Securities and Commodity Financing Transactions - Schedule of Carrying Value of Repurchase Agreements and Securities Lending Agreements by Class of Collateral Pledged (Details) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 16,527.9	$ 9,196.4
Securities loaned	1,615.9	1,117.3
Gross amount of secured financing	18,143.8	10,313.7
U.S. Treasury obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	9,673.7	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	652.0	542.2
Asset-based obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	136.1	102.9
Agency mortgage-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	5,079.6	4,371.6
Foreign government obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	649.6	148.1
Corporate bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	336.9	335.5
Equity securities
Assets Sold under Agreements to Repurchase [Line Items]
Securities loaned	$ 1,615.9	$ 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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