INTEREST INCOME AND INTEREST EXPENSE (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Interest income:
Cash and cash equivalents	$ 2	$ 16	$ 6	$ 33
Assets segregated pursuant to regulations	5	11	8	22
Available-for-sale securities	21	19	44	37
Brokerage client receivables	19	21	37	48
Bank loans, net of unearned income and deferred expenses	142	198	287	404
All other	11	20	21	38
Total Interest income	200	285	403	582
Interest expense:
Bank deposits	6	12	12	28
Brokerage client payables	1	3	2	6
Other borrowings	5	5	10	10
Senior notes payable	24	19	48	37
All other	1	4	3	13
Total interest expense	37	43	75	94
Net interest income	163	242	328	488
Bank loan (provision)/benefit for credit losses	32	(109)	18	(107)
Net interest income after bank loan (provision)/benefit for credit losses	$ 195	$ 133	$ 346	$ 381