Condensed Consolidated Income Statements - USD ($)	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenues:
Revenues	$ 7,873,000,000	$ 7,118,300,000	$ 21,617,400,000	$ 21,543,900,000
Cost of sales of physical commodities	7,589,600,000	6,858,500,000	20,798,200,000	20,811,300,000
Operating revenues	283,400,000	259,800,000	819,200,000	732,600,000
Transaction-based clearing expenses	45,700,000	49,000,000	138,500,000	136,600,000
Introducing broker commissions	29,600,000	34,100,000	87,000,000	101,400,000
Interest expense	42,500,000	22,100,000	113,900,000	55,400,000
Net operating revenues	165,600,000	154,600,000	479,800,000	439,200,000
Compensation and other expenses:
Compensation and benefits	100,900,000	86,900,000	287,900,000	252,300,000
Trading systems and market information	9,800,000	8,600,000	28,500,000	25,700,000
Occupancy and equipment rental	5,000,000	4,200,000	14,400,000	12,500,000
Professional fees	5,800,000	4,800,000	16,100,000	13,400,000
Travel and business development	4,000,000	3,700,000	11,800,000	10,200,000
Non-trading technology and support	5,800,000	3,800,000	15,000,000	10,300,000
Depreciation and amortization	3,500,000	2,800,000	9,600,000	8,400,000
Communications	1,600,000	1,300,000	4,900,000	4,100,000
Provision for Doubtful Accounts	500,000	1,600,000	1,500,000	1,900,000
(Recovery) bad debt on physical coal	0	0	(2,400,000.0)	1,000,000.0
Other	7,100,000	6,000,000	21,000,000	20,400,000
Total compensation and other expenses	144,000,000	123,700,000	408,300,000	360,200,000
Other gain	0	2,000,000	5,400,000	2,000,000
Income before tax	21,600,000	32,900,000	76,900,000	81,000,000
Income tax expense	5,300,000	8,900,000	19,000,000	41,200,000
Net income	$ 16,300,000	$ 24,000,000	$ 57,900,000	$ 39,800,000
Earnings per share:
Basic	$ 0.85	$ 1.27	$ 3.04	$ 2.10
Diluted	$ 0.84	$ 1.25	$ 2.99	$ 2.06
Weighted-average number of common shares outstanding:
Basic	18,781,401	18,597,165	18,731,203	18,524,846
Diluted	19,011,526	18,976,898	19,003,720	18,876,259
Interest Income [Member]
Revenues:
Revenues	$ 53,200,000	$ 33,700,000	$ 146,400,000	$ 85,600,000
Other Income [Member]
Revenues:
Revenues	20,700,000.0	18,400,000.0	58,900,000.0	53,400,000.0
Commission and Clearing Fees [Member]
Revenues:
Revenues	97,500,000	105,100,000	282,100,000	298,000,000
Principal or Proprietary Transactions [Member]
Revenues:
Revenues	102,300,000	94,900,000	305,400,000	270,500,000
Sales [Member]
Revenues:
Revenues	$ 7,599,300,000	$ 6,866,200,000	$ 20,824,600,000	$ 20,836,400,000