Quarterly Financial Information (Unaudited) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 11,279,600,000	$ 7,873,000,000	$ 7,192,200,000	$ 6,552,200,000	$ 6,078,800,000	$ 7,118,300,000	$ 6,507,000,000	$ 7,918,600,000	$ 32,897,000,000	$ 27,622,700,000	$ 29,423,600,000
Cost of sales of physical commodities	10,992,700,000	7,589,600,000	6,921,100,000	6,287,500,000	5,835,600,000	6,858,500,000	6,246,800,000	7,706,000,000	31,790,900,000	26,646,900,000	28,639,600,000
Operating revenues	286,900,000	283,400,000	271,100,000	264,700,000	243,200,000	259,800,000	260,200,000	212,600,000	1,106,100,000	975,800,000	784,000,000
Transaction-based clearing expenses	45,000,000	45,700,000	42,700,000	50,100,000	43,100,000	49,000,000	50,700,000	36,900,000	183,500,000	179,700,000	136,300,000
Introducing broker commissions	27,700,000	29,600,000	24,800,000	32,600,000	32,400,000	34,100,000	36,200,000	31,100,000	114,700,000	133,800,000	113,000,000
Interest expense	40,800,000	42,500,000	38,400,000	33,000,000	25,300,000	22,100,000	19,000,000	14,300,000	154,700,000	80,700,000	42,100,000
Net operating revenues	173,400,000	165,600,000	165,200,000	149,000,000	142,400,000	154,600,000	154,300,000	130,300,000	653,200,000	581,600,000	492,600,000
Compensation and benefits	105,200,000	100,900,000	97,900,000	89,100,000	85,400,000	86,900,000	88,200,000	77,200,000	393,100,000	337,700,000	295,700,000
Bad debts	1,000,000	500,000	700,000	300,000	1,200,000	1,600,000	200,000	100,000
(Recovery) bad debt on physical coal	(10,000,000)	0	0	(2,400,000)	0	0	0	1,000,000	(12,400,000.0)	1,000,000.0	47,000,000.0
Other	43,200,000	42,600,000	41,100,000	37,600,000	35,300,000	35,200,000	36,400,000	33,400,000	28,400,000	26,300,000	25,900,000
Total compensation and other expenses	139,400,000	144,000,000	139,700,000	124,600,000	121,900,000	123,700,000	124,800,000	111,700,000	547,700,000	482,100,000	477,400,000
Other gains	100,000	0	5,400,000	0	0	2,000,000	0	0	5,500,000	2,000,000	0
Income before tax	34,100,000	21,600,000	30,900,000	24,400,000	20,500,000	32,900,000	29,500,000	18,600,000	111,000,000	101,500,000	15,200,000
Income tax expense	6,900,000	5,300,000	7,500,000	6,200,000	4,800,000	8,900,000	6,800,000	25,500,000	25,900,000	46,000,000	8,800,000
Net income	$ 27,200,000	$ 16,300,000	$ 23,400,000	$ 18,200,000	$ 15,700,000	$ 24,000,000	$ 22,700,000	$ (6,900,000)	$ 85,100,000	$ 55,500,000	$ 6,400,000
Basic	$ 1.42	$ 0.85	$ 1.23	$ 0.96	$ 0.83	$ 1.27	$ 1.20	$ (0.37)	$ 4.46	$ 2.93	$ 0.32
Diluted	$ 1.40	$ 0.84	$ 1.21	$ 0.94	$ 0.81	$ 1.25	$ 1.18	$ (0.37)	$ 4.39	$ 2.87	$ 0.31