Credit Facilities - Credit Facilities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Line of Credit Facility [Line Items]
Long-term Debt, Gross	$ 185.0
Senior secured term loan, net	184.3	$ 167.6
Short-term Debt	454.3	366.1
Notes Payable to Bank	0.0	0.4
Debt, Long-term and Short-term, Combined Amount	459.3	369.9
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	766.5
IFFI Uncommitted Lines of Credit [Member] [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	100.0
Line of Credit, Current	5.0	0.0
HCO Syndicated line of credit facility [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	0.7
Senior secured term loan, net	184.3	167.6
Line of Credit Facility, Maximum Borrowing Capacity	196.5
Long-term Line of Credit, Noncurrent	100.0	70.0
Borrowing Capacity Under Credit Facilities	381.5
Long-term Debt	284.3	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
Long-term Line of Credit, Noncurrent	170.0	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
Line of Credit, Current	$ 0.0	$ 3.4