Credit Facilities - Credit Facilities (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Sep. 30, 2019
Line of Credit Facility [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ 0.8
Long-term Debt, Gross	187.5
Long-term Debt	186.7	$ 167.6
Short-term Debt	307.7	366.1
Notes Payable to Bank	0.3	0.4
Debt and Capital Lease Obligations	$ 325.4	369.9
Debt Instrument, Interest Rate During Period	2.00%
Main line of credit facilities [Member]
Line of Credit Facility [Line Items]
Credit Facilities Expiring Within One Year	$ 125.0
Borrowing Capacity Under Credit Facilities	769.0
IFFI Uncommitted Lines of Credit [Member] [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	100.0
Short-term Debt	16.7	0.0
HCO Syndicated line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	196.5
Line of Credit, Current	0.0	70.0
Borrowing Capacity Under Credit Facilities	384.0
Lenders under loans	186.7	237.6
FCS Margin line of credit facility [Member]
Line of Credit Facility [Line Items]
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 [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	260.0
Line of Credit, Current	121.0	128.5
INTL FCStone Ltd [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	50.0
Line of Credit, Current	0.0	0.0
INTL FCStone Financial Inc. Uncommitted Commodities Delivery Line [Domain]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	75.0
Rabobank Uncommited Line of Credit [Domain]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	20.0
Short-term Debt	$ 0.7	$ 3.4