Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk 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, 2012	Sep. 30, 2012
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 2,748.8	$ 4,450.9
Derivative Liability, Fair Value, Gross Liability	2,493.5	4,675.6
Derivative Asset, Fair Value, Amount Offset Against Collateral	(2,703.0)	(4,548.1)
Derivative Liability, Fair Value, Amount Offset Against Collateral	(2,456.7)	(4,631.0)
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,919.2	3,325.6
Derivative Liability, Fair Value, Gross Liability	1,665.3	3,565.3
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	454.7	823.6
Derivative Liability, Fair Value, Gross Liability	472.1	841.4
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	46.5	63.0
Derivative Liability, Fair Value, Gross Liability	40.6	47.7
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	309.2	215.4
Derivative Liability, Fair Value, Gross Liability	289.9	196.6
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1.2	0.9
Derivative Liability, Fair Value, Gross Liability	4.4	2.6
Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1.1	1.6
Derivative Liability, Fair Value, Gross Liability	0	0
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	16.9	20.8
Derivative Liability, Fair Value, Gross Liability	21.2	22.0
Deposits and receivables from exchange clearing organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(9.3)	(150.4)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	4.8	(0.7)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	50.3	53.9
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ 36.8	$ 44.6