FAIR VALUE, Recurring and Nonrecurring Fair Value Measurements (Details) - USD ($) $ in Millions	Jun. 30, 2024	Sep. 30, 2023
Assets at fair value on a recurring basis:
Available-for-sale securities	$ 8,530	$ 9,181
Total derivative assets	400	526
Total derivative assets	(315)	(261)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	85	265
Liabilities at fair value on a recurring basis:
Derivative liabilities	436	578
Total amounts offset	(82)	(88)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	354	490
Recurring
Assets at fair value on a recurring basis:
Total debt securities	1,432	1,125
Equity securities	14	22
Brokered certificates of deposit	13	36
Other	13	4
Total trading assets	1,472	1,187
Available-for-sale securities	8,530	9,181
Total derivative assets		(261)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition		265
Total all other investments	232	205
Other assets - client-owned fractional shares	124	98
Total assets at fair value on a recurring basis	10,544	11,037
Netting adjustments	(315)	(261)
Other investments - private equity - measured at net asset value (“NAV”)	101	101
Liabilities at fair value on a recurring basis:
Total debt securities	848	686
Equity securities	43	30
Total trading liabilities	891	716
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	354	490
Netting adjustments	(82)	(88)
Other payables - repurchase liabilities related to client-owned fractional shares	124	98
Total liabilities at fair value on a recurring basis	1,369	1,304
Recurring | Interest rate
Assets at fair value on a recurring basis:
Total derivative assets	(315)	(261)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	85	256
Liabilities at fair value on a recurring basis:
Total amounts offset	(82)	(88)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	352	488
Recurring | Foreign exchange
Assets at fair value on a recurring basis:
Net amounts presented on the Condensed Consolidated Statements of Financial Condition		9
Liabilities at fair value on a recurring basis:
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	2	2
Recurring | Municipal and provincial obligations
Liabilities at fair value on a recurring basis:
Total debt securities	8	10
Recurring | Corporate obligations
Liabilities at fair value on a recurring basis:
Total debt securities	595	514
Recurring | Government and agency obligations
Liabilities at fair value on a recurring basis:
Total debt securities	222	162
Recurring | Agency mortgage-backed securities (“MBS”), collateralized mortgage obligations (“CMOs”) and asset-backed securities (“ABS”)
Liabilities at fair value on a recurring basis:
Total debt securities	23
Recurring | Municipal and provincial obligations
Assets at fair value on a recurring basis:
Total debt securities	337	239
Recurring | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	671	642
Recurring | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	143	141
Total all other investments	90	71
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	218	35
Recurring | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	63	68
Recurring | Other
Assets at fair value on a recurring basis:
Total all other investments	142	134
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	10,443	10,936
Recurring | Level 1
Assets at fair value on a recurring basis:
Total debt securities	55	46
Equity securities	11	20
Brokered certificates of deposit	0	0
Other	0	0
Total trading assets	66	66
Available-for-sale securities	946	1,240
Total derivative assets		14
Total all other investments	202	173
Other assets - client-owned fractional shares	124	98
Total assets at fair value on a recurring basis	1,340	1,591
Liabilities at fair value on a recurring basis:
Total debt securities	224	171
Equity securities	32	30
Total trading liabilities	256	201
Derivative contracts liability, gross	2	13
Other payables - repurchase liabilities related to client-owned fractional shares	124	98
Total liabilities at fair value on a recurring basis	382	312
Recurring | Level 1 | Interest rate
Assets at fair value on a recurring basis:
Total derivative assets	2	14
Liabilities at fair value on a recurring basis:
Derivative liabilities	2	13
Recurring | Level 1 | Foreign exchange
Assets at fair value on a recurring basis:
Total derivative assets		0
Liabilities at fair value on a recurring basis:
Derivative liabilities		0
Derivative contracts liability, gross	0
Recurring | Level 1 | Municipal and provincial obligations
Liabilities at fair value on a recurring basis:
Total debt securities	8	10
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	216	161
Recurring | Level 1 | Agency mortgage-backed securities (“MBS”), collateralized mortgage obligations (“CMOs”) and asset-backed securities (“ABS”)
Liabilities at fair value on a recurring basis:
Total debt securities	0
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	22	22
Recurring | Level 1 | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	32	24
Total all other investments	90	71
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:
Total all other investments	112	102
Recurring | Level 2
Assets at fair value on a recurring basis:
Total debt securities	1,377	1,079
Equity securities	3	2
Brokered certificates of deposit	13	36
Other	0	0
Total trading assets	1,393	1,117
Available-for-sale securities	7,584	7,941
Total derivative assets		512
Total all other investments	1	2
Other assets - client-owned fractional shares	0	0
Total assets at fair value on a recurring basis	9,376	9,572
Liabilities at fair value on a recurring basis:
Total debt securities	624	515
Equity securities	11	0
Total trading liabilities	635	515
Derivative contracts liability, gross	434	565
Other payables - repurchase liabilities related to client-owned fractional shares	0	0
Total liabilities at fair value on a recurring basis	1,069	1,080
Recurring | Level 2 | Interest rate
Assets at fair value on a recurring basis:
Total derivative assets	398	503
Liabilities at fair value on a recurring basis:
Derivative liabilities	432	563
Recurring | Level 2 | Foreign exchange
Assets at fair value on a recurring basis:
Total derivative assets		9
Liabilities at fair value on a recurring basis:
Derivative liabilities	2	2
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	595	514
Recurring | Level 2 | Government and agency obligations
Liabilities at fair value on a recurring basis:
Total debt securities	6	1
Recurring | Level 2 | Agency mortgage-backed securities (“MBS”), collateralized mortgage obligations (“CMOs”) and asset-backed securities (“ABS”)
Liabilities at fair value on a recurring basis:
Total debt securities	23
Recurring | Level 2 | Municipal and provincial obligations
Assets at fair value on a recurring basis:
Total debt securities	336	239
Recurring | Level 2 | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	649	620
Recurring | Level 2 | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	111	117
Total all other investments	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	218	35
Recurring | Level 2 | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	63	68
Recurring | Level 2 | Other
Assets at fair value on a recurring basis:
Total all other investments	1	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	13	4
Total trading assets	13	4
Available-for-sale securities	0	0
Total derivative assets		0
Total all other investments	29	30
Other assets - client-owned fractional shares	0	0
Total assets at fair value on a recurring basis	42	34
Liabilities at fair value on a recurring basis:
Total debt securities	0	0
Equity securities	0	0
Total trading liabilities	0	0
Derivative contracts liability, gross	0	0
Other payables - repurchase liabilities related to client-owned fractional shares	0	0
Total liabilities at fair value on a recurring basis	0	0
Recurring | Level 3 | Interest rate
Assets at fair value on a recurring basis:
Total derivative assets	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:
Total derivative assets		0
Liabilities at fair value on a recurring basis:
Derivative liabilities		0
Derivative contracts liability, gross	0
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 | Agency mortgage-backed securities (“MBS”), collateralized mortgage obligations (“CMOs”) and asset-backed securities (“ABS”)
Liabilities at fair value on a recurring basis:
Total debt securities	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
Total all other investments	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:
Total all other investments	$ 29	$ 30