INTEREST INCOME AND INTEREST EXPENSE (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Interest income:
Cash and cash equivalents	$ 48	$ 12	$ 41
Assets segregated for regulatory purposes and restricted cash	96	15	28
Trading assets — debt securities	27	13	18
Available-for-sale securities	136	85	83
Brokerage client receivables	100	77	84
Bank loans, net	1,051	593	702
All other	50	28	44
Total interest income	1,508	823	1,000
Interest expense:
Bank deposits	131	23	41
Trading liabilities — debt securities	12	2	3
Brokerage client payables	24	3	11
Other borrowings	21	19	20
Senior notes payable	93	96	85
All other	24	7	18
Total interest expense	305	150	178
Net interest income	1,203	673	822
Bank loan (provision)/benefit for credit losses	(100)	32	(233)
Net interest income after bank loan (provision)/benefit for credit losses	$ 1,103	$ 705	$ 589