COLLATERALIZED AGREEMENTS AND FINANCINGS, Encumbered Assets (Details) - USD ($) $ in Millions	Jun. 30, 2024	Sep. 30, 2023
Asset pledged as collateral | Assets pledged with the FHLB or FRB:
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial instruments, owned, at fair value	$ 14,742	$ 14,063
Asset pledged as collateral | Assets pledged with the FHLB or FRB: | Available-for-sale securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial instruments, owned, at fair value	3,958	3,897
Asset pledged as collateral | Assets pledged with the FHLB or FRB: | Bank loans
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial instruments, owned, at fair value	10,784	10,166
Had the right to deliver or repledge
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial instruments, owned, at fair value	1,217	1,091
Did not have the right to deliver or repledge
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial instruments, owned, at fair value	$ 65	$ 63