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	Jun. 30, 2022 USD ($) contract	Sep. 30, 2021 USD ($) contract
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 12,309.7	$ 5,760.7
Derivative liability, fair value, gross liability	12,254.7	5,628.1
Impact of netting and collateral asset	(11,567.7)	(5,353.9)
Impact of netting and collateral liability	$ (11,144.4)	$ (4,955.4)
Derivative, number of instruments held | contract	13.2	11.1
Deposits and Receivables from Exchange Clearing Organizations
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ 593.7	$ 259.9
Receivable From Clients, Net
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(21.2)	2.6
Financial Instruments Owned
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	169.5	144.3
Payables to Clients
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	624.9	291.5
Payables to Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	6.5	12.7
Financial Instrument Sold, Not Yet Purchased
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	478.9	368.5
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	12,309.7	5,760.7
Derivative liability, fair value, gross liability	12,221.4	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	33.3	0.0
Exchange-traded commodity derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	5,579.8	3,904.1
Derivative liability, fair value, gross liability	5,587.9	3,904.7
OTC commodity derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	993.6	803.4
Derivative liability, fair value, gross liability	1,050.8	761.3
Exchange-traded foreign exchange derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	414.4	119.9
Derivative liability, fair value, gross liability	414.4	119.9
OTC foreign exchange derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	464.2	216.9
Derivative liability, fair value, gross liability	400.7	185.5
Exchange-traded interest rate derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	3,623.8	245.9
Derivative liability, fair value, gross liability	3,623.8	249.0
OTC interest rate derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	127.4	56.4
Derivative liability, fair value, gross liability	115.2	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	859.8	206.5
Derivative liability, fair value, gross liability	859.9	206.5
OTC equity and indices derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	162.8	148.5
Derivative liability, fair value, gross liability	93.1	94.1
TBA and forward settling securities | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	83.9	59.1
Derivative liability, fair value, gross liability	75.6	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	$ 33.3	$ 0.0