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	Sep. 30, 2013	Sep. 30, 2012
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 2,879.4	$ 4,450.9
Derivative Liability, Fair Value, Gross Liability	2,975.4	4,675.6
Derivative Asset, Fair Value, Amount Offset Against Collateral	(2,940.4)	(4,548.1)
Derivative Liability, Fair Value, Amount Offset Against Collateral	(2,944.7)	(4,631.0)
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,036.6	3,325.6
Derivative Liability, Fair Value, Gross Liability	2,046.3	3,565.3
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	481.4	823.6
Derivative Liability, Fair Value, Gross Liability	484.9	841.4
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	89.3	63.0
Derivative Liability, Fair Value, Gross Liability	104.2	47.7
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	132.3	215.4
Derivative Liability, Fair Value, Gross Liability	162.3	196.6
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	4.3	0.9
Derivative Liability, Fair Value, Gross Liability	36.0	2.6
Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	1.6
Derivative Liability, Fair Value, Gross Liability	0	0
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	135.5	20.8
Derivative Liability, Fair Value, Gross Liability	141.7	22.0
Deposits and receivables from exchange clearing organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(69.8)	(150.4)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(13.1)	(0.7)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	21.9	53.9
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ 30.7	$ 44.6