INTEREST INCOME AND INTEREST EXPENSE (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Interest income:
Cash and cash equivalents	$ 12	$ 41	$ 83
Assets segregated for regulatory purposes and restricted cash	15	28	59
Available-for-sale securities	85	83	69
Brokerage client receivables	77	84	122
Bank loans, net of unearned income and deferred expenses	593	702	871
All other	41	62	77
Total interest income	823	1,000	1,281
Interest expense:
Bank deposits	23	41	132
Brokerage client payables	3	11	21
Other borrowings	19	20	21
Senior notes payable	96	85	73
All other	9	21	36
Total interest expense	150	178	283
Net interest income	673	822	998
Bank loan (provision)/benefit for credit losses	32	(233)	(22)
Net interest income after bank loan (provision)/benefit for credit losses	$ 705	$ 589	$ 976