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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Gross Derivative Assets and Liabilities by Type and Balance Sheet Location (Details) - USD ($) $ in Millions | Mar. 31, 2017 | Sep. 30, 2016
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Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | $ 3,579.8 | $ 3,715.9
Derivative Liability, Fair Value, Gross Liability | 3,511.5 | 3,580.5
Derivative Asset, Fair Value, Amount Offset Against Collateral | (3,368.0) | (3,653.5)
Derivative Liability, Fair Value, Amount Offset Against Collateral | (3,272.1) | (3,366.1)
Deposits and Receivables from Exchange Clearing Organizations [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | (21.3) | (116.3)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | 34.7 | (15.2)
Financial instruments owned [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | 198.4 | 193.9
Payable to broker-dealers, clearing organizations and counterparties [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | 9.8 | 3.5
Financial instrument sold, not yet purchased [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | 229.6 | 210.9
Exchange-traded Commodity Contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 1,617.7 | 2,022.1
Derivative Liability, Fair Value, Gross Liability | 1,496.9 | 1,920.5
Over the Counter (OTC) Commodity Contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 1,336.7 | 1,217.0
Derivative Liability, Fair Value, Gross Liability | 1,518.4 | 1,188.9
Foreign Exchange Forward [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 22.6 | 12.2
Derivative Liability, Fair Value, Gross Liability | 21.0 | 7.5
Over the Counter (OTC) Foreign Exchange Contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 434.8 | 346.5
Derivative Liability, Fair Value, Gross Liability | 317.8 | 290.2
Exchange-traded interest rate contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 132.9 | 78.7
Derivative Liability, Fair Value, Gross Liability | 113.9 | 120.5
Equity Contract [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 28.9 | 39.1
Derivative Liability, Fair Value, Gross Liability | 32.7 | 50.3
TBA and forward settling securities [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 6.2 | 0.3
Derivative Liability, Fair Value, Gross Liability | $ 10.8 | $ 2.6

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