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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 | Dec. 31, 2019 | Sep. 30, 2019
--- | --- | ---
Derivative Asset, Fair Value, Gross Asset | $ 2,115.1 | $ 3,671.5
Derivative Liability, Fair Value, Gross Liability | 2,091.6 | 3,604.5
TBA securities purchased [Member] |  |
Derivative Liability, Notional Amount | 174.1 | 234.5
Derivative Asset, Notional Amount | (1,283.0) | (1,778.4)
TBA securities sold [Member] |  |
Derivative Liability, Notional Amount | 2,182.9 |
Derivative Asset, Notional Amount | (280.1) |
Over the Counter (OTC) Commodity Contracts [Member] |  |
Derivative Liability, Notional Amount | 406.7 | 1,243.5
Derivative Asset, Notional Amount | (294.4) | (581.2)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | TBA securities purchased [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 2.5 | 3.7
Derivative Liability, Fair Value, Gross Liability | (0.1) | (0.6)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | Over the Counter (OTC) Commodity Contracts [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 3.0 | 5.2
Payable to broker-dealers, clearing organizations and counterparties [Member] | TBA securities sold [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 0.4 | 0.9
Derivative Liability, Fair Value, Gross Liability | (3.7) | (5.9)
Payable to broker-dealers, clearing organizations and counterparties [Member] | Over the Counter (OTC) Commodity Contracts [Member] |  |
Derivative Liability, Fair Value, Gross Liability | $ (0.6) | $ (0.3)

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