INTEREST INCOME AND INTEREST EXPENSE (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Interest income:
Assets segregated pursuant to regulations	$ 11	$ 17	$ 22	$ 32
Trading instruments	7	6	13	13
Available-for-sale securities	19	17	37	33
Margin loans	21	31	48	63
Bank loans, net of unearned income	198	220	404	434
Loans to financial advisors	5	5	10	9
Corporate cash and all other	24	28	48	56
Total Interest income	285	324	582	640
Interest expense:
Bank deposits	12	37	28	72
Trading instruments sold but not yet purchased	1	2	2	4
Brokerage client payables	3	5	6	11
Other borrowings	5	5	10	11
Senior notes payable	19	18	37	36
Other	3	8	11	14
Total interest expense	43	75	94	148
Net interest income	242	249	488	492
Bank loan loss provision	(109)	(5)	(107)	(21)
Net interest income after bank loan loss provision	$ 133	$ 244	$ 381	$ 471