Credit Facilities - Credit Facilities (Details) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018
Line of Credit Facility [Line Items]
Short-term Debt	$ 426.3		$ 336.2
Notes Payable to Bank	0.8		1.2
Debt and Capital Lease Obligations	$ 428.6		355.2
Debt Instrument, Interest Rate During Period	2.00%
Main line of credit facilities [Member]
Line of Credit Facility [Line Items]
Credit Facilities Expiring Within One Year	$ 357.5
Line of Credit Facility, Maximum Borrowing Capacity	705.3
IFFI Uncommitted Lines of Credit [Member] [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	100.0
Short-term Debt	0.0		14.0
HCO Syndicated line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	350.0	$ 262.0
Payments of Financing Costs	3.1
Lenders under loans	254.8		208.2
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	232.5
Line of Credit, Current	171.5		128.0
INTL FCStone Ltd [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	50.0	$ 25.0
Line of Credit, Current	0.0		0.0
INTL FCStone Ltd. Uncommitted Line of Credit [Domain]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	20.0
INTL FCStone Financial Inc. Uncommitted Commodities Delivery Line [Domain]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	75.0
IFL Uncommitted Line of Credit [Member] [Member]
Line of Credit Facility [Line Items]
Short-term Debt	0.0		0.0
Rabobank Uncommited Line of Credit [Domain]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	20.0
Short-term Debt	$ 1.5		$ 3.8