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, 2022 USD ($) contract	Sep. 30, 2022 USD ($) contract
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 7,120.1	$ 9,657.2
Derivative liability, fair value, gross liability	6,984.7	9,552.4
Impact of netting and collateral asset	(7,828.4)	(10,920.8)
Impact of netting and collateral liability	$ (7,464.7)	$ (10,505.0)
Derivative, number of instruments held | contract	11.2	13.3
Deposits and Receivables from Exchange Clearing Organizations
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ (976.3)	$ (1,455.7)
Receivable From Clients, Net
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	0.3	(0.5)
Financial Instruments Owned
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	267.7	192.6
Payables to Clients
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(820.4)	(1,392.4)
Payables to Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	13.5	55.8
Financial Instrument Sold, Not Yet Purchased
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	326.9	384.0
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	7,118.5	9,657.2
Derivative liability, fair value, gross liability	6,930.2	9,481.8
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1.6	0.0
Derivative liability, fair value, gross liability	54.5	70.6
Exchange-traded commodity derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	2,232.4	4,520.4
Derivative liability, fair value, gross liability	2,224.6	4,519.3
OTC commodity derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1,330.5	756.9
Derivative liability, fair value, gross liability	1,272.2	695.6
Exchange-traded foreign exchange derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	690.6	25.6
Derivative liability, fair value, gross liability	690.6	25.7
OTC foreign exchange derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	833.1	577.1
Derivative liability, fair value, gross liability	800.2	549.3
Exchange-traded interest rate derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1,085.2	2,626.8
Derivative liability, fair value, gross liability	1,086.1	2,626.7
OTC interest rate derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	337.9	168.9
Derivative liability, fair value, gross liability	337.9	205.1
OTC interest rate derivatives | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	0.3	0.0
Derivative liability, fair value, gross liability	50.9	48.8
Exchange-traded equity index derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	391.6	609.5
Derivative liability, fair value, gross liability	391.6	609.5
OTC equity and indices derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	151.4	164.4
Derivative liability, fair value, gross liability	81.0	95.7
TBA and forward settling securities | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	65.8	207.6
Derivative liability, fair value, gross liability	46.0	154.9
Interest rate contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative liability, fair value, gross liability	50.9
Foreign currency forward contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1.3	0.0
Derivative liability, fair value, gross liability	$ 3.6	$ 21.8