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, 2021 USD ($) contract	Sep. 30, 2020 USD ($) contract
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 4,622.0	$ 3,264.4
Derivative liability, fair value, gross liability	4,606.5	3,069.5
Gross fair value of derivative contracts	(4,527.6)	(3,319.0)
Gross fair value of derivative contracts	$ (4,351.1)	$ (3,100.7)
Derivative, number of instruments held | contract	10.7	7.9
Deposits and Receivables from Exchange Clearing Organizations
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ (107.4)	$ (173.5)
Receivable From Clients, Net
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	1.0	1.5
Financial Instruments Owned
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	200.8	117.4
Payables to Clients
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(178.2)	(222.7)
Payables to Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	43.7	14.7
Financial Instrument Sold, Not Yet Purchased
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	389.9	176.8
Exchange-traded commodity derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	2,393.9	1,637.2
Derivative liability, fair value, gross liability	2,325.0	1,747.3
OTC commodity derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	855.9	553.9
Derivative liability, fair value, gross liability	858.2	433.2
Exchange-traded foreign exchange derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	411.4	9.3
Derivative liability, fair value, gross liability	475.6	13.0
OTC foreign exchange derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	279.4	520.8
Derivative liability, fair value, gross liability	227.4	461.5
Exchange-traded interest rate derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	351.4	271.1
Derivative liability, fair value, gross liability	479.9	200.7
OTC interest rate derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	59.0	96.0
Derivative liability, fair value, gross liability	58.0	96.6
Exchange-traded equity index derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	82.5	32.1
Derivative liability, fair value, gross liability	51.6	39.8
OTC equity and indices derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	133.6	113.0
Derivative liability, fair value, gross liability	83.3	55.4
TBA and forward settling securities
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	54.9	31.0
Derivative liability, fair value, gross liability	$ 47.5	$ 22.0