FAIR VALUE, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Assets, Fair Value Disclosure
Available-for-sale securities	$ 4,265	$ 3,093
Derivative assets, gross	710	465
Amount of derivative assets offset	(263)	(127)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	447	338
Liabilities, Fair Value Disclosure [Abstract]
Total trading instruments sold but not yet purchased	24	296
Derivative liabilities	645	434
Total amounts offset	(218)	(121)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	427	313
Recurring
Assets, Fair Value Disclosure
Total debt securities	391	647
Equity securities	9	13
Brokered certificates of deposit	24	45
Trading Securities, Other Security	21	3
Total trading instruments	445	708
Available-for-sale securities	4,265	3,093
Amount of derivative assets offset	(263)	(127)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	447	338
All other investments	215	219
Total assets at fair value on a recurring basis	5,473	4,504
Netting adjustments	(263)	(127)
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	21	291
Equity securities	3	4
Other	0	1
Total trading instruments sold but not yet purchased	24	296
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	427	313
Netting adjustments	(218)	(121)
Total liabilities at fair value on a recurring basis	451	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	8	22
Recurring | Government obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	6	269
Recurring | Non Agency CMOs And ABS
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	6
Recurring | Municipal and provincial obligations
Assets, Fair Value Disclosure
Total debt securities	14	267
Recurring | Corporate obligations
Assets, Fair Value Disclosure
Total debt securities	21	103
Recurring | Government and agency obligations
Assets, Fair Value Disclosure
Total debt securities	58	79
Recurring | Agency mortgage-backed securities (“MBS”) and collateralized mortgage obligations (“CMOs”)
Assets, Fair Value Disclosure
Total debt securities	281	147
Recurring | Non-agency CMOs and asset-backed securities (“ABS”)
Assets, Fair Value Disclosure
Total debt securities	17	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	5,402	4,421
Recurring | Level 1
Assets, Fair Value Disclosure
Total debt securities	42	20
Equity securities	9	12
Brokered certificates of deposit	0	0
Trading Securities, Other Security	0	0
Total trading instruments	51	32
Available-for-sale securities	16	10
Derivative assets, gross	93	3
All other investments	192	194
Total assets at fair value on a recurring basis	352	239
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	8	271
Equity securities	3	4
Other	0	0
Total trading instruments sold but not yet purchased	11	275
Derivative contracts liability, gross	102	4
Total liabilities at fair value on a recurring basis	113	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	1	2
Recurring | Level 1 | Government obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	6	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	0	0
Recurring | Level 1 | Corporate obligations
Assets, Fair Value Disclosure
Total debt securities	12	8
Recurring | Level 1 | Government and agency obligations
Assets, Fair Value Disclosure
Total debt securities	30	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	349	627
Equity securities	0	1
Brokered certificates of deposit	24	45
Trading Securities, Other Security	0	0
Total trading instruments	373	673
Available-for-sale securities	4,249	3,083
Derivative assets, gross	617	462
All other investments	1	1
Total assets at fair value on a recurring basis	5,240	4,219
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	13	20
Equity securities	0	0
Other	0	0
Total trading instruments sold but not yet purchased	13	20
Derivative contracts liability, gross	543	430
Total liabilities at fair value on a recurring basis	556	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	7	20
Recurring | Level 2 | Government 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	6
Recurring | Level 2 | Municipal and provincial obligations
Assets, Fair Value Disclosure
Total debt securities	14	267
Recurring | Level 2 | Corporate obligations
Assets, Fair Value Disclosure
Total debt securities	9	95
Recurring | Level 2 | Government and agency obligations
Assets, Fair Value Disclosure
Total debt securities	28	67
Recurring | Level 2 | Agency mortgage-backed securities (“MBS”) and collateralized mortgage obligations (“CMOs”)
Assets, Fair Value Disclosure
Total debt securities	281	147
Recurring | Level 2 | Non-agency CMOs and asset-backed securities (“ABS”)
Assets, Fair Value Disclosure
Total debt securities	17	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	21	3
Total trading instruments	21	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	73	90
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Equity securities	0	0
Other	0	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 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
Matched book	351	280
Amount of derivative assets offset	(263)	(127)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	96	58
Liabilities, Fair Value Disclosure [Abstract]
Matched book	351	280
Total amounts offset	(218)	(121)
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	69	25
Recurring | Interest rate | Level 1
Assets, Fair Value Disclosure
Matched book	0	0
Derivative assets, gross	93	3
Liabilities, Fair Value Disclosure [Abstract]
Matched book	0	0
Derivative liabilities	102	4
Recurring | Interest rate | Level 2
Assets, Fair Value Disclosure
Matched book	351	280
Derivative assets, gross	266	182
Liabilities, Fair Value Disclosure [Abstract]
Matched book	351	280
Derivative liabilities	185	142
Recurring | Interest rate | Level 3
Assets, Fair Value Disclosure
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	6	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	6	2
Recurring | Foreign exchange | Level 3
Liabilities, Fair Value Disclosure [Abstract]
Derivative contracts liability, gross	0	0
Recurring | Equity
Liabilities, Fair Value Disclosure [Abstract]
Net amounts presented on the Condensed Consolidated Statements of Financial Condition	1	6
Recurring | Equity | Level 1
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0	0
Recurring | Equity | Level 2
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	1	6
Recurring | Equity | Level 3
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0	0
Other investments | Recurring
Assets, Fair Value Disclosure
Private equity investments not measured at net asset value (NAV)	30	63
Private equity investments at NAV	71	83
Other investments | Recurring | Level 1
Assets, Fair Value Disclosure
Private equity investments not measured at net asset value (NAV)	0	0
Other investments | Recurring | Level 2
Assets, Fair Value Disclosure
Private equity investments not measured at net asset value (NAV)	0	0
Other investments | Recurring | Level 3
Assets, Fair Value Disclosure
Private equity investments not measured at net asset value (NAV)	$ 30	$ 63