Quarterly Financial Information (Unaudited) (Details) (USD $) In Millions, except Per Share data, 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
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 10,655.7	$ 7,005.1	$ 8,452.9	$ 7,908.7	$ 8,927.5	$ 9,659.1	$ 11,769.3	$ 12,137.5	$ 34,022.4	$ 42,493.4	$ 66,693.4
Cost of sales of physical commodities	10,525.1	6,886.9	8,323.7	7,795.8	8,815.6	9,536.1	11,654.4	12,019.1	33,531.5	42,025.2	66,245.3
Operating revenues	130.6	118.2	129.2	112.9	111.9	123.0	114.9	118.4	490.9	468.2	448.1
Transaction-based clearing 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
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
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 expense	2.0	0.3	2.6	1.5	(3.3)	2.0	0	3.9	6.4	2.6	5.5
Net income from continuing operations	5.8	3.7	7.7	2.4	1.7	5.1	1.7	10.1	19.6	18.6	17.0
(Loss) income from discontinued operations, net of tax	0	(0.2)	(0.2)	0.1	(0.5)	(1.8)	(0.2)	3.2	(0.3)	0.7	(4.3)
Net income	$ 5.8	$ 3.5	$ 7.5	$ 2.5	$ 1.2	$ 3.3	$ 1.5	$ 13.3	$ 19.3	$ 19.3	$ 12.7
Net basic earnings per share	$ 0.30	$ 0.19	$ 0.39	$ 0.13	$ 0.05	$ 0.18	$ 0.08	$ 0.70	$ 1.01	$ 1.01	$ 0.67
Net diluted earnings per share	$ 0.29	$ 0.18	$ 0.39	$ 0.12	$ 0.04	$ 0.17	$ 0.08	$ 0.68	$ 0.98	$ 0.97	$ 0.64