INTEREST INCOME AND INTEREST EXPENSE (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Interest income:
Cash and cash equivalents	$ 121	$ 109	$ 381	$ 239
Assets segregated for regulatory purposes and restricted cash	46	47	140	152
Trading assets — debt securities	20	13	54	40
Available-for-sale securities	55	56	167	163
Brokerage client receivables	48	42	140	124
Bank loans, net	736	698	2,197	1,954
All other	31	22	80	57
Total interest income	1,057	987	3,159	2,729
Interest expense:
Bank deposits	446	312	1,318	690
Trading liabilities — debt securities	11	9	33	26
Brokerage client payables	22	17	63	57
Other borrowings	7	12	23	30
Senior notes payable	23	23	69	69
All other	25	13	55	39
Total interest expense	534	386	1,561	911
Net interest income	523	601	1,598	1,818
Bank loan provision/(benefit) for credit losses	(10)	54	23	96
Net interest income after bank loan provision/(benefit) for credit losses	$ 533	$ 547	$ 1,575	$ 1,722