Quarterly Financial Information Quarterly Financial Information (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Quarterly Financial Information Disclosure [Abstract]
Total Revenues	$ 17,668.1	$ 17,351.1	$ 16,951.0	$ 17,290.4	$ 24,070.4	$ 20,573.4	$ 14,583.0	$ 16,270.8	$ 69,260.6	$ 75,497.6	$ 46,940.3
Cost of sales of physical commodities	17,550.1	17,227.3	16,831.4	17,194.1	23,962.3	20,468.0	14,470.0	16,174.1	68,802.9	75,074.4	46,671.3
Operating revenues	118.0	123.8	119.6	96.3	108.1	105.4	113.0	96.7	457.7	423.2	269.0
Interest expense	3.3	2.6	3.6	2.1	2.2	2.0	3.3	3.8	11.6	11.3	9.9
Net revenues	114.7	121.2	116.0	94.2	105.9	103.4	109.7	92.9	446.1	411.9	259.1
Total non-interest expenses	104.2	115.3	112.4	94.9	92.9	86.1	86.4	87.0	426.8	352.4	241.2
Income from continuing operations, before tax	10.5	5.9	3.6	(0.7)	13.0	17.3	23.3	5.9	19.3	59.5	17.9
Income tax expense	2.2	1.2	1.2	(0.2)	5.4	7.0	8.0	2.1	4.4	22.5	6.4
Net income from continuing operations	8.3	4.7	2.4	(0.5)	7.6	10.3	15.3	3.8	14.9	37.0	11.5
Income from discontinued operations					0	0	0	(0.2)	0	0.2	0.6
Net income					7.6	10.3	15.3	4.0	14.9	37.2	5.1
Add: Net loss attributable to noncontrolling interests	0	0	0	0.1	0.1	(0.1)	(0.1)	0	0.1	0.1	0.3
Net income attributable to INTL FCStone Inc. common stockholders	$ 8.3	$ 4.7	$ 2.4	$ (0.4)	$ 7.5	$ 10.4	$ 15.4	$ 4.0	$ 15.0	$ 37.3	$ 5.4
Net basic earnings per share	$ 0.44	$ 0.24	$ 0.13	$ (0.02)	$ 0.41	$ 0.58	$ 0.87	$ 0.23	$ 0.79	$ 2.07	$ 0.31
Net diluted earnings per share	$ 0.42	$ 0.23	$ 0.12	$ (0.02)	$ 0.39	$ 0.55	$ 0.81	$ 0.22	$ 0.75	$ 1.96	$ 0.30