Segment Analysis (Details) - USD ($)	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 6,552.2	$ 7,918.6
Operating revenues	264.7	212.6
Net operating revenues	149.0	130.3
Net Segment Contribution	112.3	98.8
Segment Income	71.5	58.3
Costs not allocated to operating segments	47.1	39.7
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	24.4	18.6
Assets	8,710.5		$ 7,824.7
Commercial Hedging [Member]
Segment Reporting Information [Line Items]
Total revenues	59.8	61.5
Operating revenues	59.8	61.5
Net operating revenues	45.3	49.1
Net Segment Contribution	30.6	36.5
Segment Income	13.3	21.1
Assets	1,901.3		1,935.7
Global Payments [Member]
Segment Reporting Information [Line Items]
Total revenues	29.7	24.6
Operating revenues	29.7	24.6
Net operating revenues	28.4	23.0
Net Segment Contribution	23.1	18.4
Segment Income	18.6	14.6
Assets	200.6		206.6
Securities Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	69.0	43.0
Operating revenues	69.0	43.0
Net operating revenues	32.1	23.5
Net Segment Contribution	21.8	17.8
Segment Income	16.0	11.0
Assets	4,330.6		3,058.2
Physical Commodities [Member]
Segment Reporting Information [Line Items]
Total revenues	6,301.8	7,716.6
Operating revenues	14.3	10.6
Net operating revenues	10.4	8.4
Net Segment Contribution	7.1	5.6
Segment Income	5.9	1.1
Assets	372.2		413.7
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	95.2	72.2
Operating revenues	95.2	72.2
Net operating revenues	37.5	27.4
Net Segment Contribution	29.7	20.5
Segment Income	17.7	10.5
Assets	1,821.4		2,109.9
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	(3.3)	0.7
Operating revenues	(3.3)	0.7
Net operating revenues	(4.7)	$ (1.1)
Assets	$ 84.4		$ 100.6