Segment Analysis (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Sep. 30, 2015
Segment Reporting Information [Line Items]
Revenues	$ 3,708.9	$ 14,442.0	$ 7,108.8	$ 28,069.7
Operating revenues	166.1	156.5	317.4	294.0
Net operating revenues	112.9	107.9	215.6	201.1
Net Segment Contribution	81.4	83.7	166.9	153.6
Segment Income	46.7	49.1	98.1	90.0
Costs not allocated to operating segments	26.7	31.0	66.0	58.3
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	20.0	18.1	32.1	31.7
Assets	5,657.9		5,657.9		$ 5,070.0
Commercial Hedging [Member]
Segment Reporting Information [Line Items]
Revenues	54.7	64.7	110.1	133.1
Operating revenues	54.7	64.7	110.1	133.1
Net operating revenues	43.6	53.7	87.9	110.7
Net Segment Contribution	30.5	39.3	62.2	79.9
Segment Income	13.8	21.7	28.8	46.6
Assets	1,435.4		1,435.4		1,548.1
Global Payments [Member]
Segment Reporting Information [Line Items]
Revenues	17.4	18.4	35.7	33.6
Operating revenues	17.4	18.4	35.7	33.6
Net operating revenues	15.5	16.1	31.8	29.6
Net Segment Contribution	12.4	12.8	25.4	23.6
Segment Income	9.3	9.9	19.3	18.1
Assets	195.2		195.2		207.3
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues	45.6	36.1	94.4	53.3
Operating revenues	45.6	36.1	94.4	53.3
Net operating revenues	31.8	25.5	67.8	35.5
Net Segment Contribution	25.0	19.6	54.5	26.2
Segment Income	18.2	12.2	40.1	13.9
Assets	2,427.5		2,427.5		1,861.0
Physical Commodities [Member]
Segment Reporting Information [Line Items]
Revenues	3,550.7	14,291.9	6,805.2	27,788.6
Operating revenues	7.9	6.4	13.8	12.9
Net operating revenues	6.8	5.9	11.5	11.5
Net Segment Contribution	5.3	4.5	8.7	8.9
Segment Income	1.9	2.4	2.9	4.9
Assets	333.3		333.3		190.9
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues	33.3	31.5	63.1	62.7
Operating revenues	33.3	31.5	63.1	62.7
Net operating revenues	10.4	9.2	20.8	18.7
Net Segment Contribution	8.2	7.5	16.1	15.0
Segment Income	3.5	2.9	7.0	6.5
Assets	1,177.2		1,177.2		1,163.8
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues	7.2	(0.6)	0.3	(1.6)
Operating revenues	7.2	(0.6)	0.3	(1.6)
Net operating revenues	4.8	$ (2.5)	(4.2)	$ (4.9)
Assets	$ 89.3		$ 89.3		$ 98.9