Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 14,284.9	$ 8,243.4	$ 20,366.3	$ 11,245.0	$ 11,279.6	$ 7,873.0	$ 7,192.2	$ 6,552.2	$ 54,139.6	$ 32,897.0	$ 27,622.7
Cost of sales of physical commodities	13,942.8	7,920.8	19,999.5	10,968.2	10,992.7	7,589.6	6,921.1	6,287.5	52,831.3	31,790.9	26,646.9
Operating revenues	342.1	322.6	366.8	276.8	286.9	283.4	271.1	264.7	1,308.3	1,106.1	975.8
Transaction-based clearing expenses	57.1	55.3	63.8	46.3	45.0	45.7	42.7	50.1	222.5	183.5	179.7
Introducing broker commissions	34.0	24.0	29.6	26.2	27.7	29.6	24.8	32.6	113.8	114.7	133.8
Interest expense	10.0	11.5	27.8	31.1	37.2	39.4	35.2	30.2	80.4	142.0	70.5
Interest expense on corporate funding	14.8	3.9	2.2	2.7	3.6	3.1	3.2	2.8	23.6	12.7	10.2
Net operating revenues	226.2	227.9	243.4	170.5	173.4	165.6	165.2	149.0	868.0	653.2	581.6
Compensation and benefits	145.5	132.5	136.7	104.0	105.2	100.9	97.9	89.1	518.7	393.1	337.7
Bad debts, net of recoveries and impairment	12.5	1.8	4.4	0.0	1.0	0.5	0.7	0.3	18.7	2.5	3.1
Recovery of bad debt on physical coal					(10.0)	0.0	0.0	(2.4)	0.0	(12.4)	1.0	$ 47.0
Other expenses	70.1	44.6	46.2	44.9	43.2	42.6	41.1	37.6
Total compensation and other expenses	228.1	178.9	187.3	148.9	139.4	144.0	139.7	124.6	743.2	547.7	482.1
Gain on acquisitions	81.8	0.0	0.0	0.1	0.1	0.0	5.4	0.0	81.9	5.5	2.0
Income before tax	79.9	49.0	56.1	21.7	34.1	21.6	30.9	24.4	206.7	111.0	101.5
Income tax expense	2.5	12.4	16.8	5.4	6.9	5.3	7.5	6.2	37.1	25.9	46.0
Net income (loss)	$ 77.4	$ 36.6	$ 39.3	$ 16.3	$ 27.2	$ 16.3	$ 23.4	$ 18.2	$ 169.6	$ 85.1	$ 55.5
Net basic (loss) earnings per share (in dollar per share)	$ 4.00	$ 1.90	$ 2.03	$ 0.85	$ 1.42	$ 0.85	$ 1.23	$ 0.96	$ 8.78	$ 4.46	$ 2.93
Net diluted (loss) earnings per share (in dollar per share)	$ 3.90	$ 1.87	$ 2.00	$ 0.84	$ 1.40	$ 0.84	$ 1.21	$ 0.94	$ 8.61	$ 4.39	$ 2.87
Bargain purchase gain related to acquisition	$ 81.8						$ 5.4