Quarterly Financial Information (Unaudited) Quarterly Financial Information (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended			3 Months Ended								12 Months Ended		3 Months Ended							12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2013 As Reported [Member]	Jun. 30, 2013 As Reported [Member]	Mar. 31, 2013 As Reported [Member]	Dec. 31, 2012 As Reported [Member]	Sep. 30, 2012 As Reported [Member]	Jun. 30, 2012 As Reported [Member]	Mar. 31, 2012 As Reported [Member]	Dec. 31, 2011 As Reported [Member]	Sep. 30, 2012 As Reported [Member]	Sep. 30, 2011 As Reported [Member]	Jun. 30, 2013 As Restated [Member]	Mar. 31, 2013 As Restated [Member]	Dec. 31, 2012 As Restated [Member]	Sep. 30, 2012 As Restated [Member]	Jun. 30, 2012 As Restated [Member]	Mar. 31, 2012 As Restated [Member]	Dec. 31, 2011 As Restated [Member]	Sep. 30, 2012 As Restated [Member]	Sep. 30, 2011 As Restated [Member]
Total revenues	$ 43,768.3	$ 67,959.3	$ 72,891.3	$ 9,030.4	$ 11,039.5	$ 13,113.1	$ 12,719.5	$ 17,668.1	$ 17,351.1	$ 16,951.0	$ 17,290.4	$ 69,260.6	$ 75,497.6	$ 9,996.6	$ 12,188.5	$ 12,552.8	$ 17,369.1	$ 17,192.9	$ 16,808.0	$ 16,589.3	$ 67,959.3	$ 72,891.3
Cost of sales of physical commodities	43,289.9	67,505.1	72,472.2	8,917.8	10,917.4	12,995.8	12,593.8	17,550.1	17,227.3	16,831.4	17,194.1	68,802.9	75,074.4	9,873.8	12,071.2	12,427.1	17,251.2	17,071.4	16,689.5	16,493.0	67,505.1	72,472.2
Operating revenues	478.4	454.2	419.1	112.6	122.1	117.3	125.7	118.0	123.8	119.6	96.3	457.7	423.2	122.8	117.3	125.7	117.9	121.5	118.5	96.3	454.2	419.1
Floor Brokerage, Exchange and Clearance Fees	110.1	105.4	75.6	28.0	28.9	28.6	24.6	26.0	30.3	26.9	22.2			28.9	28.6	24.6	26.0	30.3	26.9	22.2
Introducing broker commissions	40.5	31.0	24.0	11.5	11.0	9.6	8.4	9.3	7.9	8.0	5.8			11.0	9.6	8.4	9.3	7.9	8.0	5.8
Interest expense	12.5	11.6	11.3	2.9	3.2	2.9	3.5	3.3	2.6	3.6	2.1			3.2	2.9	3.5	3.3	2.6	3.6	2.1
Net operating revenues	315.3	306.2	308.2	70.2	79.0	76.2	89.2	79.4	83.0	81.1	66.2	309.7	312.3	79.7	76.2	89.2	79.3	80.7	80.0	66.2	306.2	308.2
Compensation and other expenses	292.7	290.4	252.8	72.7	74.9	74.8	70.4	68.9	77.1	77.5	66.9			74.8	74.8	70.4	68.9	77.1	77.5	66.9
Income from continuing operations, before tax	22.6	15.8	55.4	(2.5)	4.1	1.4	18.8	10.5	5.9	3.6	(0.7)	19.3	59.5	4.9	1.4	18.8	10.4	3.6	2.5	(0.7)	15.8	55.4
Income tax expense	3.3	3.1	20.9	(3.7)	1.3	(0.1)	5.5	2.2	1.2	1.2	(0.2)	4.4	22.5	1.6	(0.1)	5.5	2.1	0.4	0.8	(0.2)	3.1	20.9
Net income	19.3	12.7	34.5	1.2	2.8	1.5	13.3	8.3	4.7	2.4	(0.5)			3.3	1.5	13.3	8.3	3.2	1.7	(0.5)
Net Income (Loss) Attributable to Noncontrolling Interest	0	0.1	0.1					0	0	0	0.1						0	0	0	0.1
Net Income (Loss) Attributable to Parent	$ 19.3	$ 12.8	$ 34.8					$ 8.3	$ 4.7	$ 2.4	$ (0.4)	$ 15.0	$ 37.3				$ 8.3	$ 3.2	$ 1.7	$ (0.4)	$ 12.8	$ 34.8
Net basic earnings per share	$ 1.01	$ 0.67	$ 1.93	$ 0.05	$ 0.15	$ 0.08	$ 0.70	$ 0.44	$ 0.24	$ 0.13	$ (0.02)	$ 0.79	$ 2.07	$ 0.18	$ 0.08	$ 0.70	$ 0.44	$ 0.16	$ 0.09	$ (0.02)	$ 0.67	$ 1.93
Net diluted earnings per share	$ 0.97	$ 0.64	$ 1.83	$ 0.04	$ 0.15	$ 0.08	$ 0.68	$ 0.42	$ 0.23	$ 0.12	$ (0.02)	$ 0.75	$ 1.96	$ 0.17	$ 0.08	$ 0.68	$ 0.42	$ 0.15	$ 0.09	$ (0.02)	$ 0.64	$ 1.83