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, 2014	Sep. 30, 2013
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 6,606.8	$ 2,879.4
Derivative Liability, Fair Value, Gross Liability	6,054.5	2,975.4
Derivative Asset, Fair Value, Amount Offset Against Collateral	(6,837.5)	(2,940.4)
Derivative Liability, Fair Value, Amount Offset Against Collateral	(5,970.1)	(2,944.7)
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	3,777.7	2,036.6
Derivative Liability, Fair Value, Gross Liability	3,255.4	2,046.3
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,852.3	481.4
Derivative Liability, Fair Value, Gross Liability	1,842.9	484.9
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	93.5	89.3
Derivative Liability, Fair Value, Gross Liability	90.2	104.2
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	808.0	132.3
Derivative Liability, Fair Value, Gross Liability	741.8	162.3
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	13.4	4.3
Derivative Liability, Fair Value, Gross Liability	10.2	36.0
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	61.9	135.5
Derivative Liability, Fair Value, Gross Liability	114.0	141.7
Deposits and receivables from exchange clearing organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(273.9)	(69.8)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(1.1)	(13.1)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	44.3	21.9
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ 84.4	$ 30.7