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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 | Sep. 30, 2025 USD ($) contract | Sep. 30, 2024 USD ($) contract
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Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | $ 7,987.4 | $ 7,235.9
Derivative liability, fair value, gross liability | 8,461.7 | 7,144.1
Impact of netting and collateral asset | (7,621.7) | (6,670.6)
Impact of netting and collateral liability | $ (7,594.3) | $ (6,614.6)
Derivative, number of instruments held | contract | 19.2 | 12.2
Not Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | $ 7,977.9 | $ 7,226.3
Derivative liability, fair value, gross liability | 8,460.7 | 7,143.9
Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | 9.5 | 9.6
Derivative liability, fair value, gross liability | 1.0 | 0.2
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 | 107.7 | 336.0
Total fair value included in Receivable from clients, net |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, fair value, net | 58.1 | (8.4)
Total fair value included in Financial instruments owned, at fair value |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, fair value, net | 199.9 | 237.7
Total fair value included in Payables to clients |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, fair value, net | 530.7 | 265.9
Total fair value included in Payables to broker-dealers, clearing organizations and counterparties |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, fair value, net | 38.4 | (1.4)
Fair value included in Financial instruments sold, not yet purchased, at fair value |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, fair value, net | 298.3 | 265.0
Exchange-traded commodity derivatives | Not Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | 2,487.3 | 1,383.1
Derivative liability, fair value, gross liability | 2,528.6 | 1,415.7
OTC commodity derivatives | Not Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | 3,580.7 | 1,967.9
Derivative liability, fair value, gross liability | 3,517.4 | 1,924.3
Exchange-traded foreign exchange derivatives | Not Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | 7.5 | 2.0
Derivative liability, fair value, gross liability | 7.5 | 2.0
OTC foreign exchange derivatives | Not Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | 749.5 | 975.2
Derivative liability, fair value, gross liability | 1,264.0 | 938.2
Exchange-traded interest rate derivatives | Not Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | 361.2 | 720.1
Derivative liability, fair value, gross liability | 367.4 | 728.1
OTC interest rate derivatives | Not Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | 148.3 | 207.1
Derivative liability, fair value, gross liability | 148.3 | 207.1
OTC interest rate derivatives | Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
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 | 413.4 | 1,817.4
Derivative liability, fair value, gross liability | 413.4 | 1,817.4
OTC equity and indices derivatives | Not Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | 195.1 | 127.4
Derivative liability, fair value, gross liability | 138.1 | 86.7
TBA and forward settling securities | Not Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | 34.9 | 26.1
Derivative liability, fair value, gross liability | 76.0 | 24.4
Interest rate swaps | 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
Foreign currency forwards | Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | 9.5 | 9.6
Derivative liability, fair value, gross liability | $ 1.0 | $ 0.0

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