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	Jun. 30, 2019	Sep. 30, 2018
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 4,413.7	$ 8,035.9
Derivative Liability, Fair Value, Gross Liability	4,725.7	8,511.0
Impact of netting and collateral	4,281.8	8,118.5
Impact of netting and collateral	4,512.7	8,317.6
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(114.3)	(268.7)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	246.2	186.1
Payable to broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	14.0	0.0
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	199.0	193.4
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,577.7	2,455.7
Derivative Liability, Fair Value, Gross Liability	1,610.4	2,499.3
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	241.9	207.0
Derivative Liability, Fair Value, Gross Liability	380.8	369.9
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	76.9	49.8
Derivative Liability, Fair Value, Gross Liability	73.1	37.2
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	334.2	302.5
Derivative Liability, Fair Value, Gross Liability	308.7	303.9
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,200.7	449.3
Derivative Liability, Fair Value, Gross Liability	1,417.6	478.7
Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	20.4	24.8
Derivative Liability, Fair Value, Gross Liability	22.5	25.9
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	955.8	4,541.8
Derivative Liability, Fair Value, Gross Liability	904.2	4,794.0
TBA and forward settling securities [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	6.1	5.0
Derivative Liability, Fair Value, Gross Liability	$ 8.4	$ 2.1

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, and obligation to return cash collateral under master netting arrangements.

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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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