Credit Facilities - Credit Facilities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017
Line of Credit Facility [Line Items]
Short-term Debt	$ 311.2	$ 194.2
Lenders under loans	360.6	230.2
Notes Payable to Bank	1.4	2.0
Debt and Capital Lease Obligations	360.6	230.2
Main line of credit facilities [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	362.0
Line of Credit Facility, Maximum Borrowing Capacity	594.5
IFFI Uncommitted Lines of Credit [Member] [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	0.0
Lenders under loans	48.0	34.0
HCO Syndicated line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	262.0
Lenders under loans	208.7	150.0
FCS Margin line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	75.0
Line of Credit Facility, Maximum Borrowing Capacity	75.0
Lenders under loans	0.0	0.0
Sterne, Agee, and Leach [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	100.0
Lenders under loans	0.0	11.0
FMS Sub-note commodity line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	232.5
Lenders under loans	102.5	44.2
INTL FCStone Ltd [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	25.0
Line of Credit Facility, Maximum Borrowing Capacity	25.0
Lenders under loans	0.0	0.0
IFL Uncommitted Line of Credit [Member] [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	0.0
Lenders under loans	$ 0.0	$ 0.0