INTEREST INCOME AND INTEREST EXPENSE (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Interest income:
Cash and cash equivalents	$ 128	$ 75	$ 260	$ 130
Assets segregated for regulatory purposes and restricted cash	47	55	94	105
Trading assets — debt securities	19	13	34	27
Available-for-sale securities	56	54	112	107
Brokerage client receivables	47	41	92	82
Bank loans, net	727	657	1,461	1,256
All other	25	20	49	35
Total interest income	1,049	915	2,102	1,742
Interest expense:
Bank deposits	441	206	872	378
Trading liabilities — debt securities	11	7	22	17
Brokerage client payables	21	23	41	40
Other borrowings	8	9	16	18
Senior notes payable	23	23	46	46
All other	16	16	30	26
Total interest expense	520	284	1,027	525
Net interest income	529	631	1,075	1,217
Bank loan provision for credit losses	(21)	(28)	(33)	(42)
Net interest income after bank loan provision for credit losses	$ 508	$ 603	$ 1,042	$ 1,175