Commitments and Contingencies - Contigent Consideration for Acquisitions (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Sep. 30, 2015
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	$ 1.5		$ 1.5		$ 3.3
Change in fair value of contingent consideration for acquisitions			0.3	$ 1.7
Tradewire Acquisition [Member] [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	0.9		0.9
Present Value of Estimated Total Purchase Price	4.5		4.5
G.X. Clarke (INTL FCStone Partners) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	0.6		0.6
Present Value of Estimated Total Purchase Price	28.5		28.5
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	1.5		1.5
Contingent Consideration [Member]
Business Acquisition, Contingent Consideration [Line Items]
Change in fair value of contingent consideration for acquisitions	$ 0.1	$ 0.1	$ 0.3	$ 0.2