Securities and Commodity Financing Transactions - Underlying Collateral Types of Gross Obligations (Details) - USD ($) $ in Millions	Jun. 30, 2024	Sep. 30, 2023
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 15,997.3	$ 9,196.4
Securities loaned	1,124.0	1,117.3
Gross amount of secured financing	17,121.3	10,313.7
U.S. Treasury obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	9,815.1	3,696.1
U.S. government agency obligations and municipal bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	520.4	542.2
Asset-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	137.7	102.9
Agency mortgage-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	4,556.0	4,371.6
Foreign government obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	488.3	148.1
Corporate bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	479.8	335.5
Equity securities
Assets Sold under Agreements to Repurchase [Line Items]
Securities loaned	$ 1,124.0	$ 1,117.3

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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The value of the collateral (typically cash) that the entity receives in a financing transaction in exchange for securities lent. This item represents a liability in the financial statements of the lender of the securities.

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Amount, after the effects of master netting arrangements, of securities loaned to entities in exchange for collateral. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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