INTEREST INCOME AND INTEREST EXPENSE (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Interest income:
Cash and cash equivalents	$ 132	$ 55
Assets segregated for regulatory purposes and restricted cash	47	50
Trading assets — debt securities	15	14
Available-for-sale securities	56	53
Brokerage client receivables	45	41
Bank loans, net	734	599
All other	24	15
Total interest income	1,053	827
Interest expense:
Bank deposits	431	172
Trading liabilities — debt securities	11	10
Brokerage client payables	20	17
Other borrowings	8	9
Senior notes payable	23	23
All other	14	10
Total interest expense	507	241
Net interest income	546	586
Bank loan provision for credit losses	(12)	(14)
Net interest income after bank loan provision for credit losses	$ 534	$ 572