Segment and Geographic Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
Revenues	$ 17,668.1	$ 17,351.1	$ 16,951.0	$ 17,290.4	$ 24,070.4	$ 20,573.4	$ 14,583.0	$ 16,270.8	$ 69,260.6	$ 75,497.6	$ 46,940.3
Gross Profit	118.0	123.8	119.6	96.3	108.1	105.4	113.0	96.7	457.7	423.2	269.0
Net Contribution									228.7	229.5	128.1
Net Segment Income									110.1	133.9	65.0
Costs not allocated									90.8	74.4	47.1
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	10.5	5.9	3.6	(0.7)	13.0	17.3	23.3	5.9	19.3	59.5	17.9
Assets	2,958.9				2,635.7				2,958.9	2,635.7
Commodity and Risk Management [Member]
Segment Reporting Information [Line Items]
Revenues									68,928.3	75,274.1	46,785.2
Gross Profit									246.0	252.6	129.8
Net Contribution									143.0	151.6	70.9
Net Segment Income									69.7	93.5	32.8
Assets	1,449.2				1,540.8				1,449.2	1,540.8
Foreign Exchange Segment [Member]
Segment Reporting Information [Line Items]
Revenues									62.6	59.3	47.5
Gross Profit									62.6	59.3	47.5
Net Contribution									41.3	37.1	28.0
Net Segment Income									28.3	28.0	21.8
Assets	124.5				146.1				124.5	146.1
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues									39.9	30.5	20.8
Gross Profit									39.9	30.5	20.8
Net Contribution									19.8	16.5	11.4
Net Segment Income									4.5	1.9	5.6
Assets	88.7				104.8				88.7	104.8
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues									93.8	66.1	61.8
Gross Profit									93.8	66.1	61.8
Net Contribution									14.3	14.3	10.7
Net Segment Income									2.2	4.9	1.1
Assets	1,090.9				734.4				1,090.9	734.4
Other Segment [Member]
Segment Reporting Information [Line Items]
Revenues									136.2	67.2	24.8
Gross Profit									15.6	14.3	8.9
Net Contribution									10.3	10.0	7.1
Net Segment Income									5.4	5.6	3.7
Assets	110.8				43.4				110.8	43.4
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues									(0.2)	0.4	0.2
Gross Profit									(0.2)	0.4	0.2
Assets	$ 94.8				$ 66.2				$ 94.8	$ 66.2