FAIR VALUE, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2020
Assets, Fair Value Disclosure
Available-for-sale securities	$ 8,000	$ 7,650
Derivative assets, gross	496	573
Amount of derivative assets offset	(128)	(135)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	368	438
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	448	505
Total amounts offset	(138)	(112)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	310	393
Recurring
Assets, Fair Value Disclosure
Assets segregated pursuant to regulations	2,749
Total debt securities	327	468
Equity securities	19	16
Brokered certificates of deposit	37	17
Trading Securities, Other Security	3	12
Total trading assets	386	513
Available-for-sale securities	8,000	7,650
Amount of derivative assets offset	(128)	(135)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	368	438
Other	726	218
Total assets at fair value on a recurring basis	12,366	8,935
Netting adjustments	(128)	(135)
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	66	144
Equity securities	86	96
Total trading instruments sold but not yet purchased	152	240
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	310	393
Netting adjustments	(138)	(112)
Total liabilities at fair value on a recurring basis	462	633
Recurring | Municipal and provincial obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	1	1
Recurring | Corporate obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	4	5
Recurring | Government and agency obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	61	136
Recurring | Non Agency CMOs And ABS
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities		2
Recurring | Municipal and provincial obligations
Assets, Fair Value Disclosure
Total debt securities	32	125
Recurring | Corporate obligations
Assets, Fair Value Disclosure
Total debt securities	42	56
Recurring | Government and agency obligations
Assets, Fair Value Disclosure
Total debt securities	107	144
Other	603	103
Recurring | Agency MBS and agency CMOs
Assets, Fair Value Disclosure
Total debt securities	146	130
Recurring | Non-agency CMOs and asset-backed securities (“ABS”)
Assets, Fair Value Disclosure
Total debt securities		13
Recurring | Assets at Fair Value on a Recurring Basis, Before Investments Measured at NAV
Assets, Fair Value Disclosure
Total assets at fair value on a recurring basis	12,281	8,856
Recurring | Other assets
Assets, Fair Value Disclosure
Other	123	115
Recurring | Interest rate
Assets, Fair Value Disclosure
Interest rate - matched book	273	333
Amount of derivative assets offset	(128)	(135)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	95	105
Liabilities, Fair Value Disclosure [Abstract]
Matched book	273	333
Total amounts offset	(138)	(112)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	23	49
Recurring | Foreign exchange
Liabilities, Fair Value Disclosure [Abstract]
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	11	5
Recurring | Other
Liabilities, Fair Value Disclosure [Abstract]
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	3	6
Recurring | Private equity investments
Assets, Fair Value Disclosure
Private equity investments not measured at NAV	52	37
Private equity investments measured at NAV	85	79
Recurring | Level 1
Assets, Fair Value Disclosure
Assets segregated pursuant to regulations	2,749
Total debt securities	27	29
Equity securities	14	11
Brokered certificates of deposit	0	0
Trading Securities, Other Security	0	0
Total trading assets	41	40
Available-for-sale securities	16	16
Derivative assets, gross	34	16
Other	703	195
Total assets at fair value on a recurring basis	3,543	267
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	63	137
Equity securities	79	96
Total trading instruments sold but not yet purchased	142	233
Derivative contracts liability, gross	37	16
Total liabilities at fair value on a recurring basis	179	249
Recurring | Level 1 | Municipal and provincial obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	1	1
Recurring | Level 1 | Corporate obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	1	0
Recurring | Level 1 | Government and agency obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	61	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, Fair Value Disclosure
Total debt securities	0	5
Recurring | Level 1 | Corporate obligations
Assets, Fair Value Disclosure
Total debt securities	11	11
Recurring | Level 1 | Government and agency obligations
Assets, Fair Value Disclosure
Total debt securities	16	13
Other	603	103
