INTEREST INCOME AND INTEREST EXPENSE (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Interest income:
Cash and cash equivalents	$ 439	$ 509	$ 358
Assets segregated for regulatory purposes and restricted cash	149	183	197
Trading assets — debt securities	75	73	57
Available-for-sale securities	185	220	219
Brokerage client receivables	171	187	170
Bank loans, net	2,860	2,952	2,671
All other	115	108	76
Total interest income	3,994	4,232	3,748
Interest expense:
Bank deposits	1,548	1,780	1,080
Trading liabilities — debt securities	44	44	36
Brokerage client payables	66	83	78
Other borrowings	28	30	37
Senior notes payable	96	92	92
All other	65	73	50
Total interest expense	1,847	2,102	1,373
Net interest income	2,147	2,130	2,375
Less: Bank loan provision for credit losses	37	45	132
Net interest income after bank loan provision for credit losses	$ 2,110	$ 2,085	$ 2,243