Securities and Commodity Financing Transactions - Underlying Collateral Types of Gross Obligations (Details) - USD ($) $ in Millions	Mar. 31, 2023	Sep. 30, 2022
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 10,332.1	$ 6,133.5
Securities loaned	764.5	1,189.5
Gross amount of secured financing	11,096.6	7,323.0
U.S. Treasury obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	4,777.7	1,311.0
U.S. government agency obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	476.0	604.1
Asset-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	54.8	178.0
Agency mortgage-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	4,651.0	3,762.5
Foreign government obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	124.1	97.2
Corporate bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	248.5	180.7
Equity securities
Assets Sold under Agreements to Repurchase [Line Items]
Securities loaned	$ 764.5	$ 1,189.5

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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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Amount, before effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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