FAIR VALUE, Recurring and Nonrecurring Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Assets, Fair Value Disclosure [Abstract]
Total trading instruments	$ 513	$ 708
Available-for-sale securities	7,650	3,093
Derivative assets, gross	573	465
Amount of derivative assets offset	(135)	(127)
Derivative assets	438	338
Liabilities, Fair Value Disclosure [Abstract]
Total trading instruments sold but not yet purchased	240	296
Derivative liability	505	434
Derivative liability, collateral, amount of offset	(112)	(121)
Net amounts presented on the Consolidated Statements of Financial Condition	393	313
Recurring
Assets, Fair Value Disclosure [Abstract]
Total debt securities	468	647
Equity securities	16	13
Brokered certificates of deposit	17	45
Other	12	3
Total trading instruments	513	708
Available-for-sale securities	7,650	3,093
Amount of derivative assets offset	(135)	(127)
Derivative assets	438	338
All other investments	218	219
Total assets at fair value	8,935	4,504
Netting adjustment	(135)	(127)
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	144	291
Equity securities	96	4
Other		1
Total trading instruments sold but not yet purchased	240	296
Netting adjustment	(112)	(121)
Net amounts presented on the Consolidated Statements of Financial Condition	393	313
Total liabilities at fair value on a recurring basis	633	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	5	22
Recurring | Government and agency obligations
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	136	269
Recurring | Non-agency CMOs and ABS
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	2
Recurring | Municipal and provincial obligations
Assets, Fair Value Disclosure [Abstract]
Total debt securities	125	267
Recurring | Corporate obligations
Assets, Fair Value Disclosure [Abstract]
Total debt securities	56	103
Recurring | Government and agency obligations
Assets, Fair Value Disclosure [Abstract]
Total debt securities	144	79
Recurring | Agency MBS and agency CMOs
Assets, Fair Value Disclosure [Abstract]
Total debt securities	130	147
Recurring | Non-agency CMOs and ABS
Assets, Fair Value Disclosure [Abstract]
Total debt securities	13	51
Recurring | Assets at Fair Value on a Recurring Basis, Before Investments Measured at NAV
Assets, Fair Value Disclosure [Abstract]
Total assets at fair value	8,856	4,421
Recurring | Level 1
Assets, Fair Value Disclosure [Abstract]
Total debt securities	29	20
Equity securities	11	12
Brokered certificates of deposit	0	0
Other	0	0
Total trading instruments	40	32
Available-for-sale securities	16	10
Derivative assets, gross	16	3
All other investments	195	194
Total assets at fair value	267	239
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	137	271
Equity securities	96	4
Other		0
Total trading instruments sold but not yet purchased	233	275
Derivative contracts liability, gross	16	4
Total liabilities at fair value on a recurring basis	249	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	136	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 [Abstract]
Total debt securities	5	0
Recurring | Level 1 | Corporate obligations
Assets, Fair Value Disclosure [Abstract]
Total debt securities	11	8
Recurring | Level 1 | Government and agency obligations
Assets, Fair Value Disclosure [Abstract]
Total debt securities	13	12
Recurring | Level 1 | Agency MBS and agency CMOs
Assets, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Recurring | Level 1 | Non-agency CMOs and ABS
Assets, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Total debt securities	439	627
Equity securities	5	1
Brokered certificates of deposit	17	45
Other	0	0
Total trading instruments	461	673
Available-for-sale securities	7,634	3,083
Derivative assets, gross	557	462
All other investments	1	1
Total assets at fair value	8,653	4,219
Liabilities, Fair Value Disclosure [Abstract]
Total debt securities	7	20
Equity securities	0	0
Other		0
Total trading instruments sold but not yet purchased	7	20
Derivative contracts liability, gross	484	430
Total liabilities at fair value on a recurring basis	491	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	5	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	2
Recurring | Level 2 | Municipal and provincial obligations
Assets, Fair Value Disclosure [Abstract]
Total debt securities	120	267
Recurring | Level 2 | Corporate obligations
Assets, Fair Value Disclosure [Abstract]
Total debt securities	45	95
Recurring | Level 2 | Government and agency obligations
Assets, Fair Value Disclosure [Abstract]
Total debt securities	131	67
Recurring | Level 2 | Agency MBS and agency CMOs
Assets, Fair Value Disclosure [Abstract]
Total debt securities	130	147
Recurring | Level 2 | Non-agency CMOs and ABS
Assets, Fair Value Disclosure [Abstract]
Total debt securities	13	51
Recurring | Level 3
Assets, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Equity securities	0	0
Brokered certificates of deposit	0	0
Other	12	3
Total trading instruments	12	3
Available-for-sale securities	0	0
Derivative assets, gross	0	0
All other investments	22	24
Total assets at fair value	71	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	5	0
Total liabilities at fair value on a recurring basis	5	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 [Abstract]
Total debt securities	0	0
Recurring | Level 3 | Corporate obligations
Assets, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Recurring | Level 3 | Government and agency obligations
Assets, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Recurring | Level 3 | Agency MBS and agency CMOs
Assets, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Recurring | Level 3 | Non-agency CMOs and ABS
Assets, Fair Value Disclosure [Abstract]
Total debt securities	0	0
Interest rate contracts | Recurring
Assets, Fair Value Disclosure [Abstract]
Interest rate - matched book	333	280
Amount of derivative assets offset	(135)	(127)
Derivative assets	105	58
Liabilities, Fair Value Disclosure [Abstract]
Matched book	333	280
Derivative liability, collateral, amount of offset	(112)	(121)
Net amounts presented on the Consolidated Statements of Financial Condition	49	25
Interest rate contracts | Recurring | Level 1
Assets, Fair Value Disclosure [Abstract]
Interest rate - matched book	0	0
Derivative assets, gross	16	3
Liabilities, Fair Value Disclosure [Abstract]
Matched book	0	0
Derivative liability	16	4
Interest rate contracts | Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Interest rate - matched book	333	280
Derivative assets, gross	224	182
Liabilities, Fair Value Disclosure [Abstract]
Matched book	333	280
Derivative liability	145	142
Interest rate contracts | Recurring | Level 3
Assets, Fair Value Disclosure [Abstract]
Interest rate - matched book	0	0
Derivative assets, gross	0	0
Liabilities, Fair Value Disclosure [Abstract]
Matched book	0	0
Derivative liability	0	0
Foreign exchange | Recurring
Liabilities, Fair Value Disclosure [Abstract]
Net amounts presented on the Consolidated Statements of Financial Condition	5	2
Foreign exchange | Recurring | Level 1
Liabilities, Fair Value Disclosure [Abstract]
Derivative contracts liability, gross	0	0
Foreign exchange | Recurring | Level 2
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability	5	2
Foreign exchange | Recurring | Level 3
Liabilities, Fair Value Disclosure [Abstract]
Derivative contracts liability, gross	0	0
Other | Recurring
Liabilities, Fair Value Disclosure [Abstract]
Net amounts presented on the Consolidated Statements of Financial Condition	6	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	1	6
Other | Recurring | Level 3
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability	5	0
Other investments - private equity | Recurring
Assets, Fair Value Disclosure [Abstract]
Private equity investments not measured at NAV	37	63
Private equity investments	116	146
Private equity investments measured at NAV	79	83
Other investments - private equity | Recurring | Level 1
Assets, Fair Value Disclosure [Abstract]
Private equity investments not measured at NAV	0	0
Other investments - private equity | Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Private equity investments not measured at NAV	0	0
Other investments - private equity | Recurring | Level 3
Assets, Fair Value Disclosure [Abstract]
Private equity investments not measured at NAV	$ 37	$ 63