FAIR VALUE, Recurring and Nonrecurring Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Assets at fair value on a recurring basis:
Assets segregated for regulatory purposes and restricted cash	$ 11,348	$ 4,244
Total trading assets	610	513
Available-for-sale securities	8,315	7,650
Derivative assets, gross	342	573
Amount of derivative assets offset	(87)	(135)
Derivative assets	255	438
Liabilities, Fair Value Disclosure [Abstract]
Total trading liabilities	176	240
Derivative liability	316	505
Derivative liability, collateral, amount of offset	(88)	(112)
Net amounts presented on the Consolidated Statements of Financial Condition	228	393
Recurring
Assets at fair value on a recurring basis:
Assets segregated for regulatory purposes and restricted cash	2,100
Total debt securities	568	468
Equity securities	12	16
Brokered certificates of deposit	16	17
Other	14	12
Total trading assets	610	513
Available-for-sale securities	8,315	7,650
Amount of derivative assets offset	(87)	(135)
Derivative assets	255	438
Total all other investments	188	218
Netting adjustment	(87)	(135)
Total assets at fair value on a recurring basis	11,637	8,935
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	145	144
Equity securities	31	96
Total trading liabilities	176	240
Netting adjustment	(88)	(112)
Net amounts presented on the Consolidated Statements of Financial Condition	228	393
Total liabilities at fair value on a recurring basis	404	633
Recurring | Municipal and provincial obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	2	1
Recurring | Corporate obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	6	5
Recurring | Government and agency obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	137	136
Recurring | Non-agency CMOs and ABS
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities		2
Recurring | Municipal and provincial obligations
Assets at fair value on a recurring basis:
Total debt securities	155	125
Recurring | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	79	56
Recurring | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	109	144
Total all other investments	86	103
Recurring | Agency MBS and agency CMOs
Assets at fair value on a recurring basis:
Total debt securities	211	130
Recurring | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	14	13
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	11,543	8,856
Recurring | Other assets
Assets at fair value on a recurring basis:
Total all other investments	102	115
Recurring | Level 1
Assets at fair value on a recurring basis:
Assets segregated for regulatory purposes and restricted cash	2,100
Total debt securities	31	29
Equity securities	8	11
Brokered certificates of deposit	0	0
Other	0	0
Total trading assets	39	40
Available-for-sale securities	15	16
Derivative assets, gross	16	16
Total all other investments	163	195
Total assets at fair value on a recurring basis	2,333	267
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	139	137
Equity securities	28	96
Total trading liabilities	167	233
Derivative contracts liability, gross	16	16
Total liabilities at fair value on a recurring basis	183	249
Recurring | Level 1 | Municipal and provincial obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	2	1
Recurring | Level 1 | Corporate obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Recurring | Level 1 | Government and agency obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	137	136
Recurring | Level 1 | Non-agency CMOs and ABS
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities		0
Recurring | Level 1 | Municipal and provincial obligations
Assets at fair value on a recurring basis:
Total debt securities	0	5
Recurring | Level 1 | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	16	11
Recurring | Level 1 | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	15	13
Total all other investments	86	103
Recurring | Level 1 | Agency MBS and agency CMOs
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
Assets at fair value on a recurring basis:
Total all other investments	77	92
Recurring | Level 2
Assets at fair value on a recurring basis:
Assets segregated for regulatory purposes and restricted cash	0
Total debt securities	537	439
Equity securities	4	5
Brokered certificates of deposit	16	17
Other	0	0
Total trading assets	557	461
Available-for-sale securities	8,300	7,634
Derivative assets, gross	326	557
Total all other investments	2	1
Total assets at fair value on a recurring basis	9,185	8,653
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	6	7
Equity securities	3	0
Total trading liabilities	9	7
Derivative contracts liability, gross	299	484
Total liabilities at fair value on a recurring basis	308	491
Recurring | Level 2 | Municipal and provincial obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Recurring | Level 2 | Corporate obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	6	5
Recurring | Level 2 | Government and agency obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Recurring | Level 2 | Non-agency CMOs and ABS
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities		2
