Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Gross Derivative Assets and Liabilities by Type and Balance Sheet Location (Details) country in Millions, $ in Millions	Sep. 30, 2021 USD ($) country	Sep. 30, 2020 USD ($) country
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 5,760.7	$ 3,264.4
Derivative liability, fair value, gross liability	5,628.1	3,069.5
Gross fair value of derivative contracts	(5,353.9)	(3,319.0)
Gross fair value of derivative contracts	$ (4,955.4)	$ (3,100.7)
Derivative, number of instruments held | country	11.1	7.9
Deposits and Receivables from Exchange Clearing Organizations
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ 259.9	$ (173.5)
Receivable From Clients, Net
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	2.6	1.5
Financial Instruments Owned
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	144.3	117.4
Payables To Clients
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	291.5	(222.7)
Payables to Broker-dealers, Clearing Organizations and Counterparties
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	12.7	14.7
Financial Instrument Sold, Not Yet Purchased
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	368.5	176.8
Exchange-traded commodity derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	3,904.1	1,637.2
Derivative liability, fair value, gross liability	3,904.7	1,747.3
OTC commodity derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	803.4	553.9
Derivative liability, fair value, gross liability	761.3	433.2
Exchange-traded foreign exchange derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	119.9	9.3
Derivative liability, fair value, gross liability	119.9	13.0
OTC foreign exchange derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	216.9	520.8
Derivative liability, fair value, gross liability	185.5	461.5
Exchange-traded interest rate derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	245.9	271.1
Derivative liability, fair value, gross liability	249.0	200.7
OTC interest rate derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	56.4	96.0
Derivative liability, fair value, gross liability	54.9	96.6
Exchange-traded equity index derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	206.5	32.1
Derivative liability, fair value, gross liability	206.5	39.8
OTC equity and indices derivatives
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	148.5	113.0
Derivative liability, fair value, gross liability	94.1	55.4
TBA and forward settling securities
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	59.1	31.0
Derivative liability, fair value, gross liability	$ 52.2	$ 22.0