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, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 2,278.6	$ 2,879.4
Derivative Liability, Fair Value, Gross Liability	2,188.4	2,975.4
Derivative Asset, Fair Value, Amount Offset Against Collateral	(2,281.9)	(2,940.4)
Derivative Liability, Fair Value, Amount Offset Against Collateral	(2,154.1)	(2,944.7)
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,430.0	2,036.6
Derivative Liability, Fair Value, Gross Liability	1,311.0	2,046.3
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	411.9	481.4
Derivative Liability, Fair Value, Gross Liability	419.9	484.9
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	75.3	89.3
Derivative Liability, Fair Value, Gross Liability	77.9	104.2
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	287.7	132.3
Derivative Liability, Fair Value, Gross Liability	274.4	162.3
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	8.6	4.3
Derivative Liability, Fair Value, Gross Liability	3.9	36.0
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	65.1	135.5
Derivative Liability, Fair Value, Gross Liability	101.3	141.7
Deposits and receivables from exchange clearing organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(16.8)	(69.8)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(5.1)	(13.1)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	18.6	21.9
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ 34.3	$ 30.7