Recurring | Level 2 | Municipal and provincial obligations
Assets at fair value on a recurring basis:
Total debt securities	155	120
Recurring | Level 2 | Corporate obligations
Assets at fair value on a recurring basis:
Total debt securities	63	45
Recurring | Level 2 | Government and agency obligations
Assets at fair value on a recurring basis:
Total debt securities	94	131
Total all other investments	0	0
Recurring | Level 2 | Agency MBS and agency CMOs
Assets at fair value on a recurring basis:
Total debt securities	211	130
Recurring | Level 2 | Non-agency CMOs and ABS
Assets at fair value on a recurring basis:
Total debt securities	14	13
Recurring | Level 2 | Other assets
Assets at fair value on a recurring basis:
Total all other investments	2	1
Recurring | Level 3
Assets at fair value on a recurring basis:
Assets segregated for regulatory purposes and restricted cash	0
Total debt securities	0	0
Equity securities	0	0
Brokered certificates of deposit	0	0
Other	14	12
Total trading assets	14	12
Available-for-sale securities	0	0
Derivative assets, gross	0	0
Total all other investments	23	22
Total assets at fair value on a recurring basis	112	71
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Equity securities	0	0
Total trading liabilities	0	0
Derivative contracts liability, gross	1	5
Total liabilities at fair value on a recurring basis	1	5
Recurring | Level 3 | Municipal and provincial obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Recurring | Level 3 | Corporate obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Recurring | Level 3 | Government and agency obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Recurring | Level 3 | Non-agency CMOs and ABS
Liabilities, Fair Value Disclosure [Abstract]
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 MBS and agency CMOs
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
Assets at fair value on a recurring basis:
Total all other investments	23	22
Interest rate contracts | Recurring
Assets at fair value on a recurring basis:
Interest rate - matched book	193	333
Amount of derivative assets offset	(87)	(135)
Derivative assets	57	105
Liabilities, Fair Value Disclosure [Abstract]
Interest rate - matched book	193	333
Derivative liability, collateral, amount of offset	(88)	(112)
Net amounts presented on the Consolidated Statements of Financial Condition	34	49
Interest rate contracts | Recurring | Level 1
Assets at fair value on a recurring basis:
Interest rate - matched book	0	0
Derivative assets, gross	16	16
Liabilities, Fair Value Disclosure [Abstract]
Interest rate - matched book	0	0
Derivative liability	16	16
Interest rate contracts | Recurring | Level 2
Assets at fair value on a recurring basis:
Interest rate - matched book	193	333
Derivative assets, gross	128	224
Liabilities, Fair Value Disclosure [Abstract]
Interest rate - matched book	193	333
Derivative liability	106	145
Interest rate contracts | Recurring | Level 3
Assets at fair value on a recurring basis:
Interest rate - matched book	0	0
Derivative assets, gross	0	0
Liabilities, Fair Value Disclosure [Abstract]
Interest rate - matched book	0	0
Derivative liability	0	0
Foreign exchange | Recurring
Assets at fair value on a recurring basis:
Derivative assets	5
Liabilities, Fair Value Disclosure [Abstract]
Net amounts presented on the Consolidated Statements of Financial Condition		5
Foreign exchange | Recurring | Level 1
Assets at fair value on a recurring basis:
Derivative assets, gross	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative contracts liability, gross		0
Foreign exchange | Recurring | Level 2
Assets at fair value on a recurring basis:
Derivative assets, gross	5
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability		5
Foreign exchange | Recurring | Level 3
Assets at fair value on a recurring basis:
Derivative assets, gross	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative contracts liability, gross		0
Other | Recurring
Liabilities, Fair Value Disclosure [Abstract]
Net amounts presented on the Consolidated Statements of Financial Condition	1	6
Other | Recurring | Level 1
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability	0	0
Other | Recurring | Level 2
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability	0	1
Other | Recurring | Level 3
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability	1	5
Other investments - private equity | Recurring
Assets at fair value on a recurring basis:
Other investments - private equity - not measured at NAV	75	37
Other investments - private equity - measured at NAV	94	79
Other investments - private equity | Recurring | Level 1
Assets at fair value on a recurring basis:
Other investments - private equity - not measured at NAV	0	0
Other investments - private equity | Recurring | Level 2
Assets at fair value on a recurring basis:
Other investments - private equity - not measured at NAV	0	0
Other investments - private equity | Recurring | Level 3
Assets at fair value on a recurring basis:
Other investments - private equity - not measured at NAV	$ 75	$ 37