INTEREST INCOME AND INTEREST EXPENSE (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Interest income:
Assets segregated pursuant to regulations	$ 28	$ 59	$ 53
Trading instruments	20	26	23
Available-for-sale securities	83	69	52
Margin loans	84	122	107
Bank loans, net of unearned income and deferred expenses	702	871	722
Loans to financial advisors	20	18	15
Corporate cash and all other	63	116	72
Total interest income	1,000	1,281	1,044
Interest expense:
Bank deposits	41	132	66
Trading instruments sold but not yet purchased	3	7	7
Brokerage client payables	11	21	15
Other borrowings	20	21	22
Senior notes payable	85	73	73
Other	18	29	19
Total interest expense	178	283	202
Net interest income	822	998	842
Bank loan loss provision	(233)	(22)	(20)
Net interest income after bank loan loss provision	$ 589	$ 976	$ 822