Segment Analysis (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2013
Segment Reporting Information [Line Items]
Revenues	$ 7,938.0	$ 12,552.8
Gross Profit	113.4	125.7
Net Contribution	57.7	64.2
Net Segment Income	29.5	34.0
Costs not allocated	25.5	15.2
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	4.0	18.8
Assets	2,743.9		2,848.0
Commodity and Risk Management [Member]
Segment Reporting Information [Line Items]
Revenues	7,766.9	12,453.1
Gross Profit	48.5	60.7
Net Contribution	29.2	40.5
Net Segment Income	13.1	22.7
Assets	1,057.2		1,105.1
Foreign Exchange Segment [Member]
Segment Reporting Information [Line Items]
Revenues	18.3	16.2
Gross Profit	18.3	16.2
Net Contribution	12.1	10.4
Net Segment Income	8.1	7.1
Assets	126.8		145.0
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues	17.6	12.9
Gross Profit	17.6	12.9
Net Contribution	8.6	6.9
Net Segment Income	4.3	3.2
Assets	226.6		198.5
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues	23.8	22.4
Gross Profit	23.8	22.4
Net Contribution	3.7	3.6
Net Segment Income	1.1	(0.9)
Assets	1,164.8		1,246.4
Other Segment [Member]
Segment Reporting Information [Line Items]
Revenues	111.8	39.5
Gross Profit	5.6	4.8
Net Contribution	4.1	2.8
Net Segment Income	2.9	1.9
Assets	54.7		37.3
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues	(0.4)	8.7
Gross Profit	(0.4)	8.7
Assets	$ 113.8		$ 115.7