Acquisitions and Disposals (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended							1 Months Ended			0 Months Ended				1 Months Ended	12 Months Ended			13 Months Ended	12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2014 business	Sep. 30, 2013	Sep. 30, 2012	Feb. 28, 2013	Sep. 30, 2014 Forward Insight Commodities Acquisition [Member]	Sep. 30, 2014 Tradewire Acquisition [Member] [Member]	Sep. 30, 2013 Tradewire Acquisition [Member] [Member]	Nov. 30, 2012 Coffee Network Acquisition [Member]	Sep. 30, 2014 Coffee Network Acquisition [Member]	Nov. 30, 2011 Coffee Network Acquisition [Member] payment	Nov. 25, 2012 MF Metals Team Acquisition [Member] professional	Apr. 30, 2012 TRX Futures Acquisition [Member]	Feb. 29, 2012 DTVM Acquisition [Member]	Sep. 30, 2014 Hencorp Acquisition [Member]	Sep. 30, 2010 Provident Group Asset Acquisition [Member]	Sep. 30, 2014 Provident Group Asset Acquisition [Member]	Sep. 30, 2013 Provident Group Asset Acquisition [Member]	Sep. 30, 2011 Provident Group Asset Acquisition [Member]	Sep. 30, 2011 Provident Group Asset Acquisition [Member]	Sep. 30, 2013 Customer Relationships [Member] Tradewire Acquisition [Member] [Member]	Sep. 30, 2014 Customer Relationships [Member] Forward Insight Commodities [Member]	Sep. 30, 2013 Tradewire Acquisition [Member] [Member]
Business Acquisition [Line Items]
Present Value of Estimated Total Purchase Price													$ 0.5	$ 3.3			$ 0.3					$ 8.3
Number of Businesses Acquired									3
Goodwill, Expected Tax Deductible Amount	0.5				3.3				0.5	3.3	0.7
Business Acquisition, Percentage of Voting Interests Acquired																		100.00%		100.00%	100.00%
Business Acquisition, Cost of Acquired Entity, Cash Paid, Payment 1																		0.2
Business Combination, Contingent Consideration Arrangements, Number of Annual Payments																		3
Business Combination, Consideration Transferred																										5.0				5.6
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Amortizable Intangibles																		0.2	0.5
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life																2 years			3 years
Business Combination, Professionals Hired From Acquiree Company																			50
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangibles																			1.0	12.9	1.5
Goodwill	47.5				47.5				47.5	47.5					2.8					0.3	0.8
Business Combination, Cost of Acquired Entity, Equity Interest Issued or Issuable, Total Shares Purchased and Placed In Escrow																											214,325
Business Combination, Cost of Acquired Entity, Equity Interest Issued or Issuable, Period From Date of Closing Over Which Shares Will be Released from Escrow																							5 years
Business Combination, Cost of Acquired Entity, Equity Interest Issued or Issuable, Shares Released from Escrow																								0	6,799
Acquired Finite-lived Intangible Asset, Amount																												2.8	0.5
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds					0.8					0.8
Assets of Disposal Group, Including Discontinued Operation												0.6
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment Realized upon Sale or Liquidation, before Tax									0	0.1	0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	$ 7.8	$ 4.0	$ 10.3	$ 3.9	$ (1.6)	$ 7.1	$ 1.7	$ 14.0	$ 26.0	$ 21.2	$ 22.5