COMMITMENTS, CONTINGENCIES AND GUARANTEES, Commitments Narrative (Details) $ in Millions	12 Months Ended
	Sep. 30, 2021 USD ($) agreement	Sep. 30, 2020 USD ($)
Commitments [Line Items]
Loans to financial advisors, net	$ 1,057	$ 1,012
Underwriting commitment
Commitments [Line Items]
Number of open underwriting commitments | agreement	3
Various Investment Commitment
Commitments [Line Items]
Amount of commitment	$ 36
Commitment to lend to RJTCF
Commitments [Line Items]
Amount of commitment	$ 61
Number of days that investments in project partnerships are typically sold (in days)	90 days
Forward GNMA MBS purchase commitments
Commitments [Line Items]
Amount of commitment	$ 198
Expected time of purchase (in days)	90 days
Unfunded lending commitments
Commitments [Line Items]
Loans to financial advisors, net	$ 21