INTEREST INCOME AND INTEREST EXPENSE (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Interest income:
Cash and cash equivalents	$ 509	$ 358	$ 48
Assets segregated for regulatory purposes and restricted cash	183	197	96
Trading assets — debt securities	73	57	27
Available-for-sale securities	220	219	136
Brokerage client receivables	187	170	100
Bank loans, net	2,952	2,671	1,051
All other	108	76	50
Total interest income	4,232	3,748	1,508
Interest expense:
Bank deposits	1,780	1,080	131
Trading liabilities — debt securities	44	36	12
Brokerage client payables	83	78	24
Other borrowings	30	37	21
Senior notes payable	92	92	93
All other	73	50	24
Total interest expense	2,102	1,373	305
Net interest income	2,130	2,375	1,203
Less: Bank loan provision for credit losses	45	132	100
Net interest income after bank loan provision for credit losses	$ 2,085	$ 2,243	$ 1,103