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	Jun. 30, 2013	Sep. 30, 2012
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 3,171.1	$ 4,450.9
Derivative Liability, Fair Value, Gross Liability	3,215.3	4,675.6
Derivative Asset, Fair Value, Amount Offset Against Collateral	(3,200.9)	(4,548.1)
Derivative Liability, Fair Value, Amount Offset Against Collateral	(3,172.7)	(4,631.0)
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,123.0	3,325.6
Derivative Liability, Fair Value, Gross Liability	2,187.1	3,565.3
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	547.8	823.6
Derivative Liability, Fair Value, Gross Liability	568.1	841.4
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	142.1	63.0
Derivative Liability, Fair Value, Gross Liability	129.4	47.7
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	214.7	215.4
Derivative Liability, Fair Value, Gross Liability	178.1	196.6
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	18.5	0.9
Derivative Liability, Fair Value, Gross Liability	7.2	2.6
Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0.1	1.6
Derivative Liability, Fair Value, Gross Liability	0	0
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	124.9	20.8
Derivative Liability, Fair Value, Gross Liability	145.4	22.0
Deposits and receivables from exchange clearing organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(95.1)	(150.4)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	0.3	(0.7)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	65.0	53.9
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ 42.6	$ 44.6