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, 2014	Sep. 30, 2013
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 3,303.9	$ 2,879.4
Derivative Liability, Fair Value, Gross Liability	3,192.5	2,975.4
Derivative Asset, Fair Value, Amount Offset Against Collateral	(3,391.6)	(2,940.4)
Derivative Liability, Fair Value, Amount Offset Against Collateral	(3,135.0)	(2,944.7)
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,380.3	2,036.6
Derivative Liability, Fair Value, Gross Liability	2,220.7	2,046.3
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	546.2	481.4
Derivative Liability, Fair Value, Gross Liability	553.8	484.9
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	70.7	89.3
Derivative Liability, Fair Value, Gross Liability	84.0	104.2
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	239.6	132.3
Derivative Liability, Fair Value, Gross Liability	233.2	162.3
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	6.0	4.3
Derivative Liability, Fair Value, Gross Liability	9.6	36.0
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	61.1	135.5
Derivative Liability, Fair Value, Gross Liability	91.2	141.7
Deposits and Receivables from Exchange Clearing Organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(99.8)	(69.8)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(8.1)	(13.1)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	20.2	21.9
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ 57.5	$ 30.7