Condensed Parent Only Financial Statements - Cash Flows (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Condensed Financial Statements, Captions [Line Items]
Net income attributable to INTL FCStone Inc. common stockholders	$ 19.3	$ 19.3	$ 12.8
Depreciation and amortization	7.3	8.0	7.2
Bad debts and impairments	5.5	0.8	1.5
Deferred Income Tax Expense (Benefit)	(5.7)	(7.7)	(0.5)
Amortization of Financing Costs and Discounts	1.0	1.2	1.7
Share-based Compensation	4.3	9.3	5.9
Notes receivable, net	(27.9)	66.6	(77.9)
Income taxes receivable	4.2	(0.2)	(3.1)
Financial instruments owned, at fair value	(42.6)	3.1	58.3
Other assets	0.1	4.3	(1.6)
Increase (Decrease) in Accounts Payable and Accrued Liabilities	1.9	(0.2)	10.6
Increase (Decrease) in Payables to Customers	191.5	24.0	248.0
Increase (Decrease) in Financial Instruments Sold, Not yet Purchased	84.1	4.6	(215.5)
Net Cash Provided by (Used in) Operating Activities	132.6	44.9	(92.1)
Payments to Acquire Property, Plant, and Equipment	(4.3)	(4.9)	(8.7)
Net Cash Provided by (Used in) Investing Activities	(4.3)	5.1	(20.0)
Net change in payable to lenders under loans	(38.5)	(157.2)	140.8
Proceeds from Issuance of Senior Long-term Debt	0	45.5	0
Payments related to earn-outs on acquisitions	(1.6)	(12.0)	(9.6)
Share repurchase	(9.7)	(3.9)	(4.0)
Debt issuance costs	(0.3)	(3.7)	(0.3)
Exercise of stock options	1.4	1.5	1.9
Excess Tax Benefit (Tax Deficiency) from Share-based Compensation, Financing Activities	(0.1)	0.1	0.2
Net Cash Provided by (Used in) Financing Activities	(48.8)	(129.7)	129.0
Cash and Cash Equivalents, Period Increase (Decrease)	75.2	(80.2)	15.7
Cash and Cash Equivalents, at Carrying Value	231.3	156.1	236.3
Cash paid for interest	9.6	8.9	7.9
Income taxes paid, net of cash refunds	3.0	10.2	6.1
Additional consideration payable related to acquisitions	(1.8)	8.2	2.1
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net income attributable to INTL FCStone Inc. common stockholders	(26.2)	(24.6)	(14.5)
Depreciation and amortization	1.8	1.5	1.2
Bad debts and impairments	0.1	0	2.2
Deferred Income Tax Expense (Benefit)	(9.6)	(6.1)	(0.1)
Amortization of Financing Costs and Discounts	0.8	0.5	0.5
Share-based Compensation	4.3	9.3	5.9
Increase (Decrease) Due from Affiliates	84.6	(2.8)	21.7
Notes receivable, net	(12.8)	(17.1)	(10.2)
Income taxes receivable	4.6	(3.2)	7.1
Financial instruments owned, at fair value	0	1.5	1.7
Other assets	(1.1)	1.7	(0.3)
Increase (Decrease) in Accounts Payable and Accrued Liabilities	(1.1)	1.4	(2.2)
Increase (Decrease) in Payables to Customers	7.1	18.0	0.7
Increase (Decrease) in Financial Instruments Sold, Not yet Purchased	(0.6)	(24.7)	(33.4)
Net Cash Provided by (Used in) Operating Activities	51.9	(44.6)	(19.7)
Payments for Advance to Affiliate	(0.5)	(11.5)	(12.5)
Payments to Acquire Property, Plant, and Equipment	(1.8)	(0.8)	(2.7)
Net Cash Provided by (Used in) Investing Activities	(2.3)	(12.3)	(15.2)
Net change in payable to lenders under loans	(40.0)	7.0	48.0
Proceeds from Issuance of Senior Long-term Debt	0	45.5	0
Payments related to earn-outs on acquisitions	(1.1)	0	0
Share repurchase	(9.7)	(4.0)	(4.0)
Debt issuance costs	(0.2)	(3.2)	(0.1)
Exercise of stock options	1.4	1.5	1.9
Excess Tax Benefit (Tax Deficiency) from Share-based Compensation, Financing Activities	(0.1)	0.1	0.2
Net Cash Provided by (Used in) Financing Activities	(49.7)	46.9	46.0
Cash and Cash Equivalents, Period Increase (Decrease)	(0.1)	(10.0)	11.1
Cash and Cash Equivalents, at Carrying Value	3.0	3.1	13.1
Cash paid for interest	6.9	3.0	2.1
Income taxes paid, net of cash refunds	(5.3)	(1.6)	0.1
Additional consideration payable related to acquisitions	$ (3.0)	$ 5.6	$ 0