Retirement Plans (Notes)	12 Months Ended
Sep. 30, 2017
Retirement Benefits [Abstract]
Pension and Other Postretirement Benefits Disclosure [Text Block]	Retirement Plans Defined Benefit Retirement Plans The Company has a frozen defined benefit pension plan (the “Plan”) and recognizes its funded status, measured as the difference between the fair value of the plan assets and the projected benefit obligation, in “accounts payable and other accrued liabilities” in the consolidated balance sheets. Plan assets, which are managed in a third-party trust, primarily consist of a diversified blend of approximately 80% debt securities and 20% equity investments and had a total fair value of $36.4 million and $33.7 million as of September 30, 2017 and 2016, respectively. All plan assets fall within Level 2 of the fair value hierarchy. The benefit obligation associated with the Plan will vary over time only as a result of changes in market interest rates, the life expectancy of the plan participants, and benefit payments, since the accrual of benefits was suspended when the Plan was frozen in 2006. The benefit obligation was $36.5 million and $38.5 million and the discount rate assumption used in the measurement of this obligation was 3.75% and 3.60% as of September 30, 2017 and 2016, respectively. The Company’s unfunded pension obligation was $0.1 million and $4.8 million as of September 30, 2017 and 2016, respectively. The Company recognized a net periodic benefit $0.3 million for the year ended September 30, 2017. The net periodic benefit cost associated with the Plan was $0.2 million for the year ended September 30, 2016 and less than $0.1 million for the year ended September 30, 2015. The expected long-term return on plan assets assumption is 6.00% for 2017. The Company made contributions of $2.0 million and $1.8 million to the Plan in the years ended September 30, 2017 and 2016, respectively. The Company complies with minimum funding requirements. The estimated undiscounted future benefit payments are expected to be $2.1 million in 2018, $2.1 million in 2019, $2.0 million in 2020, $2.0 million in 2021, $1.9 million in 2022 and $9.5 million in 2023 through 2027. Defined Contribution Retirement Plans The Company offers participation in the INTL FCStone Inc. 401(k) Plan (“401(k) Plan”), a defined contribution plan providing retirement benefits, to all domestic employees who have reached 21 years of age, and provided four months of service to the Company. Employees may contribute from 1% to 80% of their annual compensation to the 401(k) Plan, limited to a maximum annual amount as set periodically by the Internal Revenue Service. The Company makes matching contributions to the 401(k) Plan in an amount equal to 62.5% of each participant’s eligible elective deferral contribution to the 401(k) Plan, up to 8% of employee compensation. Matching contributions vest, by participant, based on the following years of service schedule: less than two years – none, after two years – 33%, after three years – 66%, and after four years – 100%. U.K. based employees of INTL FCStone are eligible to participate in a defined contribution pension plan. The Company contributes double the employee’s contribution up to 10% of total base salary for this plan. For this plan, employees are 100% vested in both the employee and employer contributions at all times. For fiscal years ended September 30, 2017, 2016, and 2015, the Company’s contribution to these defined contribution plans were $6.1 million, $5.3 million and $5.1 million, respectively.

Retirement Plans

Defined Benefit Retirement Plans

The Company has a frozen defined benefit pension plan (the “Plan”) and recognizes its funded status, measured as the difference between the fair value of the plan assets and the projected benefit obligation, in “accounts payable and other accrued liabilities” in the consolidated balance sheets. Plan assets, which are managed in a third-party trust, primarily consist of a diversified blend of approximately 80% debt securities and 20% equity investments and had a total fair value of $36.4 million and $33.7 million as of September 30, 2017 and 2016, respectively. All plan assets fall within Level 2 of the fair value hierarchy. The benefit obligation associated with the Plan will vary over time only as a result of changes in market interest rates, the life expectancy of the plan participants, and benefit payments, since the accrual of benefits was suspended when the Plan was frozen in 2006. The benefit obligation was $36.5 million and $38.5 million and the discount rate assumption used in the measurement of this obligation was 3.75% and 3.60% as of September 30, 2017 and 2016, respectively. The Company’s unfunded pension obligation was $0.1 million and $4.8 million as of September 30, 2017 and 2016, respectively.

The Company recognized a net periodic benefit $0.3 million for the year ended September 30, 2017. The net periodic benefit cost associated with the Plan was $0.2 million for the year ended September 30, 2016 and less than $0.1 million for the year ended September 30, 2015. The expected long-term return on plan assets assumption is 6.00% for 2017. The Company made contributions of $2.0 million and $1.8 million to the Plan in the years ended September 30, 2017 and 2016, respectively. The Company complies with minimum funding requirements. The estimated undiscounted future benefit payments are expected to be $2.1 million in 2018, $2.1 million in 2019, $2.0 million in 2020, $2.0 million in 2021, $1.9 million in 2022 and $9.5 million in 2023 through 2027.

Defined Contribution Retirement Plans

The Company offers participation in the INTL FCStone Inc. 401(k) Plan (“401(k) Plan”), a defined contribution plan providing retirement benefits, to all domestic employees who have reached 21 years of age, and provided four months of service to the Company. Employees may contribute from 1% to 80% of their annual compensation to the 401(k) Plan, limited to a maximum annual amount as set periodically by the Internal Revenue Service. The Company makes matching contributions to the 401(k) Plan in an amount equal to 62.5% of each participant’s eligible elective deferral contribution to the 401(k) Plan, up to 8% of employee compensation. Matching contributions vest, by participant, based on the following years of service schedule: less than two years – none, after two years – 33%, after three years – 66%, and after four years – 100%.

U.K. based employees of INTL FCStone are eligible to participate in a defined contribution pension plan. The Company contributes double the employee’s contribution up to 10% of total base salary for this plan. For this plan, employees are 100% vested in both the employee and employer contributions at all times.

For fiscal years ended September 30, 2017, 2016, and 2015, the Company’s contribution to these defined contribution plans were $6.1 million, $5.3 million and $5.1 million, respectively.