Acquisitions and Disposals (Details) (USD $) In Millions, except Share data, unless otherwise specified	9 Months Ended	12 Months Ended						1 Months Ended		0 Months Ended						1 Months Ended	12 Months Ended		13 Months Ended		1 Months Ended				12 Months Ended
	Jun. 30, 2013	Sep. 30, 2013 business	Sep. 30, 2012 business	Sep. 30, 2011	Feb. 28, 2013	Sep. 30, 2013 Tradewire Acquisition [Member] [Member]	Apr. 30, 2010 Tradewire Acquisition [Member] [Member]	Nov. 30, 2012 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]	Oct. 31, 2011 Hencorp Acquisition [Member] payment	Apr. 30, 2011 Hudson Capital Asset Acquisition [Member]	Aug. 31, 2011 Arabian Commodities Limited [Member]	Sep. 30, 2010 Provident Group Asset Acquisition [Member]	Sep. 30, 2013 Provident Group Asset Acquisition [Member]	Sep. 30, 2012 Provident Group Asset Acquisition [Member]	Sep. 30, 2011 Provident Group Asset Acquisition [Member]	Dec. 12, 2012 Customer Relationships [Member] Tradewire Acquisition [Member] [Member]	Oct. 31, 2011 Customer Relationships [Member] Hencorp Acquisition [Member]	Oct. 31, 2011 Noncompete Agreements [Member] Hencorp Acquisition [Member]	Oct. 31, 2011 Trade Names [Member] Hencorp Acquisition [Member]	Sep. 30, 2013 Tradewire Acquisition [Member] [Member]	Sep. 30, 2013 INTL Commodities - Physical Base Metals [Domain]
Business Acquisition [Line Items]
Business Acquisition, Contingent Consideration, Potential Cash Payment													$ 0.7
Number of Businesses Acquired		3	2
Goodwill, Expected Tax Deductible Amount		5.6	0.7	4.9
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				2.3
Business Combination, Contingent Consideration Arrangements, Number of Annual Payments									3				2
Business Combination, Consideration Transferred									0.3		12.9	1.5		1.0	7.1	5.0								5.6
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Amortizable Intangibles									0.2	0.5			1.7								1.3	0.4
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life								2 years		3 years											20 years	2 years
Business Combination, Professionals Hired From Acquiree Company										50
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangibles										1.0
Goodwill		47.5	44.7				2.8				0.3	0.8	2.1
Business Combination, Cost of Acquired Entity, Number of Payments													2
Business Acquisition, Cost of Acquired Entity, Total of Payments 2 and 3													1.4
Business Combination, Consideration, Including Contingent Consideration						6.3							7.5
Business Acquisition, Cost of Acquired Entity, Cash Paid, Payment 4													0.3
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangibles Not Amortizable																							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																	6,799	3,255
Acquired Finite-lived Intangible Asset, Amount																				2.8
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds	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	(0.1)	(0.1)	0	0
Business Exit Costs		1.9
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest		$ 22.6	$ 15.8	$ 55.4																					$ 1.4