INTEREST INCOME AND INTEREST EXPENSE (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Interest income:
Cash and cash equivalents	$ 109	$ 10	$ 239	$ 16
Assets segregated for regulatory purposes and restricted cash	47	28	152	39
Trading assets — debt securities	13	4	40	13
Available-for-sale securities	56	37	163	84
Brokerage client receivables	42	24	124	66
Bank loans, net	698	255	1,954	590
All other	22	16	57	33
Total Interest income	987	374	2,729	841
Interest expense:
Bank deposits	312	20	690	31
Trading liabilities — debt securities	9	1	26	3
Brokerage client payables	17	3	57	4
Other borrowings	12	6	30	15
Senior notes payable	23	23	69	69
All other	13	7	39	13
Total interest expense	386	60	911	135
Net interest income	601	314	1,818	706
Bank loan provision for credit losses	(54)	(56)	(96)	(66)
Net interest income after bank loan provision for credit losses	$ 547	$ 258	$ 1,722	$ 640