Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Gross Derivative Assets and Liabilities by Type and Balance Sheet Location (Details) contract in Millions, $ in Millions	Mar. 31, 2022 USD ($) contract	Sep. 30, 2021 USD ($) contract
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 12,036.2	$ 5,760.7
Derivative liability, fair value, gross liability	11,843.3	5,628.1
Impact of netting and collateral asset	(11,962.5)	(5,353.9)
Impact of netting and collateral liability	$ (11,853.0)	$ (4,955.4)
Derivative, number of instruments held | contract	12.6	11.1
Deposits and Receivables from Exchange Clearing Organizations
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ (83.1)	$ 259.9
Receivable From Clients, Net
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(15.5)	2.6
Financial Instruments Owned
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	172.3	144.3
Payables to Clients
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(546.1)	291.5
Payables to Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	52.0	12.7
Financial Instrument Sold, Not Yet Purchased
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	484.4	368.5
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	12,036.2	5,760.7
Derivative liability, fair value, gross liability	11,819.0	5,628.1
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	0.0	0.0
Derivative liability, fair value, gross liability	24.3	0.0
Exchange-traded commodity derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	5,901.8	3,904.1
Derivative liability, fair value, gross liability	5,911.7	3,904.7
OTC commodity derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1,276.5	803.4
Derivative liability, fair value, gross liability	1,205.0	761.3
Exchange-traded foreign exchange derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	109.5	119.9
Derivative liability, fair value, gross liability	109.5	119.9
OTC foreign exchange derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	645.9	216.9
Derivative liability, fair value, gross liability	577.4	185.5
Exchange-traded interest rate derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	3,431.1	245.9
Derivative liability, fair value, gross liability	3,435.1	249.0
OTC interest rate derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	83.5	56.4
Derivative liability, fair value, gross liability	76.2	54.9
OTC interest rate derivatives | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	0.0
Exchange-traded equity index derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	291.0	206.5
Derivative liability, fair value, gross liability	291.0	206.5
OTC equity and indices derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	144.7	148.5
Derivative liability, fair value, gross liability	92.8	94.1
TBA and forward settling securities | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	152.2	59.1
Derivative liability, fair value, gross liability	120.3	52.2
Interest rate contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 0.0	0.0
Derivative liability, fair value, gross liability		$ 0.0