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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 | Mar. 31, 2017 | Sep. 30, 2016
--- | --- | ---
Derivative Asset, Fair Value, Gross Asset | $ 3,579.8 | $ 3,715.9
Derivative Liability, Fair Value, Gross Liability | 3,511.5 | 3,580.5
TBA securities purchased [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 3.2 | 0.8
Derivative Liability, Notional Amount | (180.4) | (485.5)
Derivative Liability, Fair Value, Gross Liability | (0.4) | (0.8)
Derivative Asset, Notional Amount | (795.7) | (289.8)
TBA securities sold [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 0.6 | 1.3
Derivative Liability, Notional Amount | (1,537.9) | (754.3)
Derivative Liability, Fair Value, Gross Liability | (9.6) | (1.7)
Derivative Asset, Notional Amount | (351.4) | (702.3)
Forward settling securities purchased [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 2.5 | 0.1
Derivative Asset, Notional Amount | (595.5) | (607.9)
Over the Counter (OTC) Commodity Contracts [Member] |  |
Derivative Liability, Notional Amount | (301.4) | (470.4)
Derivative Liability, Fair Value, Gross Liability | $ (0.9) | $ 0.2

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