INTEREST INCOME AND INTEREST EXPENSE (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Interest income:
Cash and cash equivalents	$ 3	$ 4
Assets segregated for regulatory purposes and restricted cash	4	3
Available-for-sale securities	22	23
Brokerage client receivables	21	18
Bank loans, net of unearned income and deferred expenses	164	145
All other	11	10
Total Interest income	225	203
Interest expense:
Bank deposits	6	6
Brokerage client payables	1	1
Other borrowings	5	5
Senior notes payable	23	24
All other	2	2
Total interest expense	37	38
Net interest income	188	165
Bank loan (provision)/benefit for credit losses	11	(14)
Net interest income after bank loan (provision)/benefit for credit losses	$ 199	$ 151