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, 2017	Sep. 30, 2016
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 4,321.2	$ 3,715.9
Derivative Liability, Fair Value, Gross Liability	4,201.0	3,580.5
Derivative, Fair Value, Net	317.0
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,094.2	2,022.1
Derivative Liability, Fair Value, Gross Liability	1,975.0	1,920.5
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,084.0	1,217.0
Derivative Liability, Fair Value, Gross Liability	1,110.3	1,188.9
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	66.0	12.2
Derivative Liability, Fair Value, Gross Liability	52.0	7.5
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	618.5	346.5
Derivative Liability, Fair Value, Gross Liability	609.8	290.2
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	228.4	78.7
Derivative Liability, Fair Value, Gross Liability	203.6	120.5
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	221.3	39.1
Derivative Liability, Fair Value, Gross Liability	245.4	50.3
TBA and forward settling securities [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	8.8	0.3
Derivative Liability, Fair Value, Gross Liability	4.9	2.6
TBA securities purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0.0
Derivative Liability, Fair Value, Gross Liability	(2.9)
Derivative Asset, Notional Amount	(51.3)
Derivative Liability, Notional Amount	(1,236.8)
TBA securities sold [Member] [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	5.8
Derivative Liability, Fair Value, Gross Liability	(0.1)
Derivative Asset, Notional Amount	(1,881.9)
Derivative Liability, Notional Amount	(404.1)
Forward settling securities purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	(2.0)
Derivative Asset, Notional Amount	(882.9)
Forward settling securities sold [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	3.0
Derivative Liability, Notional Amount	(590.2)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(46.4)	(131.5)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	162.1	193.9
Payable to broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	4.8	3.5
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	317.0	210.9
Counterparty And Cash Collateral Netting Adjustment [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Amount Offset Against Collateral	(4,205.5)	(3,653.5)
Derivative Liability, Fair Value, Amount Offset Against Collateral	$ (3,879.2)	$ (3,366.1)