Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 6,078.8	$ 7,118.3	$ 6,507.0	$ 7,918.6	$ 12,382.5	$ 5,505.9	$ 5,460.8	$ 6,074.4	$ 27,622.7	$ 29,423.6	$ 14,754.9
Cost of sales of physical commodities	5,835.6	6,858.5	6,246.8	7,706.0	12,177.4	5,308.3	5,265.0	5,888.9	26,646.9	28,639.6	14,083.9
Operating revenues	243.2	259.8	260.2	212.6	205.1	197.6	195.8	185.5	975.8	784.0	671.0
Transaction-based clearing expenses	43.1	49.0	50.7	36.9	35.1	33.9	33.7	33.6	179.7	136.3	129.9
Introducing broker commissions	32.4	34.1	36.2	31.1	26.9	29.2	28.2	28.7	133.8	113.0	68.9
Interest expense	25.3	22.1	19.0	14.3	12.0	11.2	10.0	8.9	80.7	42.1	28.3
Net operating revenues	142.4	154.6	154.3	130.3	131.1	123.3	123.9	114.3	581.6	492.6	443.9
Compensation and benefits	85.4	86.9	88.2	77.2	73.0	75.5	76.6	70.6	337.7	295.7	263.9
Bad debts	1.2	1.6	0.2	0.1	0.4	0.1	1.3	2.5	3.1	4.3	4.4
Bad debt on physical coal	0.0	0.0	0.0	1.0					1.0	47.0	0.0
Other	35.3	35.2	36.4	33.4	33.2	32.7	31.7	32.8	26.3	25.9	22.3
Total compensation and other expenses	121.9	123.7	124.8	111.7	153.6	108.3	109.6	105.9	482.1	477.4	377.4
Income before tax	20.5	32.9	29.5	18.6	(22.5)	15.0	14.3	8.4	101.5	15.2	72.7
Income tax expense	4.8	8.9	6.8	25.5	1.1	2.3	3.3	2.1	46.0	8.8	18.0
Net income	$ 15.7	$ 24.0	$ 22.7	$ (6.9)	$ (23.6)	$ 12.7	$ 11.0	$ 6.3	$ 55.5	$ 6.4	$ 54.7
Net basic earnings per share	$ 0.83	$ 1.27	$ 1.20	$ (0.37)	$ (1.27)	$ 0.67	$ 0.58	$ 0.34
Net diluted earnings per share	$ 0.81	$ 1.25	$ 1.18	$ (0.37)	$ (1.27)	$ 0.66	$ 0.58	$ 0.34