COLLATERALIZED AGREEMENTS AND FINANCINGS, Encumbered Assets (Details) - USD ($) $ in Millions	Dec. 31, 2025	Sep. 30, 2025
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	$ 34,299	$ 33,449
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	2,363	2,435
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	31,936	31,014
Had the right to deliver or repledge
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial instruments, owned, at fair value	1,287	1,265
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