INTEREST INCOME AND INTEREST EXPENSE (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Interest income:
Cash and cash equivalents	$ 4	$ 17
Assets segregated pursuant to regulations	3	11
Available-for-sale securities	23	18
Brokerage client receivables	18	27
Bank loans, net of unearned income and deferred expenses	145	206
All other	10	18
Total Interest income	203	297
Interest expense:
Bank deposits	6	16
Brokerage client payables	1	3
Other borrowings	5	5
Senior notes payable	24	18
All other	2	9
Total interest expense	38	51
Net interest income	165	246
Bank loan (provision)/benefit for credit losses	(14)	2
Net interest income after bank loan (provision)/benefit for credit losses	$ 151	$ 248