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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Schedule of Derivative Assets and Liabilities (Details) contract in Millions, $ in Millions	Dec. 31, 2024 USD ($) contract	Sep. 30, 2024 USD ($) contract
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 7,883.5	$ 7,235.9
Derivative liability, fair value, gross liability	7,724.8	7,144.1
Impact of netting and collateral asset	(7,136.0)	(6,670.6)
Impact of netting and collateral liability	$ (7,026.4)	$ (6,614.6)
Derivative, number of instruments held | contract	11.3	12.2
Total fair value included in Deposits with and receivables from broker-dealers, clearing organizations, and counterparties, net
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ 523.1	$ 336.0
Total fair value included in Receivable from clients, net
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(2.8)	(8.4)
Total fair value included in Financial instruments owned, at fair value
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	227.2	237.7
Total fair value included in Payables to clients
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	371.1	265.9
Total fair value included in Payables to broker-dealers, clearing organizations and counterparties
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	37.5	(1.4)
Total fair value included in Financial instruments sold, not yet purchased, at fair value
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	289.8	265.0
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	7,882.3	7,226.3
Derivative liability, fair value, gross liability	7,723.9	7,143.9
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1.2	9.6
Derivative liability, fair value, gross liability	0.9	0.2
Exchange-traded commodity derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1,303.8	1,383.1
Derivative liability, fair value, gross liability	1,281.3	1,415.7
OTC commodity derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	2,919.4	1,967.9
Derivative liability, fair value, gross liability	2,817.1	1,924.3
Exchange-traded foreign exchange derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	(1.7)	2.0
Derivative liability, fair value, gross liability	2.1	2.0
OTC foreign exchange derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	815.1	975.2
Derivative liability, fair value, gross liability	858.2	938.2
Exchange-traded interest rate derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	587.8	720.1
Derivative liability, fair value, gross liability	590.9	728.1
OTC interest rate derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	220.4	207.1
Derivative liability, fair value, gross liability	220.4	207.1
OTC interest rate derivatives | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	0.0	0.0
Derivative liability, fair value, gross liability	0.0	0.2
Exchange-traded equity index derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1,810.0	1,817.4
Derivative liability, fair value, gross liability	1,810.0	1,817.4
OTC equity and indices derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	153.5	127.4
Derivative liability, fair value, gross liability	113.0	86.7
TBA and forward settling securities | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	74.0	26.1
Derivative liability, fair value, gross liability	30.9	24.4
Foreign currency forward contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1.2	9.6
Derivative liability, fair value, gross liability	$ 0.9	$ 0.0

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Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, and obligation to return cash collateral under master netting arrangements.

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