Segment Analysis (Details) - USD ($)	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenues	$ 11,245,000,000	$ 6,552,200,000
Operating revenues	276,800,000	264,700,000
Net operating revenues	170,500,000	149,000,000
Net Segment Contribution	125,800,000	112,300,000
Segment Income	75,800,000	71,500,000
Net costs not allocated to operating segments	54,200,000.0	47,100,000.0
Other gain	100,000	0
Income before tax	21,700,000	24,400,000
Assets	10,129,300,000		$ 9,936,100,000
Commercial Hedging
Segment Reporting Information [Line Items]
Revenues	69,700,000.0	59,800,000.0
Operating revenues	69,700,000.0	59,800,000.0
Net operating revenues	54,400,000.0	45,300,000.0
Net Segment Contribution	38,800,000.0	30,600,000.0
Segment Income	21,500,000.0	13,300,000.0
Assets	1,991,500,000		2,041,000,000
Global Payments
Segment Reporting Information [Line Items]
Revenues	31,400,000.0	29,700,000.0
Operating revenues	31,400,000.0	29,700,000.0
Net operating revenues	29,800,000.0	28,400,000.0
Net Segment Contribution	23,800,000.0	23,100,000.0
Segment Income	18,900,000.0	18,600,000.0
Assets	257,800,000		278,300,000
Securities Segment
Segment Reporting Information [Line Items]
Revenues	81,100,000.0	69,000,000.0
Operating revenues	81,100,000.0	69,000,000.0
Net operating revenues	43,100,000.0	32,100,000.0
Net Segment Contribution	27,400,000.0	21,800,000.0
Segment Income	16,700,000.0	16,000,000.0
Assets	5,296,600,000		5,219,100,000
Physical Commodities
Segment Reporting Information [Line Items]
Revenues	10,988,300,000.0	6,301,800,000.0
Operating revenues	20,100,000.0	14,300,000.0
Net operating revenues	15,900,000.0	10,400,000.0
Net Segment Contribution	11,300,000.0	7,100,000.0
Segment Income	7,600,000.0	5,900,000.0
Assets	392,600,000		357,800,000
Clearing and Execution Services Segment
Segment Reporting Information [Line Items]
Revenues	75,900,000.0	95,200,000.0
Operating revenues	75,900,000.0	95,200,000.0
Net operating revenues	30,000,000.0	37,500,000.0
Net Segment Contribution	24,500,000.0	29,700,000.0
Segment Income	11,100,000.0	17,700,000.0
Assets	2,023,300,000		1,892,100,000
Corporate unallocated
Segment Reporting Information [Line Items]
Revenues	5,200,000	2,900,000
Operating revenues	5,200,000	2,900,000
Net operating revenues	(2,700,000.0)	(4,700,000.0)
Assets	167,500,000		$ 147,800,000
Eliminations
Segment Reporting Information [Line Items]
Revenues	(6,600,000)	(6,200,000)
Operating revenues	$ (6,600,000)	$ (6,200,000)