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, 2020	Sep. 30, 2019
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 3,875.2	$ 3,671.5
Derivative Liability, Fair Value, Gross Liability	3,892.8	3,604.5
Impact of netting and collateral	3,677.8	3,581.9
Impact of netting and collateral	3,785.3	3,540.8
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	105.0	23.1
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	92.4	66.5
Payable to broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	8.8	5.6
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	98.7	58.1
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,949.1	1,437.1
Derivative Liability, Fair Value, Gross Liability	2,085.0	1,463.4
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	359.0	84.2
Derivative Liability, Fair Value, Gross Liability	489.5	106.2
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	11.4	36.9
Derivative Liability, Fair Value, Gross Liability	14.3	33.5
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	525.5	403.2
Derivative Liability, Fair Value, Gross Liability	516.2	368.8
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	852.9	900.1
Derivative Liability, Fair Value, Gross Liability	603.7	882.0
Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	101.1	42.1
Derivative Liability, Fair Value, Gross Liability	100.6	43.6
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	64.0	758.1
Derivative Liability, Fair Value, Gross Liability	70.9	700.2
TBA and forward settling securities [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	12.2	9.8
Derivative Liability, Fair Value, Gross Liability	$ 12.6	$ 6.8

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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