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, 2018	Sep. 30, 2017
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 8,035.9	$ 4,321.2
Derivative Liability, Fair Value, Gross Liability	8,511.0	4,201.0
Derivative, Fair Value, Net	193.4
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,455.7	2,094.2
Derivative Liability, Fair Value, Gross Liability	2,499.3	1,975.0
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	207.0	1,084.0
Derivative Liability, Fair Value, Gross Liability	369.9	1,110.3
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	49.8	66.0
Derivative Liability, Fair Value, Gross Liability	37.2	52.0
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	302.5	618.5
Derivative Liability, Fair Value, Gross Liability	303.9	609.8
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	449.3	228.4
Derivative Liability, Fair Value, Gross Liability	478.7	203.6
Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	24.8	0.0
Derivative Liability, Fair Value, Gross Liability	25.9	0.0
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	4,541.8	221.3
Derivative Liability, Fair Value, Gross Liability	4,794.0	245.4
TBA and forward settling securities [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	5.0	8.8
Derivative Liability, Fair Value, Gross Liability	2.1	4.9
TBA securities purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1.2
Derivative Liability, Fair Value, Gross Liability	(0.6)
Derivative Asset, Notional Amount	(721.5)
Derivative Liability, Notional Amount	(293.2)
TBA securities sold [Member] [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	3.2
Derivative Liability, Fair Value, Gross Liability	(1.5)
Derivative Asset, Notional Amount	(1,099.5)
Derivative Liability, Notional Amount	(812.7)
Forward settling securities purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0.5
Derivative Asset, Notional Amount	(614.3)
Forward settling securities sold [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	0.1
Derivative Liability, Notional Amount	(427.2)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(268.7)	(46.4)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	186.1	162.1
Payable to broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	0.0	4.8
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	193.4	317.0
Counterparty And Cash Collateral Netting Adjustment [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Amount Offset Against Collateral	(8,118.5)	(4,205.5)
Derivative Liability, Fair Value, Amount Offset Against Collateral	$ (8,317.6)	$ (3,879.2)