Securities and Commodity Financing Transactions - Schedule of Fair Values of Collateral Pledged, Received and Repledged (Details) - USD ($) $ in Millions	Jun. 30, 2026	Sep. 30, 2025
Assets Sold under Agreements to Repurchase [Line Items]
Securities pledged or repledged to cover collateral requirements for tri-party arrangements	$ 11,135.0	$ 8,604.4
Securities received as collateral that may be repledged	38,337.5	32,452.0
Securities received as collateral that are subject to segregation rules	3,791.4	1,739.3
Securities received as collateral that may be repledged covering securities sold short	3,379.7	1,965.1
Asset Pledged as Collateral with Right
Assets Sold under Agreements to Repurchase [Line Items]
Securities received as collateral that are subject to segregation rules	468.5	0.0
Repledged securities borrowed and client securities held under custodial clearing arrangements to collateralize securities loaned agreements	2,936.7	2,491.4
Tri-party Repurchase Agreements
Assets Sold under Agreements to Repurchase [Line Items]
Securities pledged or repledged to cover collateral requirements for tri-party arrangements	$ 12,456.2	$ 9,757.7

Fair Value Of Securities Received As Collateral That Segregated Resold Or Repledged

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Fair value of collateral permitted, by contact or custom, to be sold or re-pledged.

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Fair value of the collateral, or portion thereof, that has been re-pledged, delivered, or otherwise used.

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The aggregate fair value as of the balance sheet date of financial instruments and other positions owned by the entity including: (1) mortgages, mortgage-backed and asset backed securities; (2) US government and agency obligations; (3) state and municipal government obligations; (4) other sovereign government debt; (5) corporate obligations; (6) corporate equities; (7) principal investments; (8) derivative contracts; and (9) physical commodities. Includes both pledged and unpledged holdings.

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Fair value of the securities pledged as collateral against securities loaned.

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