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, 2021 USD ($) contract	Sep. 30, 2020 USD ($) contract
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 5,640.0	$ 3,264.4
Derivative liability, fair value, gross liability	5,302.5	3,069.5
Gross fair value of derivative contracts	(5,170.8)	(3,319.0)
Gross fair value of derivative contracts	$ (5,308.7)	$ (3,100.7)
Derivative, number of instruments held | contract	11.1	7.9
Deposits and Receivables from Exchange Clearing Organizations
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ 266.1	$ (173.5)
Receivable From Clients, Net
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	5.1	1.5
Financial Instruments Owned
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	198.0	117.4
Payables to Clients
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(495.7)	(222.7)
Payables to Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	12.4	14.7
Financial Instrument Sold, Not Yet Purchased
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	477.1	176.8
Exchange-traded commodity derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	3,438.7	1,637.2
Derivative liability, fair value, gross liability	3,133.9	1,747.3
OTC commodity derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1,008.1	553.9
Derivative liability, fair value, gross liability	1,028.3	433.2
Exchange-traded foreign exchange derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	166.6	9.3
Derivative liability, fair value, gross liability	188.4	13.0
OTC foreign exchange derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	300.0	520.8
Derivative liability, fair value, gross liability	240.3	461.5
Exchange-traded interest rate derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	287.3	271.1
Derivative liability, fair value, gross liability	427.8	200.7
OTC interest rate derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	58.8	96.0
Derivative liability, fair value, gross liability	57.6	96.6
Exchange-traded equity index derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	223.3	32.1
Derivative liability, fair value, gross liability	119.5	39.8
OTC equity and indices derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	135.6	113.0
Derivative liability, fair value, gross liability	90.2	55.4
TBA and forward settling securities
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	21.6	31.0
Derivative liability, fair value, gross liability	$ 16.5	$ 22.0