Securities and Commodity Financing Transactions - Collateral Maturities of Gross Obligations (Details) - USD ($) $ in Millions	Mar. 31, 2024	Sep. 30, 2023
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 12,075.0	$ 9,196.4
Securities loaned	1,458.6	1,117.3
Gross amount of secured financing	13,533.6	10,313.7
Overnight and Open
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	10,183.8	8,300.0
Securities loaned	1,458.6	1,117.3
Gross amount of secured financing	11,642.4	9,417.3
Less than 30 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	1,183.1	786.8
Securities loaned	0.0	0.0
Gross amount of secured financing	1,183.1	786.8
30-90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	610.6	107.0
Securities loaned	0.0	0.0
Gross amount of secured financing	610.6	107.0
Over 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	97.5	2.6
Securities loaned	0.0	0.0
Gross amount of secured financing	$ 97.5	$ 2.6

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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