FAIR VALUE, Recurring and Nonrecurring Fair Value Measurements (Details) - USD ($) $ in Millions	Mar. 31, 2024	Sep. 30, 2023
Assets at fair value on a recurring basis:
Available-for-sale securities	$ 9,031	$ 9,181
Total derivative assets	408	526
Total derivative assets	(202)	(261)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	206	265
Liabilities at fair value on a recurring basis:
Derivative liabilities	450	578
Total amounts offset	(85)	(88)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	365	490
Recurring
Assets at fair value on a recurring basis:
Total debt securities	1,411	1,125
Equity securities	13	22
Brokered certificates of deposit	6	36
Other	4	4
Total trading assets	1,434	1,187
Available-for-sale securities	9,031	9,181
Total derivative assets	(202)	(261)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	206	265
Total all other investments	207	205
Other assets - client-owned fractional shares	119	98
Total assets at fair value on a recurring basis	11,096	11,037
Netting adjustments	(202)	(261)
Other investments - private equity - measured at net asset value (“NAV”)	99	101
Liabilities at fair value on a recurring basis:
Total debt securities	879	686
Equity securities	25	30
Total trading liabilities	904	716
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	365	490
Netting adjustments	(85)	(88)
Other payables - repurchase liabilities related to client-owned fractional shares	119	98
Total liabilities at fair value on a recurring basis	1,388	1,304
Recurring | Interest rate
Assets at fair value on a recurring basis:
Total derivative assets	(202)	(261)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	205	256
Liabilities at fair value on a recurring basis:
Total amounts offset	(85)	(88)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	360	488
Recurring | Foreign exchange
Assets at fair value on a recurring basis:
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	1	9
Liabilities at fair value on a recurring basis:
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	5	2
Recurring | Municipal and provincial obligations
Liabilities at fair value on a recurring basis:
Total debt securities	6	10
Recurring | Corporate obligations
Liabilities at fair value on a recurring basis:
Total debt securities	583	514
Recurring | Government and agency obligations
Liabilities at fair value on a recurring basis:
Total debt securities	232	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	58
Recurring | Municipal and provincial obligations
Assets at fair value on a recurring basis:
Total debt securities	266	239
Recurring | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	693	642
Recurring | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	187	141
Total all other investments	71	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	198	35
Recurring | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	67	68
Recurring | Other
Assets at fair value on a recurring basis:
Total all other investments	136	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	10,997	10,936
Recurring | Level 1
Assets at fair value on a recurring basis:
Total debt securities	55	46
Equity securities	10	20
Brokered certificates of deposit	0	0
Other	0	0
Total trading assets	65	66
Available-for-sale securities	1,180	1,240
Total derivative assets	4	14
Total all other investments	177	173
Other assets - client-owned fractional shares	119	98
Total assets at fair value on a recurring basis	1,545	1,591
Liabilities at fair value on a recurring basis:
Total debt securities	237	171
Equity securities	25	30
Total trading liabilities	262	201
Derivative contracts liability, gross	4	13
Other payables - repurchase liabilities related to client-owned fractional shares	119	98
Total liabilities at fair value on a recurring basis	385	312
Recurring | Level 1 | Interest rate
Assets at fair value on a recurring basis:
Total derivative assets	4	14
Liabilities at fair value on a recurring basis:
Derivative liabilities	4	13
Recurring | Level 1 | Foreign exchange
Assets at fair value on a recurring basis:
Total derivative assets	0	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	6	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	231	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	0	0
Recurring | Level 1 | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	20	22
Recurring | Level 1 | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	35	24
Total all other investments	71	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	106	102
Recurring | Level 2
Assets at fair value on a recurring basis:
Total debt securities	1,356	1,079
Equity securities	3	2
Brokered certificates of deposit	6	36
Other	0	0
Total trading assets	1,365	1,117
Available-for-sale securities	7,851	7,941
Total derivative assets	404	512
Total all other investments	1	2
Other assets - client-owned fractional shares	0	0
Total assets at fair value on a recurring basis	9,621	9,572
Liabilities at fair value on a recurring basis:
Total debt securities	642	515
Equity securities	0	0
Total trading liabilities	642	515
Derivative contracts liability, gross	446	565
Other payables - repurchase liabilities related to client-owned fractional shares	0	0
Total liabilities at fair value on a recurring basis	1,088	1,080
Recurring | Level 2 | Interest rate
Assets at fair value on a recurring basis:
Total derivative assets	403	503
Liabilities at fair value on a recurring basis:
Derivative liabilities	441	563
Recurring | Level 2 | Foreign exchange
Assets at fair value on a recurring basis:
Total derivative assets	1	9
Liabilities at fair value on a recurring basis:
Derivative liabilities	5	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	583	514
Recurring | Level 2 | Government and agency obligations
Liabilities at fair value on a recurring basis:
Total debt securities	1	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	58
Recurring | Level 2 | Municipal and provincial obligations
Assets at fair value on a recurring basis:
Total debt securities	266	239
Recurring | Level 2 | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	673	620
Recurring | Level 2 | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	152	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	198	35
Recurring | Level 2 | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	67	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	4	4
Total trading assets	4	4
Available-for-sale securities	0	0
Total derivative assets	0	0
Total all other investments	29	30
Other assets - client-owned fractional shares	0	0
Total assets at fair value on a recurring basis	33	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	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