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, 2014	Sep. 30, 2014
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 8,958.5	$ 6,606.8
Derivative Liability, Fair Value, Gross Liability	8,817.9	6,054.5
Derivative Asset, Fair Value, Amount Offset Against Collateral	(9,262.8)	(6,837.5)
Derivative Liability, Fair Value, Amount Offset Against Collateral	(8,737.6)	(5,970.1)
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	6,546.2	3,777.7
Derivative Liability, Fair Value, Gross Liability	6,401.7	3,255.4
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,291.3	1,852.3
Derivative Liability, Fair Value, Gross Liability	1,191.5	1,842.9
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	28.4	93.5
Derivative Liability, Fair Value, Gross Liability	23.3	90.2
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	969.4	808.0
Derivative Liability, Fair Value, Gross Liability	1,010.5	741.8
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	100.7	13.4
Derivative Liability, Fair Value, Gross Liability	115.2	10.2
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	22.5	61.9
Derivative Liability, Fair Value, Gross Liability	75.7	114.0
Deposits and Receivables from Exchange Clearing Organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(334.6)	(273.9)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(5.6)	(1.1)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	35.9	44.3
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ 80.3	$ 84.4