Condensed Parent Only Financial Statements - Income Statement (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Condensed Financial Statements, Captions [Line Items]
Trading gains, net									$ 244.5	$ 244.0	$ 246.8
Commission and clearing fees									180.7	173.3	161.0
Interest income									8.0	8.9	8.0
Other Income									0.7	0.9	0.3
Operating revenues	130.6	118.2	129.2	112.9	111.9	123.0	114.9	118.4	490.9	468.2	448.1
Interest expense	2.5	2.5	2.8	2.7	2.4	1.8	1.6	2.1	10.5	7.9	5.6
Net operating revenues	86.7	76.0	85.9	73.4	70.0	81.3	75.1	83.3	322.0	309.7	306.2
Non-interest expenses:
Compensation and benefits									201.9	198.7	197.2
Clearing and related expenses	27.5	28.1	27.7	25.2	28.0	28.9	28.6	24.6	108.5	110.1	105.3
Introducing broker commissions	13.9	11.6	12.8	11.6	11.5	11.0	9.6	8.4	49.9	40.5	31.0
Communication and data services									25.8	23.1	22.4
Occupancy and equipment rental									12.3	12.0	11.0
Professional fees									14.9	12.4	12.6
Travel and business development									9.9	10.4	10.4
Depreciation and amortization									7.3	8.0	7.2
Bad debts and impairments									5.5	0.8	1.5
Other									18.4	23.1	21.4
Total compensation and other expenses	78.9	72.0	75.6	69.5	71.6	74.2	73.4	69.3	296.0	288.5	283.7
Income from continuing operations, before tax	7.8	4.0	10.3	3.9	(1.6)	7.1	1.7	14.0	26.0	21.2	22.5
Income tax benefit	(2.0)	(0.3)	(2.6)	(1.5)	3.3	(2.0)	0	(3.9)	(6.4)	(2.6)	(5.5)
Net income attributable to INTL FCStone Inc. common stockholders									19.3	19.3	12.8
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Trading gains, net									9.5	7.3	7.1
Commission and clearing fees									1.6	1.6	0.2
Interest income									4.3	3.6	2.1
Other Income									0	1.9	0.1
Operating revenues									15.4	14.4	9.5
Interest expense									10.6	7.1	5.6
Net operating revenues									4.8	7.3	3.9
Non-interest expenses:
Compensation and benefits									29.8	30.5	12.4
Clearing and related expenses									0.3	0.5	0.3
Introducing broker commissions									0.3	0.5	0
Communication and data services									1.3	0.9	0.6
Occupancy and equipment rental									2.0	1.4	1.0
Professional fees									5.0	2.3	3.1
Travel and business development									1.1	1.2	1.4
Depreciation and amortization									1.8	1.5	1.2
Bad debts and impairments									0.1	0	2.2
Other									6.4	6.3	5.4
Total compensation and other expenses									48.1	45.1	27.6
Income from continuing operations, before tax									(43.3)	(37.8)	(23.7)
Income tax benefit									17.1	13.2	9.2
Net income attributable to INTL FCStone Inc. common stockholders									(26.2)	(24.6)	(14.5)
Adjustment to Investment in Subs for Equity Method Accounting									$ 45.5	$ 43.9	$ 27.3