INTEREST INCOME AND INTEREST EXPENSE (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Interest income:
Assets segregated pursuant to regulations	$ 3	$ 14	$ 25	$ 46
Trading instruments	4	7	17	20
Available-for-sale securities	23	18	60	51
Margin loans	18	30	66	93
Bank loans, net of unearned income and deferred expenses	157	221	561	655
Loans to financial advisors	5	5	15	14
Corporate cash and all other	7	26	55	82
Total Interest income	217	321	799	961
Interest expense:
Bank deposits	7	33	35	105
Trading instruments sold but not yet purchased	1	2	3	6
Brokerage client payables	3	5	9	16
Other borrowings	5	5	15	16
Senior notes payable	24	19	61	55
Other	2	9	13	23
Total interest expense	42	73	136	221
Net interest income	175	248	663	740
Bank loan loss (provision)/benefit	(81)	5	(188)	(16)
Net interest income after bank loan loss (provision)/benefit	$ 94	$ 253	$ 475	$ 724