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, 2019	Sep. 30, 2018
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 3,671.5	$ 8,035.9
Derivative Liability, Fair Value, Gross Liability	3,604.5	8,511.0
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(3,581.9)	(8,118.5)
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	(3,540.8)	(8,317.6)
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,437.1	2,455.7
Derivative Liability, Fair Value, Gross Liability	1,463.4	2,499.3
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	84.2	207.0
Derivative Liability, Fair Value, Gross Liability	106.2	369.9
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	36.9	49.8
Derivative Liability, Fair Value, Gross Liability	33.5	37.2
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	403.2	302.5
Derivative Liability, Fair Value, Gross Liability	368.8	303.9
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	900.1	449.3
Derivative Liability, Fair Value, Gross Liability	882.0	478.7
Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	42.1	24.8
Derivative Liability, Fair Value, Gross Liability	43.6	25.9
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	758.1	4,541.8
Derivative Liability, Fair Value, Gross Liability	700.2	4,794.0
TBA and forward settling securities [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	9.8	5.0
Derivative Liability, Fair Value, Gross Liability	6.8	2.1
Deposits and receivables from exchange clearing organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	23.1	(268.7)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | TBA securities purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	3.7	1.2
Derivative Liability, Fair Value, Gross Liability	(0.6)	(0.6)
Derivative Asset, Notional Amount	(1,778.4)	(721.5)
Derivative Liability, Notional Amount	(234.5)	(293.2)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | TBA securities sold [Member] [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0.0	3.2
Derivative Liability, Fair Value, Gross Liability	0.0	(1.5)
Derivative Asset, Notional Amount	0.0	(1,099.5)
Derivative Liability, Notional Amount	0.0	(812.7)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | Forward settling securities purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0.0	0.5
Derivative Asset, Notional Amount	0.0	(614.3)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | Forward settling securities sold [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	5.2	0.1
Derivative Asset, Notional Amount	(581.2)	(427.2)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	66.5	186.1
Payable to broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	5.6	0.0
Payable to broker-dealers, clearing organizations and counterparties [Member] | TBA securities sold [Member] [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0.9	0.0
Derivative Liability, Fair Value, Gross Liability	(5.9)	0.0
Derivative Asset, Notional Amount	(451.6)	0.0
Derivative Liability, Notional Amount	(2,788.0)	0.0
Payable to broker-dealers, clearing organizations and counterparties [Member] | Forward settling securities sold [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(0.3)	0.0
Derivative Liability, Notional Amount	(1,243.5)	0.0
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ 58.1	$ 193.4