Commitments and Contingencies - Contigent Consideration for Acquisitions (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2015
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	$ 3.5		$ 3.3
Change in fair value of contingent consideration for acquisitions	0.2	$ 0.1
Tradewire Acquisition [Member] [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	2.4
Present Value of Estimated Total Purchase Price	4.4
G.X. Clarke (INTL FCStone Partners) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	1.1
Present Value of Estimated Total Purchase Price	28.5
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	1.5
Contingent Consideration [Member]
Business Acquisition, Contingent Consideration [Line Items]
Change in fair value of contingent consideration for acquisitions	$ 0.2	$ 0.1