Segment Analysis (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 6,507.0	$ 5,460.8	$ 14,425.6	$ 11,535.2
Operating revenues	260.2	195.8	472.8	381.3
Net operating revenues	154.3	123.9	284.6	238.2
Net Segment Contribution	115.9	97.2	214.7	190.6
Segment Income	72.2	53.9	130.5	104.1
Costs not allocated to operating segments	42.7	39.6	82.4	81.4
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	29.5	14.3	48.1	22.7
Assets	7,203.7		7,203.7		$ 6,243.4
Commercial Hedging [Member]
Segment Reporting Information [Line Items]
Revenues	78.3	62.7	139.8	120.2
Operating revenues	78.3	62.7	139.8	120.2
Net operating revenues	61.9	50.5	111.0	96.1
Net Segment Contribution	44.7	36.9	81.2	70.3
Segment Income	27.6	18.7	48.7	34.1
Assets	1,636.1		1,636.1		1,650.3
Global Payments [Member]
Segment Reporting Information [Line Items]
Revenues	23.4	21.5	48.0	44.6
Operating revenues	23.4	21.5	48.0	44.6
Net operating revenues	21.9	19.1	44.9	39.7
Net Segment Contribution	17.6	15.3	36.0	31.7
Segment Income	13.5	11.7	28.1	24.9
Assets	208.8		208.8		199.5
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues	55.5	37.9	98.5	75.3
Operating revenues	55.5	37.9	98.5	75.3
Net operating revenues	27.6	24.1	51.1	49.2
Net Segment Contribution	20.3	18.9	38.1	38.7
Segment Income	12.8	11.7	23.8	24.6
Assets	2,901.3		2,901.3		2,101.7
Physical Commodities [Member]
Segment Reporting Information [Line Items]
Revenues	6,262.3	5,276.2	13,978.9	11,174.9
Operating revenues	15.5	11.2	26.1	21.0
Net operating revenues	12.5	9.0	20.9	17.1
Net Segment Contribution	9.4	6.5	15.0	12.3
Segment Income	5.6	3.9	6.7	6.9
Assets	378.0		378.0		339.5
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues	88.0	64.2	160.2	127.8
Operating revenues	88.0	64.2	160.2	127.8
Net operating revenues	31.8	25.4	59.2	49.5
Net Segment Contribution	23.9	19.6	44.4	37.6
Segment Income	12.7	7.9	23.2	13.6
Assets	1,996.5		1,996.5		1,818.9
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues	(0.5)	(1.7)	0.2	(7.6)
Operating revenues	(0.5)	(1.7)	0.2	(7.6)
Net operating revenues	(1.4)	$ (4.2)	(2.5)	$ (13.4)
Assets	$ 83.0		$ 83.0		$ 133.5