at fair value on a recurring basis:
Other	153	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	20,179	11,543
Recurring | Private equity investments
Assets at fair value on a recurring basis:
Private equity investments not measured at NAV	23	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	10,394	2,100
Total debt securities	30	31
Equity securities	13	8
Brokered certificates of deposit	0	0
Other	0	0
Total trading assets	43	39
Available-for-sale securities	213	15
Derivative assets, gross	61	16
Other	257	163
Total assets at fair value on a recurring basis	10,968	2,333
Liabilities at fair value on a recurring basis:
Total debt securities	167	139
Equity securities	15	28
Other	0
Total trading instruments sold but not yet purchased	182	167
Derivative contracts liability, gross	56	16
Total liabilities at fair value on a recurring basis	238	183
Recurring | Level 1 | Interest rate
Assets at fair value on a recurring basis:
Interest rate - matched book	0	0
Derivative assets, gross	61	16
Liabilities at fair value on a recurring basis:
Interest rate - matched book	0	0
Derivative liabilities	56	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
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	1	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	166	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	10	16
Recurring | Level 1 | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	20	15
Other	136	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	121	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	377	537
Equity securities	1	4
Brokered certificates of deposit	41	16
Other	0	0
Total trading assets	419	557
Available-for-sale securities	8,602	8,300
Derivative assets, gross	232	326
Other	2	2
Total assets at fair value on a recurring basis	9,255	9,185
Liabilities at fair value on a recurring basis:
Total debt securities	3	6
Equity securities	0	3
Other	0
Total trading instruments sold but not yet purchased	3	9
Derivative contracts liability, gross	278	299
Total liabilities at fair value on a recurring basis	281	308
Recurring | Level 2 | Interest rate
Assets at fair value on a recurring basis:
Interest rate - matched book	124	193
Derivative assets, gross	108	128
Liabilities at fair value on a recurring basis:
Interest rate - matched book	124	193
Derivative liabilities	141	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	13
Recurring | Level 2 | Other
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	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	115	155
Recurring | Level 2 | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	47	63
Recurring | Level 2 | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	74	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	118	211
Recurring | Level 2 | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	23	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	13	14
Total trading assets	13	14
Available-for-sale securities	0	0
Derivative assets, gross	0	0
Other	30	23
Total assets at fair value on a recurring basis	66	112
Liabilities at fair value on a recurring basis:
Total debt securities	0	0
Equity securities	0	0
Other	1
Total trading instruments sold but not yet purchased	1	0
Derivative contracts liability, gross	0	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
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		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	30	23
Recurring | Level 3 | Private equity investments
Assets at fair value on a recurring basis:
Private equity investments not measured at NAV	$ 23	$ 75