Credit Facilities - Credit Facilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017
Line of Credit Facility [Line Items]
Short-term Debt	$ 350.0	$ 194.2
Lenders under loans	422.9	230.2
Notes Payable to Bank	1.8	2.0
Debt and Capital Lease Obligations	422.9	230.2
Main line of credit facilities [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	270.0
Line of Credit Facility, Maximum Borrowing Capacity	532.0
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	71.1	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	210.0	150.0
FCS Margin line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	75.0
Lenders under loans	36.5	0.0
FMS Sub-note commodity line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	170.0
Lenders under loans	103.5	44.2
INTL FCStone Ltd [Member]
Line of Credit Facility [Line Items]
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