Revenue from Contracts with Clients - Revenue from Contracts with Clients (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									$ 487.3	$ 452.0	$ 462.9
Sales of physical commodities									52,899.2	31,830.3	26,682.4
Principal gains, net									622.2	415.8	354.1
Interest income									130.9	198.9	123.3
Total revenues	$ 14,284.9	$ 8,243.4	$ 20,366.3	$ 11,245.0	$ 11,279.6	$ 7,873.0	$ 7,192.2	$ 6,552.2	54,139.6	32,897.0	27,622.7
Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									396.1	362.7	379.7
Time Elapsed [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									91.2	89.3	83.2
Exchange-Traded Futures and Options [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									143.7	144.9	163.4
OTC Derivative Brokerage [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									19.7	32.1	30.3
Equities and Fixed Income [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									39.9	16.1	11.2
Mutual Fund Sales Based [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									5.2	7.5	7.2
Insurance and Annuity Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									8.4	7.3	5.8
Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									1.4	1.3	0.9
Sales Based Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									218.3	209.2	218.8
Mutual Fund [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									12.9	12.4	13.2
Insurance Annuity Trailing [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									15.3	14.4	14.6
Trailing Commissions [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									28.2	26.8	27.8
Clearing Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									139.0	118.8	123.3
Trade Conversion Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									8.9	7.5	6.8
Other Commission and Clearing Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									9.2	10.1	15.1
Commission and Clearing Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									403.6	372.4	391.8
Underwriting Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0.6	0.7	1.7
Asset Management Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									31.3	26.2	24.8
Advisory and Consulting Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									22.2	20.0	19.0
Sweep Program Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									9.5	16.3	11.6
Client Account Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									12.3	10.6	11.1
Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									7.8	5.8	2.9
Consulting, Management, and Account Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									83.7	79.6	71.1
Precious Precious Metals Trading [Member]
Disaggregation of Revenue [Line Items]
Sales of physical commodities									51,598.5	30,694.5	25,762.9
Physical Agricultural and Energy Product trading [Member]
Disaggregation of Revenue [Line Items]
Sales of physical commodities									$ 1,300.7	$ 1,135.8	$ 919.5