Credit Facilities - Number of Credit Facilities (Details) $ in Millions	6 Months Ended
	Mar. 31, 2020 USD ($) Rate	Sep. 30, 2019 USD ($)
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	766.5
HCO Syndicated line of credit facility [Member]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	0.7
Line of Credit Facility, Maximum Borrowing Capacity	196.5
Borrowing Capacity Under Credit Facilities	381.5
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	5.0	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	50.0
Line of Credit, Current	$ 0.0	$ 0.0