COLLATERALIZED AGREEMENTS AND FINANCINGS, Encumbered Assets (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
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	$ 33,449	$ 15,773
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	2,435	3,979
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	31,014	11,794
Had the right to deliver or repledge
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial instruments, owned, at fair value	1,265	1,281
Did not have the right to deliver or repledge
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial instruments, owned, at fair value	$ 66	$ 66