Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended									12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 12,382.5	$ 5,505.9	$ 5,460.8	$ 6,074.4	$ 2,777.6	$ 4,868.5	$ 3,708.9	$ 3,399.9		$ 29,423.6	$ 14,754.9	$ 34,693.2
Cost of sales of physical commodities	12,177.4	5,308.3	5,265.0	5,888.9	2,599.0	4,693.5	3,542.8	3,248.6		28,639.6	14,083.9	34,068.9
Operating revenues	205.1	197.6	195.8	185.5	178.6	175.0	166.1	151.3		784.0	671.0	624.3
Transaction-based clearing expenses	35.1	33.9	33.7	33.6	32.0	35.2	32.9	29.8		136.3	129.9	122.7
Introducing broker commissions	26.9	29.2	28.2	28.7	28.1	14.8	13.2	12.8		113.0	68.9	52.7
Interest expense	12.0	11.2	10.0	8.9	7.5	7.7	7.1	6.0		42.1	28.3	17.1
Net operating revenues	131.1	123.3	123.9	114.3	111.0	117.3	112.9	102.7		492.6	443.9	431.8
Compensation and benefits	73.0	75.5	76.6	70.6	66.2	69.4	65.2	63.1		295.7	263.9	251.1
Bad debts	0.4	0.1	1.3	2.5	(0.2)	0.0	2.6	2.0		4.3	4.4	7.3
Provision for bad debt on physical coal	47.0	0.0	0.0	0.0						47.0	0.0	0.0
Other	33.2	32.7	31.7	32.8	32.0	26.5	25.1	25.5		37.5	29.4	23.5
Total compensation and other expenses	153.6	108.3	109.6	105.9	98.0	95.9	92.9	90.6		477.4	377.4	353.7
Gain on acquisition					6.2	0.0	0.0	0.0		0.0	6.2	0.0
Income before tax	(22.5)	15.0	14.3	8.4	19.2	21.4	20.0	12.1		15.2	72.7	78.1
Income tax expense	1.1	2.3	3.3	2.1	2.4	6.8	5.5	3.3		8.8	18.0	22.4
Net income	$ (23.6)	$ 12.7	$ 11.0	$ 6.3	$ 16.8	$ 14.6	$ 14.5	$ 8.8		$ 6.4	$ 54.7	$ 55.7
Net basic earnings per share	$ (1.27)	$ 0.67	$ 0.58	$ 0.34	$ 0.91	$ 0.79	$ 0.77	$ 0.47
Net diluted earnings per share	$ (1.27)	$ 0.66	$ 0.58	$ 0.34	$ 0.90		$ 0.76	$ 0.46	$ 0.78