Segment and Geographic Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Revenues	$ 2,777.6	$ 4,868.5	$ 3,708.9	$ 3,399.9	$ 2,628.4	$ 3,995.1	$ 14,442.0	$ 13,627.7	$ 14,754.9	$ 34,693.2	$ 34,022.4
Operating revenues	178.6	175.0	166.1	151.3	178.7	151.6	156.5	137.5	671.0	624.3	490.9
Revenues, Net of Interest Expense	111.0	117.3	112.9	102.7	127.3	103.4	107.9	93.2	443.9	431.8	322.0
Net Segment Contribution									347.0	320.6	249.2
Segment Income									206.0	188.1	128.8
Costs not allocated to operating segments									133.3	110.0	102.8
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	19.2	$ 21.4	$ 20.0	$ 12.1	29.2	$ 17.2	$ 18.1	$ 13.6	72.7	78.1	26.0
Assets	5,951.3				5,070.0				5,951.3	5,070.0	3,039.7
Commercial Hedging [Member]
Segment Reporting Information [Line Items]
Revenues									236.1	262.4	224.0
Operating revenues									236.1	262.4	224.0
Revenues, Net of Interest Expense									188.2	214.7	180.5
Net Segment Contribution									134.4	151.7	132.6
Segment Income									68.7	85.6	67.3
Assets	1,637.5				1,548.1				1,637.5	1,548.1	1,400.9
Global Payments [Member]
Segment Reporting Information [Line Items]
Revenues									73.2	77.1	55.4
Operating revenues									73.2	77.1	55.4
Revenues, Net of Interest Expense									65.3	68.5	48.2
Net Segment Contribution									52.2	54.5	37.6
Segment Income									39.8	43.3	28.3
Assets	191.4				207.3				191.4	207.3	51.9
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues									175.2	129.8	80.3
Operating revenues									175.2	129.8	80.3
Revenues, Net of Interest Expense									121.9	88.6	54.6
Net Segment Contribution									97.5	67.4	40.9
Segment Income									69.4	40.5	21.0
Assets	2,130.7				1,861.0				2,130.7	1,861.0	235.5
Physical Commodities Segment [Member]
Segment Reporting Information [Line Items]
Revenues									14,120.5	34,092.0	33,552.1
Operating revenues									36.6	23.1	20.6
Revenues, Net of Interest Expense									31.5	21.2	17.9
Net Segment Contribution									23.4	16.9	14.1
Segment Income									13.3	5.8	5.9
Assets	258.0				190.9				258.0	190.9	116.8
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues									151.1	123.4	113.7
Operating revenues									151.1	123.4	113.7
Revenues, Net of Interest Expense									48.8	38.3	29.7
Net Segment Contribution									39.5	30.1	24.0
Segment Income									14.8	12.9	6.3
Assets	1,617.4				1,163.8				1,617.4	1,163.8	1,136.2
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues									(1.2)	8.5	(3.1)
Operating revenues									(1.2)	8.5	(3.1)
Revenues, Net of Interest Expense									(11.8)	0.5	(8.9)
Assets	$ 116.3				$ 98.9				$ 116.3	$ 98.9	$ 98.4