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, 2020 USD ($) contract	Sep. 30, 2020 USD ($) contract
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 3,170.4	$ 3,264.4
Derivative liability, fair value, gross liability	2,916.5	3,069.5
Gross fair value of derivative contracts	(3,053.1)	(3,319.0)
Gross fair value of derivative contracts	$ (2,767.4)	$ (3,100.7)
Derivative, number of instruments held | contract	7.4	7.9
Deposits and Receivables from Exchange Clearing Organizations
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ (11.8)	$ (173.5)
Receivable From Clients, Net
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	2.2	1.5
Financial Instruments Owned
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	126.9	117.4
Payables To Clients
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(121.3)	(222.7)
Payables to Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	30.6	14.7
Financial Instrument Sold, Not Yet Purchased
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	239.8	176.8
Exchange-traded commodity derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1,594.0	1,637.2
Derivative liability, fair value, gross liability	1,600.9	1,747.3
OTC commodity derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	763.2	553.9
Derivative liability, fair value, gross liability	647.4	433.2
Exchange-traded foreign exchange derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	32.8	9.3
Derivative liability, fair value, gross liability	44.4	13.0
OTC foreign exchange derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	378.1	520.8
Derivative liability, fair value, gross liability	303.2	461.5
Exchange-traded interest rate derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	88.6	271.1
Derivative liability, fair value, gross liability	86.3	200.7
OTC interest rate derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	84.5	96.0
Derivative liability, fair value, gross liability	84.9	96.6
Exchange-traded equity index derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	51.1	32.1
Derivative liability, fair value, gross liability	28.2	39.8
OTC equity and indices derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	149.9	113.0
Derivative liability, fair value, gross liability	90.4	55.4
TBA and forward settling securities
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	28.2	31.0
Derivative liability, fair value, gross liability	$ 30.8	$ 22.0