Segment and Geographic Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Revenues	$ 2,628.4	$ 3,995.1	$ 14,442.0	$ 13,627.7	$ 10,655.7	$ 7,005.1	$ 8,452.9	$ 7,908.7	$ 34,693.2	$ 34,022.4	$ 42,493.4
Operating revenues	178.7	151.6	156.5	137.5	130.6	118.2	129.2	112.9	624.3	490.9	468.2
Revenues, Net of Interest Expense	127.3	103.4	107.9	93.2	86.7	76.0	85.9	73.4	431.8	322.0	309.7
Net Segment Contribution									320.6	249.2	228.7
Segment Income									188.1	128.8	113.0
Costs not allocated to operating segments									110.0	102.8	91.8
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	29.2	$ 17.2	$ 18.1	$ 13.6	7.8	$ 4.0	$ 10.3	$ 3.9	78.1	26.0	21.2
Assets	5,070.0				3,039.7				5,070.0	3,039.7	2,848.0
Commercial Hedging [Member]
Segment Reporting Information [Line Items]
Revenues									262.4	224.0	202.0
Operating revenues									262.4	224.0	202.0
Revenues, Net of Interest Expense									214.7	180.5	163.0
Net Segment Contribution									151.7	132.6	119.7
Segment Income									85.6	67.3	57.1
Assets	1,548.1				1,400.9				1,548.1	1,400.9	1,005.1
Global Payments [Member]
Segment Reporting Information [Line Items]
Revenues									77.1	55.4	40.9
Operating revenues									77.1	55.4	40.9
Revenues, Net of Interest Expense									68.5	48.2	36.7
Net Segment Contribution									54.5	37.6	28.0
Segment Income									43.3	28.3	20.5
Assets	207.3				51.9				207.3	51.9	57.2
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues									129.8	80.3	70.0
Operating revenues									129.8	80.3	70.0
Revenues, Net of Interest Expense									88.6	54.6	48.0
Net Segment Contribution									67.4	40.9	37.6
Segment Income									40.5	21.0	19.5
Assets	1,861.0				235.5				1,861.0	235.5	204.2
Physical Commodities Segment [Member]
Segment Reporting Information [Line Items]
Revenues									34,092.0	33,552.1	42,052.0
Operating revenues									23.1	20.6	26.8
Revenues, Net of Interest Expense									21.2	17.9	23.4
Net Segment Contribution									16.9	14.1	17.5
Segment Income									5.8	5.9	10.0
Assets	190.9				116.8				190.9	116.8	132.5
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues									123.4	113.7	121.3
Operating revenues									123.4	113.7	121.3
Revenues, Net of Interest Expense									38.3	29.7	34.1
Net Segment Contribution									30.1	24.0	25.9
Segment Income									12.9	6.3	5.9
Assets	1,163.8				1,136.2				1,163.8	1,136.2	1,333.3
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues									8.5	(3.1)	7.2
Operating revenues									8.5	(3.1)	7.2
Revenues, Net of Interest Expense									0.5	(8.9)	4.5
Assets	$ 98.9				$ 98.4				$ 98.9	$ 98.4	$ 115.7