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	Mar. 31, 2014	Sep. 30, 2013
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 3,363.1	$ 2,879.4
Derivative Liability, Fair Value, Gross Liability	3,819.2	2,975.4
Derivative Asset, Fair Value, Amount Offset Against Collateral	(3,348.8)	(2,940.4)
Derivative Liability, Fair Value, Amount Offset Against Collateral	(3,754.3)	(2,944.7)
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,121.6	2,036.6
Derivative Liability, Fair Value, Gross Liability	2,535.8	2,046.3
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	844.8	481.4
Derivative Liability, Fair Value, Gross Liability	849.0	484.9
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	74.7	89.3
Derivative Liability, Fair Value, Gross Liability	70.3	104.2
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	238.6	132.3
Derivative Liability, Fair Value, Gross Liability	243.4	162.3
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	8.1	4.3
Derivative Liability, Fair Value, Gross Liability	6.8	36.0
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	75.3	135.5
Derivative Liability, Fair Value, Gross Liability	113.9	141.7
Deposits and Receivables from Exchange Clearing Organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(6.1)	(69.8)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(5.2)	(13.1)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	25.6	21.9
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ 64.9	$ 30.7