Securities and Commodity Financing Transactions - Underlying Collateral Types of Gross Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2024	Sep. 30, 2024
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 17,016.7	$ 16,527.9
Securities loaned	1,826.5	1,615.9
Gross amount of secured financing	18,843.2	18,143.8
U.S. Treasury obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	9,287.3	9,673.7
U.S. government agency obligations and municipal bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	459.3	652.0
Asset-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	220.8	136.1
Agency mortgage-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	5,479.2	5,079.6
Foreign government obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	969.0	649.6
Corporate bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	601.1	336.9
Equity securities
Assets Sold under Agreements to Repurchase [Line Items]
Securities loaned	$ 1,826.5	$ 1,615.9

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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