Commodity and Other Repurchase Agreements and Collateralized Transactions Gross Financings Collateral Maturities Table (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 2,800.3	$ 2,773.7
Securities Loaned	1,068.8	1,459.9
Collateralized transactions:	3,869.1	4,233.6
Maturity Overnight and on Demand [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	1,535.1	1,553.9
Securities Loaned	1,068.8	1,459.9
Collateralized transactions:	2,603.9	3,013.8
Maturity Less than 30 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	737.2	565.8
Securities Loaned	0.0	0.0
Collateralized transactions:	737.2	565.8
Maturity 30 to 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	483.0	654.0
Securities Loaned	0.0	0.0
Collateralized transactions:	483.0	654.0
Maturity Greater than 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	45.0	0.0
Securities Loaned	0.0	0.0
Collateralized transactions:	$ 45.0	$ 0.0

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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Amount, after the 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 not subject to a master netting arrangement and not elected to be offset.

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