Securities and Commodity Financing Transactions - Underlying Collateral Types of Gross Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2020
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 3,635.3	$ 3,155.5
Securities loaned	1,237.8	1,441.9
Collateralized Financings, Total	4,873.1	4,597.4
U.S. Treasury obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	932.5	815.8
U.S. government agency obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	223.7	279.5
Asset-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	26.1	18.0
Agency mortgage-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	2,357.6	1,990.0
Corporate bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	95.4	52.2
Equity securities
Assets Sold under Agreements to Repurchase [Line Items]
Securities loaned	$ 1,237.8	$ 1,441.9

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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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