Credit Facilities - Number of Credit Facilities (Details)	12 Months Ended
	Sep. 30, 2019 USD ($) Rate	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2013 USD ($)	Dec. 11, 2019 USD ($)
Line of Credit Facility [Line Items]
Long-term Debt, Gross	$ 168,400,000
Payments of Debt Issuance Costs	$ 3,300,000	$ 400,000	$ 300,000	$ 1,700,000
Number of credit facilities	4
Line of Credit Facility, Maximum Borrowing Capacity	$ 100.0
Maximum Borrowing Capacity Under Credit Facilities	$ 743,900,000
Term loan periodic principal payment percentage | Rate	1.30%
Rabobank Uncommited Line of Credit [Domain]
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate at Period End | Rate	5.00%
Line of Credit Facility, Maximum Borrowing Capacity	$ 20,000,000.0
Rabobank Uncommited Line of Credit [Domain] | Base Rate [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.00%
INTL FCStone Inc. Bank of America Syndicated Credit Facility [Member]
Line of Credit Facility [Line Items]
Payments of Debt Issuance Costs	$ 3,100,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage | Rate	0.625%
Maximum Borrowing Capacity Under Credit Facilities	$ 350,000,000	$ 262,000,000
INTL FCStone, Ltd [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 50,000,000
INTL FCStone Financial Margin line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage | Rate	0.50%
Line of Credit Facility, Interest Rate at Period End | Rate	7.25%
Line of Credit Facility, Maximum Borrowing Capacity	$ 75,000,000
FMS Sub-note commodity line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage | Rate	0.38%
Line of Credit Facility, Maximum Borrowing Capacity	$ 232,500,000
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
Debt Instrument, Face Amount					196,500,000
Maximum Borrowing Capacity Under Credit Facilities					$ 386,400,000
Cost of Funds Rate [Domain] | Rabobank Uncommited Line of Credit [Domain]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	3.00%
Base Rate [Member] | INTL FCStone Inc. Bank of America Syndicated Credit Facility [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate		2.00%
Line of Credit Facility, Interest Rate at Period End | Rate	7.25%
Base Rate [Member] | INTL FCStone, Ltd [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.50%
Base Rate [Member] | INTL FCStone Financial Margin line of credit facility [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate		2.00%
Base Rate [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 | Rate	5.00%
Eurodollar [Member] | INTL FCStone Inc. Bank of America Syndicated Credit Facility [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate		3.00%
Line of Credit Facility, Interest Rate at Period End | Rate	4.95%
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 | Rate	4.50%