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, 2012	Sep. 30, 2011
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 4,450.9	$ 9,116.3
Derivative Liability, Fair Value, Gross Liability	4,675.6	8,133.7
Derivative Asset, Fair Value, Amount Offset Against Collateral	(4,548.1)	(9,262.6)
Derivative Liability, Fair Value, Amount Offset Against Collateral	(4,631.0)	(8,010.8)
Derivative, Collateral, Right to Reclaim Cash	0	53.5
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	3,325.6	7,074.2
Derivative Liability, Fair Value, Gross Liability	3,565.3	6,062.4
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	823.6	763.7
Derivative Liability, Fair Value, Gross Liability	841.4	780.1
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	63.0	126.9
Derivative Liability, Fair Value, Gross Liability	47.7	89.8
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	215.4	1,074.3
Derivative Liability, Fair Value, Gross Liability	196.6	1,118.9
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0.9	2.3
Derivative Liability, Fair Value, Gross Liability	2.6	2.8
Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1.6	3.2
Derivative Liability, Fair Value, Gross Liability	0	0
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	20.8	71.7
Derivative Liability, Fair Value, Gross Liability	22.0	79.7
Deposits and receivables from exchange clearing organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(150.4)	(264.3)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(0.7)	16.1
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	53.9	101.9
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ 44.6	$ 122.9