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	Sep. 30, 2016	Sep. 30, 2015
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 3,715.9	$ 6,135.6
Derivative Liability, Fair Value, Gross Liability	3,580.5	6,010.1
Derivative, Fair Value, Net	210.9
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,022.1	3,443.6
Derivative Liability, Fair Value, Gross Liability	1,920.5	3,313.8
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,217.0	1,621.2
Derivative Liability, Fair Value, Gross Liability	1,188.9	1,650.7
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	12.2	27.8
Derivative Liability, Fair Value, Gross Liability	7.5	20.6
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	346.5	892.2
Derivative Liability, Fair Value, Gross Liability	290.2	865.4
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	78.7	126.8
Derivative Liability, Fair Value, Gross Liability	120.5	136.0
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	39.1	22.8
Derivative Liability, Fair Value, Gross Liability	50.3	21.0
TBA and forward settling securities [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0.3	1.2
Derivative Liability, Fair Value, Gross Liability	2.6	2.6
TBA securities purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0.8
Derivative Liability, Fair Value, Gross Liability	(0.8)
Derivative Asset, Notional Amount	(289.8)
Derivative Liability, Notional Amount	(485.5)
TBA securities sold [Member] [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1.3
Derivative Liability, Fair Value, Gross Liability	(1.7)
Derivative Asset, Notional Amount	(702.3)
Derivative Liability, Notional Amount	(754.3)
Forward settling securities purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0.1
Derivative Asset, Notional Amount	(607.9)
Forward settling securities sold [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	0.2
Derivative Liability, Notional Amount	(470.4)
Deposits and receivables from exchange clearing organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(116.3)	76.2
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(15.2)	(52.9)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	193.9	30.6
Payable to broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	3.5	1.6
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	210.9	54.1
Counterparty And Cash Collateral Netting Adjustment [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Amount Offset Against Collateral	(3,653.5)	(6,081.7)
Derivative Liability, Fair Value, Amount Offset Against Collateral	$ (3,366.1)	$ (5,954.4)