Condensed Parent Only Financial Statements [Schedule] Cash Flows (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Condensed Financial Statements, Captions [Line Items]
Net income attributable to INTL FCStone Inc. common stockholders	$ 19.3	$ 12.8	$ 34.8
Depreciation, Depletion and Amortization, Nonproduction	8.0	7.2	4.7
Deferred Income Tax Expense (Benefit)	(7.7)	(0.5)	1.8
Amortization of Financing Costs and Discounts	1.2	1.7	1.6
Convertible debt interest settled in company stock upon conversion	0	0	0.2
Share-based Compensation	9.3	5.9	2.3
Gain on acquisition of INTL Provident	0	0	(0.4)
Notes receivable, net	66.6	(77.9)	2.9
Income taxes receivable	(0.2)	(3.1)	0.6
Financial instruments owned, at fair value	3.1	58.3	280.7
Other assets	4.3	(1.6)	(12.1)
Increase (Decrease) in Accounts Payable and Accrued Liabilities	(0.2)	10.6	30.8
Increase (Decrease) in Payables to Customers	24.0	248.0	366.2
Increase (Decrease) in Financial Instruments Sold, Not yet Purchased	4.6	(215.5)	202.9
Net Cash Provided by (Used in) Operating Activities	44.9	(92.1)	209.0
Cash paid for other acquisitions, net	0	(11.7)	(9.3)
Payments to Acquire Property, Plant, and Equipment	(4.9)	(8.7)	(10.1)
Net Cash Provided by (Used in) Investing Activities	5.1	(20.0)	(21.5)
Net change in payable to lenders under loans	(157.2)	140.8	(37.5)
Proceeds from Issuance of Senior Long-term Debt	45.5	0	0
Share repurchase	(3.9)	(4.0)	0
Debt issuance costs	(3.7)	(0.3)	(2.4)
Exercise of stock options	1.5	1.9	1.4
Excess Tax Benefit (Tax Deficiency) from Share-based Compensation, Financing Activities	0.1	0.2	0
Net Cash Provided by (Used in) Financing Activities	(129.7)	129.0	(48.4)
Cash and Cash Equivalents, Period Increase (Decrease)	(80.2)	15.7	138.7
Cash and Cash Equivalents, at Carrying Value	156.1	236.3	220.6
Cash paid for interest	8.9	7.9	9.5
Income taxes paid, net of cash refunds	10.2	6.1	17.6
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net income attributable to INTL FCStone Inc. common stockholders	(24.6)	(14.5)	(10.0)
Depreciation, Depletion and Amortization, Nonproduction	1.5	1.2	0.6
Tangible Asset Impairment Charges	0	2.2	0
Deferred Income Tax Expense (Benefit)	(6.1)	(0.1)	1.1
Amortization of Financing Costs and Discounts	0.5	0.5	0.2
Convertible debt interest settled in company stock upon conversion	0	0	0.2
Share-based Compensation	9.3	5.9	2.3
Gain on acquisition of INTL Provident	0	0	(0.4)
Increase (Decrease) Due from Affiliates	(2.8)	21.7	35.7
Notes receivable, net	(17.1)	(10.2)	0
Income taxes receivable	(3.2)	7.1	(13.6)
Financial instruments owned, at fair value	1.5	1.7	(0.4)
Other assets	1.7	(0.3)	(0.8)
Increase (Decrease) in Accounts Payable and Accrued Liabilities	1.4	(2.2)	1.7
Increase (Decrease) in Payables to Customers	18.0	0.7	0
Increase (Decrease) in Financial Instruments Sold, Not yet Purchased	(24.7)	(33.4)	(0.5)
Net Cash Provided by (Used in) Operating Activities	(44.6)	(19.7)	16.1
Payments for Advance to Affiliate	(11.5)	(12.5)	(1.0)
Payments to Acquire Property, Plant, and Equipment	(0.8)	(2.7)	(1.7)
Net Cash Provided by (Used in) Investing Activities	(12.3)	(15.2)	(2.7)
Net change in payable to lenders under loans	7.0	48.0	(11.9)
Proceeds from Issuance of Senior Long-term Debt	45.5	0	0
Share repurchase	(4.0)	(4.0)	0
Debt issuance costs	(3.2)	(0.1)	(1.2)
Exercise of stock options	1.5	1.9	1.3
Excess Tax Benefit (Tax Deficiency) from Share-based Compensation, Financing Activities	0.1	0.2	0
Net Cash Provided by (Used in) Financing Activities	46.9	46.0	(11.8)
Cash and Cash Equivalents, Period Increase (Decrease)	(10.0)	11.1	1.6
Cash and Cash Equivalents, at Carrying Value	3.1	13.1	2.0
Cash paid for interest	3.0	2.1	3.0
Income taxes paid, net of cash refunds	(1.6)	0.1	10.8
Convertible note conversions	$ 0	$ 0	$ 16.7