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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, 2018 | Sep. 30, 2017
--- | --- | ---
Derivative Asset, Fair Value, Gross Asset | $ 7,634.7 | $ 4,321.2
Derivative Liability, Fair Value, Gross Liability | 7,677.1 | 4,201.0
TBA securities purchased [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 2.9 | 0.0
Derivative Liability, Notional Amount | (38.7) | (1,236.8)
Derivative Liability, Fair Value, Gross Liability | 0.0 | (2.9)
Derivative Asset, Notional Amount | 829.5 | 51.3
TBA securities sold [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 0.1 | 5.8
Derivative Liability, Notional Amount | (1,838.2) | (404.1)
Derivative Liability, Fair Value, Gross Liability | (6.9) | (0.1)
Derivative Asset, Notional Amount | 90.7 | 1,881.9
Forward settling securities purchased [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 1.4 | (2.0)
Derivative Asset, Notional Amount | 563.8 | 882.9
Over the Counter (OTC) Commodity Contracts [Member] |  |
Derivative Liability, Notional Amount | (245.5) | (590.2)
Derivative Liability, Fair Value, Gross Liability | $ (0.3) | $ 3.0

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