Credit Facilities - Number of Credit Facilities (Details) $ in Millions	12 Months Ended
	Sep. 30, 2017 USD ($) Rate	May 05, 2015 USD ($)
Line of Credit Facility [Line Items]
Number of credit facilities	4
HCO Syndicated line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.625%
Line of Credit Facility, Interest Rate at Period End	4.20%
Line of Credit Facility, Maximum Borrowing Capacity | $	$ 262.0
HCO Syndicated line of credit facility [Member] | Eurodollar Rate [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	3.00%
HCO Syndicated line of credit facility [Member] | Base Rate [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.00%
INTL FCStone, Ltd [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity | $	$ 25.0
INTL FCStone, Ltd [Member] | Base Rate [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.50%
INTL FCStone Financial Margin line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.50%
Line of Credit Facility, Interest Rate at Period End	6.25%
Line of Credit Facility, Maximum Borrowing Capacity | $	$ 75.0	$ 50.0
INTL FCStone Financial Margin line of credit facility [Member] | Base Rate [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.00%