Recurring | Level 1 | Agency MBS and agency CMOs
Assets, Fair Value Disclosure
Total debt securities	0	0
Recurring | Level 1 | Non-agency CMOs and asset-backed securities (“ABS”)
Assets, Fair Value Disclosure
Total debt securities		0
Recurring | Level 1 | Other assets
Assets, Fair Value Disclosure
Other	100	92
Recurring | Level 1 | Interest rate
Assets, Fair Value Disclosure
Interest rate - matched book	0	0
Derivative assets, gross	34	16
Liabilities, Fair Value Disclosure [Abstract]
Matched book	0	0
Derivative liabilities	37	16
Recurring | Level 1 | Foreign exchange
Liabilities, Fair Value Disclosure [Abstract]
Derivative contracts liability, gross	0	0
Recurring | Level 1 | Other
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0	0
Recurring | Level 1 | Private equity investments
Assets, Fair Value Disclosure
Private equity investments not measured at NAV	0	0
Recurring | Level 2
Assets, Fair Value Disclosure
Assets segregated pursuant to regulations	0
Total debt securities	300	439
Equity securities	5	5
Brokered certificates of deposit	37	17
Trading Securities, Other Security	0	0
Total trading assets	342	461
Available-for-sale securities	7,984	7,634
Derivative assets, gross	462	557
Other	1	1
Total assets at fair value on a recurring basis	8,789	8,653
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	3	7
Equity securities	7	0
Total trading instruments sold but not yet purchased	10	7
Derivative contracts liability, gross	410	484
Total liabilities at fair value on a recurring basis	420	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	3	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, Fair Value Disclosure
Total debt securities	32	120
Recurring | Level 2 | Corporate obligations
Assets, Fair Value Disclosure
Total debt securities	31	45
Recurring | Level 2 | Government and agency obligations
Assets, Fair Value Disclosure
Total debt securities	91	131
Other	0	0
Recurring | Level 2 | Agency MBS and agency CMOs
Assets, Fair Value Disclosure
Total debt securities	146	130
Recurring | Level 2 | Non-agency CMOs and asset-backed securities (“ABS”)
Assets, Fair Value Disclosure
Total debt securities		13
Recurring | Level 2 | Other assets
Assets, Fair Value Disclosure
Other	1	1
Recurring | Level 2 | Interest rate
Assets, Fair Value Disclosure
Interest rate - matched book	273	333
Derivative assets, gross	189	224
Liabilities, Fair Value Disclosure [Abstract]
Matched book	273	333
Derivative liabilities	124	145
Recurring | Level 2 | Foreign exchange
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	11	5
Recurring | Level 2 | Other
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	2	1
Recurring | Level 2 | Private equity investments
Assets, Fair Value Disclosure
Private equity investments not measured at NAV	0	0
Recurring | Level 3
Assets, Fair Value Disclosure
Assets segregated pursuant to regulations	0
Total debt securities	0	0
Equity securities	0	0
Brokered certificates of deposit	0	0
Trading Securities, Other Security	3	12
Total trading assets	3	12
Available-for-sale securities	0	0
Derivative assets, gross	0	0
Other	22	22
Total assets at fair value on a recurring basis	77	71
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Equity securities	0	0
Total trading instruments sold but not yet purchased	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, Fair Value Disclosure
Total debt securities	0	0
Recurring | Level 3 | Corporate obligations
Assets, Fair Value Disclosure
Total debt securities	0	0
Recurring | Level 3 | Government and agency obligations
Assets, Fair Value Disclosure
Total debt securities	0	0
Other	0	0
Recurring | Level 3 | Agency MBS and agency CMOs
Assets, Fair Value Disclosure
Total debt securities	0	0
Recurring | Level 3 | Non-agency CMOs and asset-backed securities (“ABS”)
Assets, Fair Value Disclosure
Total debt securities		0
Recurring | Level 3 | Other assets
Assets, Fair Value Disclosure
Other	22	22
Recurring | Level 3 | Interest rate
Assets, Fair Value Disclosure
Interest rate - matched book	0	0
Derivative assets, gross	0	0
Liabilities, Fair Value Disclosure [Abstract]
Matched book	0	0
Derivative liabilities	0	0
Recurring | Level 3 | Foreign exchange
Liabilities, Fair Value Disclosure [Abstract]
Derivative contracts liability, gross	0	0
Recurring | Level 3 | Other
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	1	5
Recurring | Level 3 | Private equity investments
Assets, Fair Value Disclosure
Private equity investments not measured at NAV	$ 52	$ 37