Segment Analysis (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016	Sep. 30, 2016
Segment Reporting Information [Line Items]
Revenues	$ 5,460.8	$ 3,708.9	$ 11,535.2	$ 7,108.8
Operating revenues	195.8	166.1	381.3	317.4
Net operating revenues	123.9	112.9	238.2	215.6
Net Segment Contribution	97.2	81.4	190.6	166.9
Segment Income	53.9	46.7	104.1	98.1
Costs not allocated to operating segments	39.6	26.7	81.4	66.0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	14.3	20.0	22.7	32.1
Assets	6,528.6		6,528.6		$ 5,950.3
Commercial Hedging [Member]
Segment Reporting Information [Line Items]
Revenues	62.7	54.7	120.2	110.1
Operating revenues	62.7	54.7	120.2	110.1
Net operating revenues	50.5	43.6	96.1	87.9
Net Segment Contribution	36.9	30.5	70.3	62.2
Segment Income	18.7	13.8	34.1	28.8
Assets	1,654.0		1,654.0		1,637.5
Global Payments [Member]
Segment Reporting Information [Line Items]
Revenues	21.5	17.4	44.6	35.7
Operating revenues	21.5	17.4	44.6	35.7
Net operating revenues	19.1	15.5	39.7	31.8
Net Segment Contribution	15.3	12.4	31.7	25.4
Segment Income	11.7	9.3	24.9	19.3
Assets	172.4		172.4		191.4
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues	37.9	45.6	75.3	94.4
Operating revenues	37.9	45.6	75.3	94.4
Net operating revenues	24.1	31.8	49.2	67.8
Net Segment Contribution	18.9	25.0	38.7	54.5
Segment Income	11.7	18.2	24.6	40.1
Assets	2,611.0		2,611.0		2,130.7
Physical Commodities [Member]
Segment Reporting Information [Line Items]
Revenues	5,276.2	3,550.7	11,174.9	6,805.2
Operating revenues	11.2	7.9	21.0	13.8
Net operating revenues	9.0	6.8	17.1	11.5
Net Segment Contribution	6.5	5.3	12.3	8.7
Segment Income	3.9	1.9	6.9	2.9
Assets	309.1		309.1		258.0
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues	64.2	33.3	127.8	63.1
Operating revenues	64.2	33.3	127.8	63.1
Net operating revenues	25.4	10.4	49.5	20.8
Net Segment Contribution	19.6	8.2	37.6	16.1
Segment Income	7.9	3.5	13.6	7.0
Assets	1,651.2		1,651.2		1,617.4
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues	(1.7)	7.2	(7.6)	0.3
Operating revenues	(1.7)	7.2	(7.6)	0.3
Net operating revenues	(4.2)	$ 4.8	(13.4)	$ (4.2)
Assets	$ 130.9		$ 130.9		$ 115.3

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