FAIR VALUE, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions	Jun. 30, 2020	Sep. 30, 2019
Assets, Fair Value Disclosure
Available-for-sale securities	$ 5,630	$ 3,093
Derivative assets, gross	605	465
Amount of derivative assets offset	(153)	(127)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	452	338
Liabilities, Fair Value Disclosure [Abstract]
Total trading instruments sold but not yet purchased	154	296
Derivative liabilities	535	434
Total amounts offset	(141)	(121)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	394	313
Recurring
Assets, Fair Value Disclosure
Total debt securities	325	647
Equity securities	9	13
Brokered certificates of deposit	12	45
Trading Securities, Other Security	15	3
Total trading instruments	361	708
Available-for-sale securities	5,630	3,093
Amount of derivative assets offset	(153)	(127)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	452	338
All other investments	224	219
Total assets at fair value on a recurring basis	6,770	4,504
Netting adjustments	(153)	(127)
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	112	291
Equity securities	42	4
Other		1
Total trading instruments sold but not yet purchased	154	296
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	394	313
Netting adjustments	(141)	(121)
Total liabilities at fair value on a recurring basis	548	609
Recurring | Municipal and provincial obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	1
Recurring | Corporate obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	3	22
Recurring | Government and agency obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	105	269
Recurring | Non Agency CMOs And ABS
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	3
Recurring | Municipal and provincial obligations
Assets, Fair Value Disclosure
Total debt securities	82	267
Recurring | Corporate obligations
Assets, Fair Value Disclosure
Total debt securities	52	103
Recurring | Government and agency obligations
Assets, Fair Value Disclosure
Total debt securities	82	79
Recurring | Agency mortgage-backed securities (“MBS”) and collateralized mortgage obligations (“CMOs”)
Assets, Fair Value Disclosure
Total debt securities	102	147
Recurring | Non-agency CMOs and asset-backed securities (“ABS”)
Assets, Fair Value Disclosure
Total debt securities	7	51
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	6,697	4,421
Recurring | Level 1
Assets, Fair Value Disclosure
Total debt securities	27	20
Equity securities	9	12
Brokered certificates of deposit	0	0
Trading Securities, Other Security	0	0
Total trading instruments	36	32
Available-for-sale securities	16	10
Derivative assets, gross	15	3
All other investments	201	194
Total assets at fair value on a recurring basis	268	239
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	106	271
Equity securities	42	4
Other		0
Total trading instruments sold but not yet purchased	148	275
Derivative contracts liability, gross	17	4
Total liabilities at fair value on a recurring basis	165	279
Recurring | Level 1 | Municipal and provincial obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	1
Recurring | Level 1 | Corporate obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	2
Recurring | Level 1 | Government and agency obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	105	269
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	5	0
Recurring | Level 1 | Corporate obligations
Assets, Fair Value Disclosure
Total debt securities	6	8
Recurring | Level 1 | Government and agency obligations
Assets, Fair Value Disclosure
Total debt securities	16	12
Recurring | Level 1 | Agency mortgage-backed securities (“MBS”) and collateralized mortgage obligations (“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	0
Recurring | Level 2
Assets, Fair Value Disclosure
Total debt securities	298	627
Equity securities	0	1
Brokered certificates of deposit	12	45
Trading Securities, Other Security	0	0
Total trading instruments	310	673
Available-for-sale securities	5,614	3,083
Derivative assets, gross	590	462
All other investments	1	1
Total assets at fair value on a recurring basis	6,515	4,219
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	6	20
Equity securities	0	0
Other		0
Total trading instruments sold but not yet purchased	6	20
Derivative contracts liability, gross	518	430
Total liabilities at fair value on a recurring basis	524	450
Recurring | Level 2 | Municipal and provincial obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0
Recurring | Level 2 | Corporate obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	3	20
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	3
Recurring | Level 2 | Municipal and provincial obligations
Assets, Fair Value Disclosure
Total debt securities	77	267
Recurring | Level 2 | Corporate obligations
Assets, Fair Value Disclosure
Total debt securities	46	95
Recurring | Level 2 | Government and agency obligations
Assets, Fair Value Disclosure
Total debt securities	66	67
Recurring | Level 2 | Agency mortgage-backed securities (“MBS”) and collateralized mortgage obligations (“CMOs”)
Assets, Fair Value Disclosure
Total debt securities	102	147
Recurring | Level 2 | Non-agency CMOs and asset-backed securities (“ABS”)
Assets, Fair Value Disclosure
Total debt securities	7	51
Recurring | Level 3
Assets, Fair Value Disclosure
Total debt securities	0	0
Equity securities	0	0
Brokered certificates of deposit	0	0
Trading Securities, Other Security	15	3
Total trading instruments	15	3
Available-for-sale securities	0	0
Derivative assets, gross	0	0
All other investments	22	24
Total assets at fair value on a recurring basis	67	90
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Equity securities	0	0
Other		1
Total trading instruments sold but not yet purchased	0	1
Derivative contracts liability, gross	0	0
Total liabilities at fair value on a recurring basis	0	1
Recurring | Level 3 | Municipal and provincial obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	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
Recurring | Level 3 | Agency mortgage-backed securities (“MBS”) and collateralized mortgage obligations (“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	0
Recurring | Interest rate
Assets, Fair Value Disclosure
Interest rate - matched book	351	280
Amount of derivative assets offset	(153)	(127)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	101	58
Liabilities, Fair Value Disclosure [Abstract]
Matched book	351	280
Total amounts offset	(141)	(121)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	30	25
Recurring | Interest rate | Level 1
Assets, Fair Value Disclosure
Interest rate - matched book	0	0
Derivative assets, gross	15	3
Liabilities, Fair Value Disclosure [Abstract]
Matched book	0	0
Derivative liabilities	17	4
Recurring | Interest rate | Level 2
Assets, Fair Value Disclosure
Interest rate - matched book	351	280
Derivative assets, gross	239	182
Liabilities, Fair Value Disclosure [Abstract]
Matched book	351	280
Derivative liabilities	154	142
Recurring | Interest rate | Level 3
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 | Foreign exchange
Liabilities, Fair Value Disclosure [Abstract]
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	12	2
Recurring | Foreign exchange | Level 1
Liabilities, Fair Value Disclosure [Abstract]
Derivative contracts liability, gross	0	0
Recurring | Foreign exchange | Level 2
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	12	2
Recurring | Foreign exchange | Level 3
Liabilities, Fair Value Disclosure [Abstract]
Derivative contracts liability, gross	0	0
Recurring | Other
Liabilities, Fair Value Disclosure [Abstract]
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	1	6
Recurring | Other | Level 1
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0	0
Recurring | Other | Level 2
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	1	6
Recurring | Other | Level 3
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0	0
Other investments | Recurring
Assets, Fair Value Disclosure
Private equity investments not measured at NAV	30	63
Private equity investments measured at NAV	73	83
Other investments | Recurring | Level 1
Assets, Fair Value Disclosure
Private equity investments not measured at NAV	0	0
Other investments | Recurring | Level 2
Assets, Fair Value Disclosure
Private equity investments not measured at NAV	0	0
Other investments | Recurring | Level 3
Assets, Fair Value Disclosure
Private equity investments not measured at NAV	$ 30	$ 63