Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk Level 4 - Gross Derivative Assets and Liabilities by Type and Balance Sheet Location (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Sep. 30, 2011
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 4,495.9	$ 9,116.3
Derivative Liability, Fair Value, Gross Liability	4,943.5	8,133.7
Derivative Asset, Fair Value, Amount Offset Against Collateral	(4,685.6)	(9,262.6)
Derivative Liability, Fair Value, Amount Offset Against Collateral	(4,900.4)	(8,010.8)
Derivative, Collateral, Right to Reclaim Cash	34.8	53.5
Deposits and receivables from exchange clearing organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(238.5)	(264.3)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	1.0	16.1
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	47.8	101.9
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	43.1	122.9
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	3,164.4	7,074.2
Derivative Liability, Fair Value, Gross Liability	3,594.6	6,062.4
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	818.7	763.7
Derivative Liability, Fair Value, Gross Liability	813.3	780.1
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	73.6	126.9
Derivative Liability, Fair Value, Gross Liability	54.2	89.8
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	423.2	1,074.3
Derivative Liability, Fair Value, Gross Liability	462.4	1,118.9
Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	3.0	5.5
Derivative Liability, Fair Value, Gross Liability	1.6	2.8
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	13.0	71.7
Derivative Liability, Fair Value, Gross Liability	$ 17.4	$ 79.7