Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Gross Derivative Assets and Liabilities by Type and Balance Sheet Location (Details) position in Millions, $ in Millions	Sep. 30, 2020 USD ($) position	Sep. 30, 2019 USD ($) position
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 3,264.4	$ 3,671.5
Derivative liability, fair value, gross liability	3,069.5	3,604.5
Gross fair value of derivative contracts	(3,319.0)	(3,581.9)
Gross fair value of derivative contracts	$ (3,100.7)	$ (3,540.8)
Derivative, number of instruments held | position	7.9	10.6
Deposits and Receivables from Exchange Clearing Organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ (173.5)	$ 23.1
Receivable From Clients, Net [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	1.5	0.0
Financial Instruments Owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	117.4	66.5
Payables To Clients [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(222.7)	0.0
Payables to Broker-dealers, Clearing Organizations and Counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	14.7	5.6
Financial Instrument Sold, Not Yet Purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	176.8	58.1
Exchange-traded Commodity Derivatives [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	1,637.2	1,437.1
Derivative liability, fair value, gross liability	1,747.3	1,463.4
OTC Commodity Derivatives [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	553.9	84.2
Derivative liability, fair value, gross liability	433.2	106.2
Exchange-traded Foreign Exchange Derivatives [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	9.3	36.9
Derivative liability, fair value, gross liability	13.0	33.5
OTC Foreign Exchange Derivatives [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	520.8	403.2
Derivative liability, fair value, gross liability	461.5	368.8
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	271.1	900.1
Derivative liability, fair value, gross liability	200.7	882.0
OTC Interest Rate Derivatives [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	96.0	42.1
Derivative liability, fair value, gross liability	96.6	43.6
Exchange-traded Equity Index Derivatives [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	32.1	758.1
Derivative liability, fair value, gross liability	39.8	700.2
OTC Equity And Indices Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	113.0	0.0
Derivative liability, fair value, gross liability	55.4	0.0
TBA and Forward Settling Securities [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	31.0	9.8
Derivative liability, fair value, gross liability	$ 22.0	$ 6.8