Segment Analysis Segment Analysis (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Sep. 30, 2012
Segment Reporting Information [Line Items]
Revenues	$ 12,719.5	$ 17,290.4
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	18.8	(0.7)
Costs not allocated	15.2	24.6
Net Segment Income	34.0	23.9
Gross Profit	125.7	96.3
Net Contribution	64.2	50.6
Assets	2,939.6		2,958.9
Commodity and Risk Management [Member]
Segment Reporting Information [Line Items]
Revenues	12,619.8	17,202.0
Net Segment Income	22.7	13.1
Gross Profit	60.7	50.0
Net Contribution	40.5	29.8
Assets	1,422.1		1,449.2
Foreign Exchange Segment [Member]
Segment Reporting Information [Line Items]
Revenues	16.2	17.9
Net Segment Income	7.1	9.0
Gross Profit	16.2	17.9
Net Contribution	10.4	11.7
Assets	134.7		124.5
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues	12.9	9.9
Net Segment Income	3.2	1.1
Gross Profit	12.9	9.9
Net Contribution	6.9	4.5
Assets	137.7		88.7
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues	22.4	17.7
Net Segment Income	(0.9)	(0.1)
Gross Profit	22.4	17.7
Net Contribution	3.6	2.5
Assets	1,032.4		1,090.9
Other Segment [Member]
Segment Reporting Information [Line Items]
Revenues	39.5	45.2
Net Segment Income	1.9	0.8
Gross Profit	4.8	3.1
Net Contribution	2.8	2.1
Assets	95.0		110.8
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues	8.7	(2.3)
Gross Profit	8.7	(2.3)
Assets	$ 117.7		$ 94.8