Segment Analysis (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Sep. 30, 2012
Segment Reporting Information [Line Items]
Revenues	$ 11,039.5	$ 17,351.1	$ 36,872.1	$ 51,592.5
Gross Profit	122.1	123.8	365.1	339.7
Net Contribution	58.3	61.3	179.1	169.0
Net Segment Income	28.4	30.2	88.2	80.1
Costs not allocated	24.3	24.3	63.9	71.3
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	4.1	5.9	24.3	8.8
Assets	2,907.4		2,907.4		2,958.9
Commodity and Risk Management [Member]
Segment Reporting Information [Line Items]
Revenues	10,874.3	17,266.8	36,497.0	51,341.5
Gross Profit	56.2	67.0	173.0	182.7
Net Contribution	31.2	38.3	102.9	105.1
Net Segment Income	13.8	18.9	49.1	50.0
Assets	1,258.5		1,258.5		1,449.2
Foreign Exchange Segment [Member]
Segment Reporting Information [Line Items]
Revenues	19.0	16.5	51.3	47.4
Gross Profit	19.0	16.5	51.3	47.4
Net Contribution	12.6	11.3	33.2	31.6
Net Segment Income	8.5	7.7	22.5	21.9
Assets	145.1		145.1		124.5
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues	13.8	8.6	42.7	28.8
Gross Profit	13.8	8.6	42.7	28.8
Net Contribution	6.3	4.8	22.4	14.4
Net Segment Income	1.8	1.1	9.2	3.0
Assets	169.1		169.1		88.7
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues	27.8	27.7	75.5	70.8
Gross Profit	27.8	27.7	75.5	70.8
Net Contribution	4.5	4.2	11.7	10.5
Net Segment Income	1.7	1.0	1.5	1.6
Assets	1,186.8		1,186.8		1,090.9
Other Segment [Member]
Segment Reporting Information [Line Items]
Revenues	105.3	31.3	198.1	104.5
Gross Profit	6.0	3.8	15.1	10.5
Net Contribution	3.7	2.7	8.9	7.4
Net Segment Income	2.6	1.5	5.9	3.6
Assets	42.7		42.7		110.8
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues	(0.7)	0.2	7.5	(0.5)
Gross Profit	(0.7)	0.2	7.5	(0.5)
Assets	$ 105.2		$ 105.2		$ 94.8