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	Jun. 30, 2015	Sep. 30, 2014
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 4,837.8	$ 6,606.8
Derivative Liability, Fair Value, Gross Liability	5,163.9	6,054.5
Derivative Asset, Fair Value, Amount Offset Against Collateral	(4,580.3)	(6,837.5)
Derivative Liability, Fair Value, Amount Offset Against Collateral	(5,111.3)	(5,970.1)
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,929.1	3,777.7
Derivative Liability, Fair Value, Gross Liability	3,116.0	3,255.4
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,264.8	1,852.3
Derivative Liability, Fair Value, Gross Liability	1,361.3	1,842.9
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	23.4	93.5
Derivative Liability, Fair Value, Gross Liability	20.5	90.2
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	507.6	808.0
Derivative Liability, Fair Value, Gross Liability	489.1	741.8
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	92.4	13.4
Derivative Liability, Fair Value, Gross Liability	118.2	10.2
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	20.5	61.9
Derivative Liability, Fair Value, Gross Liability	58.8	114.0
Deposits and Receivables from Exchange Clearing Organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	234.9	(273.9)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(17.5)	(1.1)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	40.1	44.3
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ 52.6	$ 84.4

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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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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