LOANS TO FINANCIAL ADVISORS, NET, Balance of Loans to Financial Advisors (Details) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2021	Sep. 30, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
Currently affiliated with the firm	$ 1,006	$ 1,001
No longer affiliated with the firm	10	15
Total loans to financial advisors	1,016	1,016
Allowance for credit losses	(28)	(4)
Loans to financial advisors, net	988	1,012
Accrued interest receivable on loans to financial advisors	$ 4	4
Past due period	180 days
Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit losses		$ (25)