Credit Facilities - Credit Facilities (Details) - USD ($)	Dec. 31, 2018	Sep. 30, 2018
Line of Credit Facility [Line Items]
Short-term Debt	$ 408,500,000	$ 336,200,000
Lenders under loans	435.4	355.2
Notes Payable to Bank	1,000,000	1,200,000
Debt and Capital Lease Obligations	435,400,000	355,200,000
Main line of credit facilities [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	595,000,000
Line of Credit Facility, Maximum Borrowing Capacity	594,500,000
IFFI Uncommitted Lines of Credit [Member] [Member]
Line of Credit Facility [Line Items]
Lenders under loans	20,000,000	14,000,000
HCO Syndicated line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	262,000,000
Lenders under loans	220,000,000	208,200,000
FCS Margin line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	75,000,000
Lenders under loans	0	0
FMS Sub-note commodity line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	232,500,000
Lenders under loans	163,500,000	128,000,000
INTL FCStone Ltd [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	25,000,000
Lenders under loans	25,000,000	0
IFL Uncommitted Line of Credit [Member] [Member]
Line of Credit Facility [Line Items]
Lenders under loans	0	0
FMS Uncommitted Line of Credit [Member]
Line of Credit Facility [Line Items]
Lenders under loans	$ 5,900,000	$ 3,800,000