Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 2,777.6	$ 4,868.5	$ 3,708.9	$ 3,399.9	$ 2,628.4	$ 3,995.1	$ 14,442.0	$ 13,627.7	$ 14,754.9	$ 34,693.2	$ 34,022.4
Cost of sales of physical commodities	2,599.0	4,693.5	3,542.8	3,248.6	2,449.7	3,843.5	14,285.5	13,490.2	14,083.9	34,068.9	33,531.5
Operating revenues	178.6	175.0	166.1	151.3	178.7	151.6	156.5	137.5	671.0	624.3	490.9
Transaction-based clearing expenses	32.0	35.2	32.9	29.8	31.3	30.2	31.8	29.4	129.9	122.7	108.5
Introducing broker commissions	28.1	14.8	13.2	12.8	15.1	13.1	12.3	12.2	68.9	52.7	49.9
Interest expense	7.5	7.7	7.1	6.0	5.0	4.9	4.5	2.7	28.3	17.1	10.5
Net operating revenues	111.0	117.3	112.9	102.7	127.3	103.4	107.9	93.2	443.9	431.8	322.0
Compensation and other expenses	98.0	95.9	92.9	90.6	98.1	86.2	89.8	79.6	377.4	353.7	296.0
Gain on acquisition	6.2	0.0	0.0	0.0					6.2	0.0	0.0
Income before tax	19.2	21.4	20.0	12.1	29.2	17.2	18.1	13.6	72.7	78.1	26.0
Income tax expense	2.4	6.8	5.5	3.3	8.1	5.0	5.1	4.2	18.0	22.4	6.4
Net income	$ 16.8	$ 14.6	$ 14.5	$ 8.8	$ 21.1	$ 12.2	$ 13.0	$ 9.4	$ 54.7	$ 55.7	$ 19.3
Net basic earnings per share	$ 0.91	$ 0.79	$ 0.77	$ 0.47	$ 1.12	$ 0.64	$ 0.68	$ 0.50	$ 2.94	$ 2.94	$ 1.01
Net diluted earnings per share	$ 0.90	$ 0.78	$ 0.76	$ 0.46	$ 1.09	$ 0.62	$ 0.67	$ 0.49	$ 2.90	$ 2.87	$ 0.98