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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - TBAs and Forward Settling Securities (Details) - USD ($) $ in Millions | Jun. 30, 2020 | Sep. 30, 2019
--- | --- | ---
Derivative Asset, Fair Value, Gross Asset | $ 3,875.2 | $ 3,671.5
Derivative Liability, Fair Value, Gross Liability | (3,892.8) | (3,604.5)
TBA securities purchased [Member] |  |
Derivative Liability, Notional Amount | (928.5) | (234.5)
Derivative Asset, Notional Amount | (2,170.9) | (1,778.4)
TBA securities sold [Member] |  |
Derivative Liability, Notional Amount | (2,982.1) | (2,788.0)
Derivative Asset, Notional Amount | (1,566.8) | (451.6)
Forward settling securities purchased [Member] |  |
Derivative Asset, Notional Amount | (1,690.5) | 0.0
Over the Counter (OTC) Commodity Contracts [Member] |  |
Derivative Asset, Notional Amount | 0.0 | (581.2)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | TBA securities purchased [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 7.7 | 3.7
Derivative Liability, Fair Value, Gross Liability | 1.9 | 0.6
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | Forward settling securities purchased [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 1.1 | 0.0
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | Over the Counter (OTC) Commodity Contracts [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 0.0 | 5.2
Payable to broker-dealers, clearing organizations and counterparties [Member] | TBA securities sold [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 3.4 | 0.9
Derivative Liability, Fair Value, Gross Liability | 10.2 | 5.9
Payable to broker-dealers, clearing organizations and counterparties [Member] | Forward settling securities purchased [Member] |  |
Derivative Liability, Notional Amount | 0.0 | (1,243.5)
Derivative Liability, Fair Value, Gross Liability | 0.0 | 0.3
Payable to broker-dealers, clearing organizations and counterparties [Member] | Over the Counter (OTC) Commodity Contracts [Member] |  |
Derivative Liability, Notional Amount | (1,268.3) | 0.0
Derivative Liability, Fair Value, Gross Liability | $ 0.5 | $ 0.0

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