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, 2016	Sep. 30, 2015
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 3,907.1	$ 6,135.6
Derivative Liability, Fair Value, Gross Liability	3,895.3	6,010.1
Derivative Asset, Fair Value, Amount Offset Against Collateral	(3,669.1)	(6,081.7)
Derivative Liability, Fair Value, Amount Offset Against Collateral	(3,713.6)	(5,954.4)
Deposits and Receivables from Exchange Clearing Organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	27.8	76.2
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(16.0)	(52.9)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	226.2	30.6
Payable to broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	1.9	1.6
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	179.8	54.1
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,849.6	3,443.6
Derivative Liability, Fair Value, Gross Liability	1,962.8	3,313.8
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,277.8	1,621.2
Derivative Liability, Fair Value, Gross Liability	1,234.1	1,650.7
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	18.9	27.8
Derivative Liability, Fair Value, Gross Liability	25.4	20.6
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	623.2	892.2
Derivative Liability, Fair Value, Gross Liability	510.9	865.4
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	105.1	126.8
Derivative Liability, Fair Value, Gross Liability	120.2	136.0
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	29.5	22.8
Derivative Liability, Fair Value, Gross Liability	37.8	21.0
TBA and forward settling securities [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	3.0	1.2
Derivative Liability, Fair Value, Gross Liability	$ 4.1	$ 2.6

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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-Name Accounting Standards Codification
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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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