Segment Analysis (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 7,118.3	$ 5,505.9	$ 21,543.9	$ 17,041.1
Operating revenues	259.8	197.6	732.6	578.9
Net operating revenues	154.6	123.3	439.2	361.5
Net Segment Contribution	114.2	95.7	328.9	286.3
Segment Income	70.4	52.9	200.9	157.0
Costs not allocated to operating segments	39.5	37.9	121.9	119.3
Other gain	2.0	0.0	2.0	0.0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	32.9	15.0	81.0	37.7
Assets	7,284.9		7,284.9		$ 6,243.4
Commercial Hedging [Member]
Segment Reporting Information [Line Items]
Revenues	77.9	57.1	217.7	177.3
Operating revenues	77.9	57.1	217.7	177.3
Net operating revenues	61.0	44.2	172.0	140.3
Net Segment Contribution	44.3	32.6	125.5	102.9
Segment Income	25.3	16.3	74.0	50.4
Assets	1,848.5		1,848.5		1,650.3
Global Payments [Member]
Segment Reporting Information [Line Items]
Revenues	26.0	22.5	74.0	67.1
Operating revenues	26.0	22.5	74.0	67.1
Net operating revenues	24.7	20.3	69.6	60.0
Net Segment Contribution	19.9	16.2	55.9	47.9
Segment Income	16.0	12.9	44.1	37.8
Assets	284.3		284.3		199.5
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues	49.9	40.0	148.4	115.3
Operating revenues	49.9	40.0	148.4	115.3
Net operating revenues	23.6	24.6	74.7	73.8
Net Segment Contribution	17.5	20.2	55.6	58.9
Segment Income	10.3	12.9	34.1	37.5
Assets	2,712.7		2,712.7		2,101.7
Physical Commodities [Member]
Segment Reporting Information [Line Items]
Revenues	6,873.4	5,320.3	20,852.3	16,495.2
Operating revenues	14.9	12.0	41.0	33.0
Net operating revenues	11.4	10.2	32.3	27.3
Net Segment Contribution	8.1	7.6	23.1	19.9
Segment Income	5.1	4.3	11.8	11.2
Assets	307.4		307.4		339.5
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues	88.9	65.4	249.1	193.2
Operating revenues	88.9	65.4	249.1	193.2
Net operating revenues	32.7	25.6	91.9	75.1
Net Segment Contribution	24.4	19.1	68.8	56.7
Segment Income	13.7	6.5	36.9	20.1
Assets	2,026.3		2,026.3		1,818.9
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues	2.2	0.6	2.4	(7.0)
Operating revenues	2.2	0.6	2.4	(7.0)
Net operating revenues	1.2	$ (1.6)	(1.3)	$ (15.0)
Assets	$ 105.7		$ 105.7		$ 133.5