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	Dec. 31, 2021 USD ($) contract	Sep. 30, 2021 USD ($) contract
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 4,895.0	$ 5,760.7
Derivative liability, fair value, gross liability	4,702.2	5,628.1
Impact of netting and collateral asset	(4,437.8)	(5,353.9)
Impact of netting and collateral liability	$ (4,372.5)	$ (4,955.4)
Derivative, number of instruments held | contract	10.3	11.1
Deposits and Receivables from Exchange Clearing Organizations
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ 318.8	$ 259.9
Receivable From Clients, Net
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	2.0	2.6
Financial Instruments Owned
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	136.4	144.3
Payables to Clients
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	54.5	291.5
Payables to Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	0.7	12.7
Financial Instrument Sold, Not Yet Purchased
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	274.5	368.5
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	4,894.9	5,760.7
Derivative liability, fair value, gross liability	4,702.1	5,628.1
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	0.1	0.0
Derivative liability, fair value, gross liability	0.1	0.0
Exchange-traded commodity derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	2,795.0	3,904.1
Derivative liability, fair value, gross liability	2,783.6	3,904.7
OTC commodity derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	814.6	803.4
Derivative liability, fair value, gross liability	730.8	761.3
Exchange-traded foreign exchange derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	33.6	119.9
Derivative liability, fair value, gross liability	33.7	119.9
OTC foreign exchange derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	201.1	216.9
Derivative liability, fair value, gross liability	157.1	185.5
Exchange-traded interest rate derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	601.0	245.9
Derivative liability, fair value, gross liability	608.3	249.0
OTC interest rate derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	47.6	56.4
Derivative liability, fair value, gross liability	45.6	54.9
Exchange-traded equity index derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	235.0	206.5
Derivative liability, fair value, gross liability	235.0	206.5
OTC equity and indices derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	152.7	148.5
Derivative liability, fair value, gross liability	96.8	94.1
TBA and forward settling securities | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	14.3	59.1
Derivative liability, fair value, gross liability	11.2	52.2
Interest rate contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	0.1	0.0
Derivative liability, fair value, gross liability	$ 0.1	$ 0.0