Credit Facilities - Number of Credit Facilities (Details) $ in Millions	6 Months Ended
	Mar. 31, 2020 USD ($) Rate	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)
Term Loan Initial Balance	$ 196.5
Term Loan Periodic Principal Payment, Percent | Rate	1.25%
Number of credit facilities	4
Maximum available under credit facility	$ 393.0
Main line of credit facilities [Member]
Borrowing Capacity Under Credit Facilities	739.0
HCO Syndicated line of credit facility [Member]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net		$ 0.6
Line of Credit Facility, Maximum Borrowing Capacity	196.5
Borrowing Capacity Under Credit Facilities	379.0
FCS Margin line of credit facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity	75.0
Line of Credit, Current		0.0	$ 0.0
FMS Sub-note commodity line of credit facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity	260.0
INTL FCStone Ltd [Member]
Line of Credit Facility, Maximum Borrowing Capacity	100.0
Line of Credit, Current		24.4	0.0
Rabobank Uncommited Line of Credit [Domain]
Line of Credit Facility, Maximum Borrowing Capacity	20.0
Line of Credit, Current		0.0	3.4
INTL FCStone Ltd [Member]
Line of Credit Facility, Maximum Borrowing Capacity	$ 25.0
Line of Credit, Current		$ 0.0	$ 0.0