Credit Facilities (Details) - USD ($)	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2013	Sep. 30, 2015	May 05, 2015	Apr. 15, 2015
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 150.0
Payables to lenders under loans	355,200,000	$ 230,200,000
Notes Payable to Bank	$ 1,200,000	2,000,000					$ 4,000,000
Debt Instrument, Interest Rate, Stated Percentage	8.50%
Debt Instrument, Call Feature, Redemption Price Percentage of Principal	100.00%
Payments of Debt Issuance Costs	$ 400,000	300,000	$ 2,100,000	$ 1,700,000
Senior Notes					$ 45,500,000
Debt and Capital Lease Obligations	$ 355,200,000	230,200,000
Write off of Deferred Debt Issuance Cost		1,000,000
Debt, Weighted Average Interest Rate	2.00%
HCO Syndicated line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate at Period End	5.17%
Line of Credit Facility, Maximum Borrowing Capacity	$ 262,000,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.625%
Payables to lenders under loans	$ 208,200,000	150,000,000
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					$ 75,000,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.50%
Payables to lenders under loans	$ 0	0
INTL FCStone Financial Inc. Uncommitted Commodities Delivery Line [Domain]
Line of Credit Facility [Line Items]
Payables to lenders under loans	0	23,000,000
INTL FCStone Ltd. Uncommitted Line of Credit [Domain]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	20,000,000.0
Payables to lenders under loans	$ 0	0
Rabobank Uncommited Line of Credit [Domain]
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate at Period End	5.40%
Line of Credit Facility, Maximum Borrowing Capacity	$ 20,000,000.0
Letter of Credit Interest Rate	2.75%
Payables to lenders under loans	$ 3,800,000	0
FMS Sub-note commodity line of credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate at Period End	4.77%
Line of Credit Facility, Maximum Borrowing Capacity	$ 232,500,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.38%
BMO Committed Credit Facility [Member] [Domain]
Line of Credit Facility [Line Items]
Payables to lenders under loans	$ 128,000,000	44,200,000
INTL FCStone, Ltd [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	25,000,000
Payables to lenders under loans	$ 0	0
INTL FCStone Financial Inc. Uncommitted Lines of Credit [Domain]
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate at Period End	2.88%
Payables to lenders under loans	$ 14,000,000	34,000,000
INTL FCStone Financial Inc. Uncommitted Sterne Legacy BNYM Line of Credit [Domain]
Line of Credit Facility [Line Items]
Payables to lenders under loans		0
INTL FCStone Financial Inc. Uncommitted Sterne BMO Line [Domain]
Line of Credit Facility [Line Items]
Payables to lenders under loans	0	$ 11,000,000
Main line of credit facilities [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	594,500,000
Line of Credit Facility, Current Borrowing Capacity	$ 362,000,000
Base Rate [Member] | HCO Syndicated line of credit facility [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.00%
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] | INTL FCStone Financial Inc. Uncommitted Commodities Delivery Line [Domain]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.50%
Base Rate [Member] | Sterne Agee [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.00%
Base Rate [Member] | Rabobank Uncommited Line of Credit [Domain]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.00%
Base Rate [Member] | INTL FCStone, Ltd [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.50%
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%