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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, 2018 | Sep. 30, 2017
--- | --- | ---
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | $ 8,035.9 | $ 4,321.2
Derivative Liability, Fair Value, Gross Liability | 8,511.0 | 4,201.0
Derivative, Fair Value, Net | 193.4 |
Exchange-traded Commodity Contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 2,455.7 | 2,094.2
Derivative Liability, Fair Value, Gross Liability | 2,499.3 | 1,975.0
Over the Counter (OTC) Commodity Contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 207.0 | 1,084.0
Derivative Liability, Fair Value, Gross Liability | 369.9 | 1,110.3
Foreign Exchange Forward [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 49.8 | 66.0
Derivative Liability, Fair Value, Gross Liability | 37.2 | 52.0
Over the Counter (OTC) Foreign Exchange Contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 302.5 | 618.5
Derivative Liability, Fair Value, Gross Liability | 303.9 | 609.8
Exchange-traded interest rate contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 449.3 | 228.4
Derivative Liability, Fair Value, Gross Liability | 478.7 | 203.6
Interest Rate Contract [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 24.8 | 0.0
Derivative Liability, Fair Value, Gross Liability | 25.9 | 0.0
Equity Contract [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 4,541.8 | 221.3
Derivative Liability, Fair Value, Gross Liability | 4,794.0 | 245.4
TBA and forward settling securities [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 5.0 | 8.8
Derivative Liability, Fair Value, Gross Liability | 2.1 | 4.9
TBA securities purchased [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 1.2 |
Derivative Liability, Fair Value, Gross Liability | (0.6) |
Derivative Asset, Notional Amount | (721.5) |
Derivative Liability, Notional Amount | (293.2) |
TBA securities sold [Member] [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 3.2 |
Derivative Liability, Fair Value, Gross Liability | (1.5) |
Derivative Asset, Notional Amount | (1,099.5) |
Derivative Liability, Notional Amount | (812.7) |
Forward settling securities purchased [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 0.5 |
Derivative Asset, Notional Amount | (614.3) |
Forward settling securities sold [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Liability, Fair Value, Gross Liability | 0.1 |
Derivative Liability, Notional Amount | (427.2) |
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | (268.7) | (46.4)
Financial instruments owned [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | 186.1 | 162.1
Payable to broker-dealers, clearing organizations and counterparties [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | 0.0 | 4.8
Financial instrument sold, not yet purchased [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | 193.4 | 317.0
Counterparty And Cash Collateral Netting Adjustment [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Amount Offset Against Collateral | (8,118.5) | (4,205.5)
Derivative Liability, Fair Value, Amount Offset Against Collateral | $ (8,317.6) | $ (3,879.2)

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