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, 2023 USD ($) contract	Sep. 30, 2022 USD ($) contract
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 9,275.0	$ 9,657.2
Derivative liability, fair value, gross liability	9,133.7	9,552.4
Impact of netting and collateral asset	(8,496.6)	(10,920.8)
Impact of netting and collateral liability	$ (8,181.9)	$ (10,505.0)
Derivative, number of instruments held | contract	16.4	13.3
Deposits and Receivables from Exchange Clearing Organizations
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ 486.0	$ (1,455.7)
Receivable From Clients, Net
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	2.3	(0.5)
Financial Instruments Owned
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	290.1	192.6
Payables to Clients
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	669.5	(1,392.4)
Payables to Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	34.1	55.8
Financial Instrument Sold, Not Yet Purchased
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	248.2	384.0
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	9,270.3	9,657.2
Derivative liability, fair value, gross liability	9,095.2	9,481.8
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	4.7	0.0
Derivative liability, fair value, gross liability	38.5	70.6
Exchange-traded commodity derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	2,052.1	4,520.4
Derivative liability, fair value, gross liability	2,040.6	4,519.3
OTC commodity derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1,476.1	756.9
Derivative liability, fair value, gross liability	1,395.6	695.6
Exchange-traded foreign exchange derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	213.0	25.6
Derivative liability, fair value, gross liability	213.0	25.7
OTC foreign exchange derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	762.5	577.1
Derivative liability, fair value, gross liability	716.0	549.3
Exchange-traded interest rate derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	2,097.1	2,626.8
Derivative liability, fair value, gross liability	2,099.7	2,626.7
OTC interest rate derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	287.3	168.9
Derivative liability, fair value, gross liability	287.3	205.1
OTC interest rate derivatives | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1.8	0.0
Derivative liability, fair value, gross liability	38.4	48.8
Exchange-traded equity index derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	2,194.3	609.5
Derivative liability, fair value, gross liability	2,194.3	609.5
OTC equity and indices derivatives | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	132.5	164.4
Derivative liability, fair value, gross liability	76.2	95.7
TBA and forward settling securities | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	55.4	207.6
Derivative liability, fair value, gross liability	72.5	154.9
Foreign currency forward contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	2.9	0.0
Derivative liability, fair value, gross liability	$ 0.1	$ 21.8