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, 2015	Sep. 30, 2014
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 7,855.5	$ 6,606.8
Derivative Liability, Fair Value, Gross Liability	7,613.2	6,054.5
Derivative Asset, Fair Value, Amount Offset Against Collateral	(7,698.6)	(6,837.5)
Derivative Liability, Fair Value, Amount Offset Against Collateral	(7,531.2)	(5,970.1)
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	4,985.2	3,777.7
Derivative Liability, Fair Value, Gross Liability	4,709.6	3,255.4
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,252.0	1,852.3
Derivative Liability, Fair Value, Gross Liability	2,227.7	1,842.9
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	30.3	93.5
Derivative Liability, Fair Value, Gross Liability	27.7	90.2
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	456.3	808.0
Derivative Liability, Fair Value, Gross Liability	492.6	741.8
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	102.8	13.4
Derivative Liability, Fair Value, Gross Liability	141.6	10.2
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	28.9	61.9
Derivative Liability, Fair Value, Gross Liability	14.0	114.0
Deposits and Receivables from Exchange Clearing Organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	135.5	(273.9)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(29.8)	(1.1)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	51.2	44.3
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ 82.0	$ 84.4