Securities and Commodity Financing Transactions - Collateral Maturities of Gross Obligations (Details) - USD ($) $ in Millions	Jun. 30, 2021	Sep. 30, 2020
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 3,627.6	$ 3,155.5
Securities loaned	1,700.3	1,441.9
Gross amount of secured financing	5,327.9	4,597.4
Overnight and Open
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	2,348.1	1,736.3
Securities loaned	1,700.3	1,441.9
Gross amount of secured financing	4,048.4	3,178.2
Less than 30 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	1,086.2	1,069.2
Securities loaned	0.0	0.0
Gross amount of secured financing	1,086.2	1,069.2
30-90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	189.1	325.0
Securities loaned	0.0	0.0
Gross amount of secured financing	189.1	325.0
Over 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	4.2	25.0
Securities loaned	0.0	0.0
Gross amount of secured financing	$ 4.2	$ 25.0

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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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