COMMITMENTS, CONTINGENCIES AND GUARANTEES, Commitments and Contingencies (Details) $ in Millions	6 Months Ended
	Mar. 31, 2020 USD ($) commitment
Underwriting commitment
Commitments [Line Items]
Number of open underwriting commitments | commitment	4
Independent venture capital or private equity investment commitment
Commitments [Line Items]
Amount of commitment	$ 41
Commitment to lend to RJTCF
Commitments [Line Items]
Amount of commitment	$ 204
Number of days that investments in project partnerships typically sold	90 days
Forward GNMA MBS purchase commitments
Commitments [Line Items]
Amount of commitment	$ 331
Expected time of purchase (in days)	90 days