Credit Facilities (Details) - USD ($)	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2013	Dec. 11, 2019	Sep. 30, 2015	Apr. 15, 2015
Line of Credit Facility [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ 800,000
Repayments of Long-term Debt	6,600,000	$ 0	$ 0
Line of Credit Facility, Maximum Borrowing Capacity	100.0
Maximum Borrowing Capacity Under Credit Facilities	743,900,000
Notes Payable to Bank	400,000	$ 1,200,000					$ 4,000,000
Debt Instrument, Interest Rate, Stated Percentage		8.50%
Payments of Debt Issuance Costs	3,300,000	$ 400,000	300,000	$ 1,700,000
Senior secured term loan, net	$ 167,600,000	0				$ 45,500,000
Write off of Deferred Debt Issuance Cost			$ 1,000,000
Debt, Weighted Average Interest Rate	2.00%
INTL FCStone Financial Margin line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate at Period End	7.25%
Line of Credit Facility, Maximum Borrowing Capacity	$ 75,000,000
Line of Credit, Current	$ 0	0
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.50%
Rabobank Uncommited Line of Credit [Domain]
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate at Period End	5.00%
Line of Credit Facility, Maximum Borrowing Capacity	$ 20,000,000.0
Short-term Debt	$ 3,400,000	3,800,000
Letter of Credit Interest Rate	2.75%
INTL FCStone Inc. Bank of America Syndicated Credit Facility [Member]
Line of Credit Facility [Line Items]
Long-term Debt	$ 167,600,000	0
Line of Credit, Current	70,000,000	208,200,000
Maximum Borrowing Capacity Under Credit Facilities	$ 350,000,000	262,000,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.625%
Payments of Debt Issuance Costs	$ 3,100,000
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
Line of Credit, Current	$ 128,500,000	128,000,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.38%
INTL FCStone, Ltd [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 50,000,000
Line of Credit, Current	0	0
Main line of credit facilities [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	358,000,000
IFFI Uncommitted Lines of Credit [Member] [Member]
Line of Credit Facility [Line Items]
Short-term Debt	0	14,000,000
IFL Uncommitted Line of Credit [Member] [Member]
Line of Credit Facility [Line Items]
Short-term Debt	$ 0	$ 0
Base Rate [Member] | Rabobank Uncommited Line of Credit [Domain]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.00%
Eurodollar [Member] | INTL FCStone Inc. Bank of America Syndicated Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate at Period End	4.95%
Debt Instrument, Basis Spread on Variable Rate		3.00%
Eurodollar [Member] | FMS Sub-note commodity line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate at Period End	4.50%
Subsequent Event [Member] | INTL FCStone Inc. Bank of America Syndicated Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity					$ 196,500,000
Long-term Debt					189,900,000
Maximum Borrowing Capacity Under Credit Facilities					$ 386,400,000