INTEREST INCOME AND INTEREST EXPENSE (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Interest income:
Cash and cash equivalents	$ 3	$ 2	$ 6	$ 6
Assets segregated for regulatory purposes and restricted cash	7	5	11	8
Available-for-sale securities	25	21	47	44
Brokerage client receivables	21	19	42	37
Bank loans, net of unearned income and deferred expenses	171	142	335	287
All other	15	11	26	21
Total Interest income	242	200	467	403
Interest expense:
Bank deposits	5	6	11	12
Brokerage client payables	0	1	1	2
Other borrowings	4	5	9	10
Senior notes payable	23	24	46	48
All other	6	1	8	3
Total interest expense	38	37	75	75
Net interest income	204	163	392	328
Bank loan (provision)/benefit for credit losses	(21)	32	(10)	18
Net interest income after bank loan (provision)/benefit for credit losses	$ 183	$ 195	$ 382	$ 346