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, 2023 USD ($) contract	Sep. 30, 2022 USD ($) contract
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 8,673.1	$ 9,657.2
Derivative liability, fair value, gross liability	8,487.8	9,552.4
Impact of netting and collateral asset	(8,519.4)	(10,920.8)
Impact of netting and collateral liability	$ (8,374.5)	$ (10,505.0)
Derivative, number of instruments held | contract	12.8	13.3
Deposits and Receivables from Exchange Clearing Organizations
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ (151.1)	$ (1,455.7)
Receivable From Clients, Net
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(0.1)	(0.5)
Financial Instruments Owned
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	304.9	192.6
Payables to Clients
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(296.5)	(1,392.4)
Payables to Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	3.2	55.8
Financial Instrument Sold, Not Yet Purchased
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	406.6	384.0
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	8,665.6	9,657.2
Derivative liability, fair value, gross liability	8,450.2	9,481.8
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	7.5	0.0
Derivative liability, fair value, gross liability	37.6	70.6
Exchange-traded commodity derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1,644.0	4,520.4
Derivative liability, fair value, gross liability	1,630.6	4,519.3
OTC commodity derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1,865.3	756.9
Derivative liability, fair value, gross liability	1,790.4	695.6
Exchange-traded foreign exchange derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	2.4	25.6
Derivative liability, fair value, gross liability	2.4	25.7
OTC foreign exchange derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	756.3	577.1
Derivative liability, fair value, gross liability	726.5	549.3
Exchange-traded interest rate derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1,835.3	2,626.8
Derivative liability, fair value, gross liability	1,840.2	2,626.7
OTC interest rate derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	343.4	168.9
Derivative liability, fair value, gross liability	343.4	205.1
OTC interest rate derivatives | 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	37.6	48.8
Exchange-traded equity index derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	2,006.9	609.5
Derivative liability, fair value, gross liability	2,006.9	609.5
OTC equity and indices derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	176.7	164.4
Derivative liability, fair value, gross liability	85.8	95.7
TBA and forward settling securities | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	35.3	207.6
Derivative liability, fair value, gross liability	24.0	154.9
Foreign currency forward contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	7.5	0.0
Derivative liability, fair value, gross liability	$ 0.0	$ 21.8