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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, 2019 | Sep. 30, 2018
--- | --- | ---
Derivative Asset, Fair Value, Gross Asset | $ 4,044.2 | $ 8,035.9
Derivative Liability, Fair Value, Gross Liability | 4,019.0 | 8,511.0
TBA securities purchased [Member] |  |
Derivative Liability, Notional Amount | (451.8) | (293.2)
Derivative Asset, Notional Amount | (1,397.6) | (721.5)
TBA securities sold [Member] |  |
Derivative Liability, Notional Amount | (2,260.8) | (812.7)
Derivative Asset, Notional Amount | (425.4) | (1,099.5)
Forward settling securities purchased [Member] |  |
Derivative Asset, Notional Amount | (1,258.1) | (614.3)
Over the Counter (OTC) Commodity Contracts [Member] |  |
Derivative Liability, Notional Amount | (1,172.7) | 0.0
Derivative Asset, Notional Amount | 0.0 | (427.2)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | TBA securities purchased [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 7.1 | 1.2
Derivative Liability, Fair Value, Gross Liability | (0.4) | (0.6)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | TBA securities sold [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 0.0 | 3.2
Derivative Liability, Fair Value, Gross Liability | 0.0 | (1.5)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | Forward settling securities purchased [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 4.5 | 0.5
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 | 0.1
Payable to broker-dealers, clearing organizations and counterparties [Member] | TBA securities sold [Member] |  |
Derivative Asset, Fair Value, Gross Asset | 0.5 | 0.0
Derivative Liability, Fair Value, Gross Liability | (13.7) | 0.0
Payable to broker-dealers, clearing organizations and counterparties [Member] | Over the Counter (OTC) Commodity Contracts [Member] |  |
Derivative Liability, Fair Value, Gross Liability | $ (2.6) | $ 0.0

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