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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 | Sep. 30, 2017 | Sep. 30, 2016
--- | --- | ---
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | $ 4,321.2 | $ 3,715.9
Derivative Liability, Fair Value, Gross Liability | 4,201.0 | 3,580.5
Derivative, Fair Value, Net | 317.0 |
Exchange-traded Commodity Contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 2,094.2 | 2,022.1
Derivative Liability, Fair Value, Gross Liability | 1,975.0 | 1,920.5
Over the Counter (OTC) Commodity Contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 1,084.0 | 1,217.0
Derivative Liability, Fair Value, Gross Liability | 1,110.3 | 1,188.9
Foreign Exchange Forward [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 66.0 | 12.2
Derivative Liability, Fair Value, Gross Liability | 52.0 | 7.5
Over the Counter (OTC) Foreign Exchange Contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 618.5 | 346.5
Derivative Liability, Fair Value, Gross Liability | 609.8 | 290.2
Exchange-traded interest rate contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 228.4 | 78.7
Derivative Liability, Fair Value, Gross Liability | 203.6 | 120.5
Equity Contract [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 221.3 | 39.1
Derivative Liability, Fair Value, Gross Liability | 245.4 | 50.3
TBA and forward settling securities [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 8.8 | 0.3
Derivative Liability, Fair Value, Gross Liability | 4.9 | 2.6
TBA securities purchased [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 0.0 |
Derivative Liability, Fair Value, Gross Liability | (2.9) |
Derivative Asset, Notional Amount | (51.3) |
Derivative Liability, Notional Amount | (1,236.8) |
TBA securities sold [Member] [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 5.8 |
Derivative Liability, Fair Value, Gross Liability | (0.1) |
Derivative Asset, Notional Amount | (1,881.9) |
Derivative Liability, Notional Amount | (404.1) |
Forward settling securities purchased [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | (2.0) |
Derivative Asset, Notional Amount | (882.9) |
Forward settling securities sold [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Liability, Fair Value, Gross Liability | 3.0 |
Derivative Liability, Notional Amount | (590.2) |
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | (46.4) | (131.5)
Financial instruments owned [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | 162.1 | 193.9
Payable to broker-dealers, clearing organizations and counterparties [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | 4.8 | 3.5
Financial instrument sold, not yet purchased [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | 317.0 | 210.9
Counterparty And Cash Collateral Netting Adjustment [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Amount Offset Against Collateral | (4,205.5) | (3,653.5)
Derivative Liability, Fair Value, Amount Offset Against Collateral | $ (3,879.2) | $ (3,366.1)

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