Segment Analysis (Details) - USD ($)	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 7,873,000,000	$ 7,118,300,000	$ 21,617,400,000	$ 21,543,900,000
Operating revenues	283,400,000	259,800,000	819,200,000	732,600,000
Net operating revenues	165,600,000	154,600,000	479,800,000	439,200,000
Net Segment Contribution	119,600,000	114,200,000	357,200,000	328,900,000
Segment Income	69,600,000	70,400,000	218,300,000	200,900,000
Costs not allocated to operating segments	48,000,000.0	39,500,000.0	146,800,000	121,900,000
Other gain	0	2,000,000	5,400,000	2,000,000
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	21,600,000	32,900,000	76,900,000	81,000,000
Assets	10,054,900,000		10,054,900,000		$ 7,824,700,000
Commercial Hedging [Member]
Segment Reporting Information [Line Items]
Revenues	86,400,000.0	77,900,000.0	226,800,000	217,700,000
Operating revenues	86,400,000.0	77,900,000.0	226,800,000	217,700,000
Net operating revenues	66,900,000.0	61,000,000.0	178,000,000	172,000,000
Net Segment Contribution	48,000,000.0	44,300,000.0	127,900,000	125,500,000
Segment Income	29,700,000.0	25,300,000.0	73,200,000	74,000,000
Assets	2,084,400,000		2,084,400,000		1,935,700,000
Global Payments [Member]
Segment Reporting Information [Line Items]
Revenues	28,900,000.0	26,000,000.0	86,000,000	74,000,000
Operating revenues	28,900,000.0	26,000,000.0	86,000,000	74,000,000
Net operating revenues	27,500,000.0	24,700,000.0	81,700,000	69,600,000
Net Segment Contribution	22,300,000.0	19,900,000.0	66,400,000	55,900,000
Segment Income	17,000,000.0	16,000,000.0	51,400,000	44,100,000
Assets	262,700,000		262,700,000		206,600,000
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues	74,200,000.0	49,900,000.0	215,800,000	148,400,000
Operating revenues	74,200,000.0	49,900,000.0	215,800,000	148,400,000
Net operating revenues	30,300,000.0	23,600,000.0	94,400,000	74,700,000
Net Segment Contribution	17,900,000.0	17,500,000.0	59,800,000	55,600,000
Segment Income	8,200,000.0	10,300,000.0	36,000,000	34,100,000
Assets	5,178,100,000		5,178,100,000		3,058,200,000
Physical Commodities [Member]
Segment Reporting Information [Line Items]
Revenues	7,604,300,000.0	6,873,400,000.0	20,847,000,000	20,852,300,000
Operating revenues	14,700,000.0	14,900,000.0	48,800,000	41,000,000
Net operating revenues	9,700,000.0	11,400,000.0	35,200,000	32,300,000
Net Segment Contribution	6,400,000.0	8,100,000.0	25,000,000	23,100,000
Segment Income	2,800,000.0	5,100,000.0	16,500,000	11,800,000
Assets	488,000,000		488,000,000		413,700,000
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues	78,900,000.0	88,900,000.0	247,700,000	249,100,000
Operating revenues	78,900,000.0	88,900,000.0	247,700,000	249,100,000
Net operating revenues	32,500,000.0	32,700,000.0	101,000,000	91,900,000
Net Segment Contribution	25,000,000.0	24,400,000.0	78,100,000	68,800,000
Segment Income	11,900,000.0	13,700,000.0	41,200,000	36,900,000
Assets	1,914,000,000		1,914,000,000		2,109,900,000
Corporate [Member]
Segment Reporting Information [Line Items]
Net operating revenues	(1,300,000.0)	$ 1,200,000.0	(10,500,000)	$ (1,300,000)
Assets	$ 127,700,000		$ 127,700,000		$ 100,600,000