Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 2,628.4	$ 3,995.1	$ 14,442.0	$ 13,627.7	$ 10,655.7	$ 7,005.1	$ 8,452.9	$ 7,908.7	$ 34,693.2	$ 34,022.4	$ 42,493.4
Cost of sales of physical commodities	2,449.7	3,843.5	14,285.5	13,490.2	10,525.1	6,886.9	8,323.7	7,795.8	34,068.9	33,531.5	42,025.2
Operating revenues	178.7	151.6	156.5	137.5	130.6	118.2	129.2	112.9	624.3	490.9	468.2
Transaction-based clearing expenses	31.3	30.2	31.8	29.4	27.5	28.1	27.7	25.2	122.7	108.5	110.1
Introducing broker commissions	15.1	13.1	12.3	12.2	13.9	11.6	12.8	11.6	52.7	49.9	40.5
Interest expense	5.0	4.9	4.5	2.7	2.5	2.5	2.8	2.7	17.1	10.5	7.9
Net operating revenues	127.3	103.4	107.9	93.2	86.7	76.0	85.9	73.4	431.8	322.0	309.7
Compensation and other expenses	98.1	86.2	89.8	79.6	78.9	72.0	75.6	69.5	353.7	296.0	288.5
Income from continuing operations, before tax	29.2	17.2	18.1	13.6	7.8	4.0	10.3	3.9	78.1	26.0	21.2
Income tax expense	8.1	5.0	5.1	4.2	2.0	0.3	2.6	1.5	22.4	6.4	2.6
Net income from continuing operations					5.8	3.7	7.7	2.4	55.7	19.6	18.6
(Loss) income from discontinued operations, net of tax					0.0	(0.2)	(0.2)	0.1	0.0	(0.3)	0.7
Net income	$ 21.1	$ 12.2	$ 13.0	$ 9.4	$ 5.8	$ 3.5	$ 7.5	$ 2.5	$ 55.7	$ 19.3	$ 19.3
Net basic earnings per share	$ 1.12	$ 0.64	$ 0.68	$ 0.50	$ 0.30	$ 0.19	$ 0.39	$ 0.13	$ 2.94	$ 1.01	$ 1.01
Net diluted earnings per share	$ 1.09	$ 0.62	$ 0.67	$ 0.49	$ 0.29	$ 0.18	$ 0.39	$ 0.12	$ 2.87	$ 0.98	$ 0.97