Condensed Parent Only Financial Statements [Schedule] Income Statement (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]
Trading gains, net	$ 265.0	$ 244.9	$ 201.6
Commission and clearing fees	173.3	161.0	134.5
Interest income	10.3	10.3	10.3
Other Income	0.9	0.5	1.0
Operating revenues	478.4	454.2	419.1
Interest expense	12.5	11.6	11.3
Net operating revenues	315.3	306.2	308.2
Non-interest expenses:
Compensation and benefits	201.6	202.4	176.6
Clearing and related expenses	110.1	105.4	75.6
Introducing broker commissions	40.5	31.0	24.0
Communication and data services	23.3	22.6	15.5
Occupancy and equipment rental	12.0	11.0	8.9
Professional fees	13.1	12.9	10.6
Travel and business development	10.7	11.2	8.9
Depreciation and amortization	8.0	7.2	4.7
Bad debts and impairments	0.8	1.5	6.2
Other	23.2	21.6	21.4
Total compensation and other expenses	292.7	290.4	252.8
Income from continuing operations, before tax	22.6	15.8	55.4
Income tax benefit	(3.3)	(3.1)	(20.9)
Net income attributable to INTL FCStone Inc. common stockholders	19.3	12.8	34.8
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Trading gains, net	7.2	7.1	3.1
Commission and clearing fees	1.6	0.2	0
Interest income	3.6	2.1	4.3
Other Income	2.0	0.1	0.4
Operating revenues	14.4	9.5	7.8
Interest expense	7.1	5.6	9.7
Net operating revenues	7.3	3.9	(1.9)
Non-interest expenses:
Compensation and benefits	30.5	12.4	8.6
Clearing and related expenses	0.5	0.3	0.1
Introducing broker commissions	0.5	0	0.2
Communication and data services	0.9	0.6	0.5
Occupancy and equipment rental	1.4	1.0	1.2
Professional fees	2.3	3.1	3.8
Travel and business development	1.2	1.4	0.5
Depreciation and amortization	1.5	1.2	0.6
Bad debts and impairments	0	2.2	0
Other	6.3	5.4	(1.0)
Total compensation and other expenses	45.1	27.6	14.5
Income from continuing operations, before tax	(37.8)	(23.7)	(16.4)
Income tax benefit	13.2	9.2	6.4
Net income attributable to INTL FCStone Inc. common stockholders	(24.6)	(14.5)	(10.0)
Adjustment to Investment in Subs for Equity Method Accounting	$ 43.9	$ 27.3	$ 44.8