Segment Analysis (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 7,918.6	$ 6,074.4
Operating revenues	212.6	185.5
Net operating revenues	130.3	114.3
Net Segment Contribution	98.8	93.4
Segment Income	58.3	50.2
Costs not allocated to operating segments	39.7	41.8
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	18.6	8.4
Assets	6,808.9		$ 6,243.4
Commercial Hedging [Member]
Segment Reporting Information [Line Items]
Revenues	61.5	57.5
Operating revenues	61.5	57.5
Net operating revenues	49.1	45.6
Net Segment Contribution	36.5	33.4
Segment Income	21.1	15.4
Assets	1,533.3		1,650.3
Global Payments [Member]
Segment Reporting Information [Line Items]
Revenues	24.6	23.1
Operating revenues	24.6	23.1
Net operating revenues	23.0	20.6
Net Segment Contribution	18.4	16.4
Segment Income	14.6	13.2
Assets	188.2		199.5
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues	43.0	37.4
Operating revenues	43.0	37.4
Net operating revenues	23.5	25.1
Net Segment Contribution	17.8	19.8
Segment Income	11.0	12.9
Assets	2,569.1		2,101.7
Physical Commodities [Member]
Segment Reporting Information [Line Items]
Revenues	7,716.6	5,898.7
Operating revenues	10.6	9.8
Net operating revenues	8.4	8.1
Net Segment Contribution	5.6	5.8
Segment Income	1.1	3.0
Assets	414.7		339.5
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues	72.2	63.6
Operating revenues	72.2	63.6
Net operating revenues	27.4	24.1
Net Segment Contribution	20.5	18.0
Segment Income	10.5	5.7
Assets	2,005.1		1,818.9
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues	0.7	(5.9)
Operating revenues	0.7	(5.9)
Net operating revenues	(1.1)	$ (9.2)
Assets	$ 98.5		$ 133.5