Segment Analysis (Details) - USD ($)	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenues	$ 8,243,400,000	$ 7,873,000,000.0	$ 39,854,700,000	$ 21,617,400,000
Operating revenues	322,600,000	283,400,000	966,200,000	819,200,000
Net operating revenues	227,900,000	165,600,000	641,800,000	479,800,000
Net Segment Contribution	163,600,000	119,600,000	464,300,000	357,200,000
Segment Income	110,700,000	69,600,000	303,800,000	218,300,000
Net costs not allocated to operating segments	61,700,000	48,000,000.0	177,100,000	146,800,000
Other gain	0	0	100,000	5,400,000
Income before tax	49,000,000.0	21,600,000	126,800,000	76,900,000
Assets	12,389,500,000		12,389,500,000		$ 9,936,100,000
Commodity and Risk Management [Member]
Segment Reporting Information [Line Items]
Revenues	65,100,000	86,400,000	236,500,000	226,800,000
Operating revenues	65,100,000	86,400,000	236,500,000	226,800,000
Net operating revenues	50,800,000	66,900,000	189,400,000	178,000,000.0
Net Segment Contribution	36,000,000.0	48,000,000.0	133,900,000	127,900,000
Segment Income	19,200,000	29,700,000	76,500,000	73,200,000
Assets	2,112,600,000		2,112,600,000		2,041,000,000.0
Global Payments
Segment Reporting Information [Line Items]
Revenues	27,400,000	28,900,000	88,200,000	86,000,000.0
Operating revenues	27,400,000	28,900,000	88,200,000	86,000,000.0
Net operating revenues	26,000,000.0	27,500,000	83,800,000	81,700,000
Net Segment Contribution	20,900,000	22,300,000	67,300,000	66,400,000
Segment Income	15,900,000	17,000,000.0	52,000,000.0	51,400,000
Assets	234,800,000		234,800,000		278,300,000
Securities Segment
Segment Reporting Information [Line Items]
Revenues	123,000,000.0	74,200,000	322,000,000.0	215,800,000
Operating revenues	123,000,000.0	74,200,000	322,000,000.0	215,800,000
Net operating revenues	97,100,000	30,300,000	218,300,000	94,400,000
Net Segment Contribution	64,300,000	17,900,000	142,200,000	59,800,000
Segment Income	51,600,000	8,200,000	106,700,000	36,000,000.0
Assets	6,284,300,000		6,284,300,000		5,219,100,000
Physical Commodities
Segment Reporting Information [Line Items]
Revenues	7,960,000,000.0	7,604,300,000	38,971,800,000	20,847,000,000.0
Operating revenues	39,200,000	14,700,000	83,300,000	48,800,000
Net operating revenues	35,700,000	9,700,000	71,100,000	35,200,000
Net Segment Contribution	24,200,000	6,400,000	49,300,000	25,000,000.0
Segment Income	20,300,000	2,800,000	37,500,000	16,500,000
Assets	425,300,000		425,300,000		357,800,000
Clearing and Execution Services Segment
Segment Reporting Information [Line Items]
Revenues	68,900,000	78,900,000	241,300,000	247,700,000
Operating revenues	68,900,000	78,900,000	241,300,000	247,700,000
Net operating revenues	22,800,000	32,500,000	89,900,000	101,000,000.0
Net Segment Contribution	18,200,000	25,000,000.0	71,600,000	78,100,000
Segment Income	3,700,000	11,900,000	31,100,000	41,200,000
Assets	2,546,100,000		2,546,100,000		1,892,100,000
Corporate unallocated
Segment Reporting Information [Line Items]
Revenues	2,300,000	6,500,000	11,500,000	13,500,000
Operating revenues	2,300,000	6,500,000	11,500,000	13,500,000
Net operating revenues	(4,500,000)	(1,300,000)	(10,700,000)	(10,500,000)
Assets	786,400,000		786,400,000		$ 147,800,000
Eliminations
Segment Reporting Information [Line Items]
Revenues	(3,300,000)	(6,200,000)	(16,600,000)	(19,400,000)
Operating revenues	$ (3,300,000)	$ (6,200,000)	$ (16,600,000)	$ (19,400,000)