Segment Analysis (Details) - USD ($)	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 7,192,200,000	$ 6,507,000,000	$ 13,744,400,000	$ 14,425,600,000
Operating revenues	271,100,000	260,200,000	535,800,000	472,800,000
Net operating revenues	165,200,000	154,300,000	314,200,000	284,600,000
Net Segment Contribution	125,300,000	115,900,000	237,600,000	214,700,000
Segment Income	77,200,000	72,200,000	148,700,000	130,500,000
Costs not allocated to operating segments	51,700,000.0	42,700,000.0	98,800,000	82,400,000
Other gain	5,400,000	0	5,400,000	0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	30,900,000	29,500,000	55,300,000	48,100,000
Assets	9,407,700,000		9,407,700,000		$ 7,824,700,000
Commercial Hedging [Member]
Segment Reporting Information [Line Items]
Revenues	80,600,000.0	78,300,000.0	140,400,000	139,800,000
Operating revenues	80,600,000.0	78,300,000.0	140,400,000	139,800,000
Net operating revenues	65,800,000.0	61,900,000.0	111,100,000	111,000,000
Net Segment Contribution	49,300,000.0	44,700,000.0	79,900,000	81,200,000
Segment Income	30,200,000.0	27,600,000.0	43,500,000	48,700,000
Assets	1,704,700,000		1,704,700,000		1,935,700,000
Global Payments [Member]
Segment Reporting Information [Line Items]
Revenues	27,400,000.0	23,400,000.0	57,100,000	48,000,000
Operating revenues	27,400,000.0	23,400,000.0	57,100,000	48,000,000
Net operating revenues	25,800,000.0	21,900,000.0	54,200,000	44,900,000
Net Segment Contribution	21,000,000.0	17,600,000.0	44,100,000	36,000,000
Segment Income	15,800,000.0	13,500,000.0	34,400,000	28,100,000
Assets	234,400,000		234,400,000		206,600,000
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues	72,600,000.0	55,500,000.0	141,600,000	98,500,000
Operating revenues	72,600,000.0	55,500,000.0	141,600,000	98,500,000
Net operating revenues	32,000,000.0	27,600,000.0	64,100,000	51,100,000
Net Segment Contribution	20,100,000.0	20,300,000.0	41,900,000	38,100,000
Segment Income	11,800,000.0	12,800,000.0	27,800,000	23,800,000
Assets	5,150,000,000		5,150,000,000		3,058,200,000
Physical Commodities [Member]
Segment Reporting Information [Line Items]
Revenues	6,940,900,000.0	6,262,300,000.0	13,242,700,000	13,978,900,000
Operating revenues	19,800,000.0	15,500,000.0	34,100,000	26,100,000
Net operating revenues	15,100,000.0	12,500,000.0	25,500,000	20,900,000
Net Segment Contribution	11,500,000.0	9,400,000.0	18,600,000	15,000,000
Segment Income	7,800,000.0	5,600,000.0	13,700,000	6,700,000
Assets	406,200,000		406,200,000		413,700,000
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues	73,600,000.0	88,000,000.0	168,800,000	160,200,000
Operating revenues	73,600,000.0	88,000,000.0	168,800,000	160,200,000
Net operating revenues	31,000,000.0	31,800,000.0	68,500,000	59,200,000
Net Segment Contribution	23,400,000.0	23,900,000.0	53,100,000	44,400,000
Segment Income	11,600,000.0	12,700,000.0	29,300,000	23,200,000
Assets	1,773,100,000		1,773,100,000		2,109,900,000
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues	(2,900,000.0)	(500,000.0)	(6,200,000)	200,000
Operating revenues	(2,900,000.0)	(500,000.0)	(6,200,000)	200,000
Net operating revenues	(4,500,000.0)	$ (1,400,000.0)	(9,200,000)	$ (2,500,000)
Assets	$ 139,300,000		$ 139,300,000		$ 100,600,000