Credit Facilities - Credit Facilities (Details) - USD ($) $ in Millions	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019
Line of Credit Facility [Line Items]
Long-term Debt, Gross		$ 182.5
Senior secured borrowings, net	$ 518.9		$ 167.6
Short-term Debt	471.4		366.1
Notes Payable to Bank	0.0
Senior Notes	336.9		0.0
Debt, Long-term and Short-term, Combined Amount	832.7		369.9
Main line of credit facilities [Member]
Line of Credit Facility [Line Items]
Credit Facilities Expiring Within One Year	100.0
Borrowing Capacity Under Credit Facilities		739.0
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	24.4		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	0.6
Senior secured borrowings, net	182.0		167.6
Line of Credit Facility, Maximum Borrowing Capacity		196.5
Long-term Line of Credit, Noncurrent	127.0		70.0
Borrowing Capacity Under Credit Facilities		379.0
Long-term Debt	309.0		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	162.4		128.5
INTL FCStone Ltd [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		25.0
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