Commitments and Contingencies - Contigent Consideration for Acquisitions (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2016	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015	Sep. 30, 2015
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	$ 0.8		$ 0.8		$ 3.3
Change in fair value of contingent consideration for acquisitions			0.3	$ 1.9
G.X. Clarke (INTL FCStone Partners) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	0.8		0.8
Present Value of Estimated Total Purchase Price	28.7		28.7
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	$ 0.0	$ 0.2	$ 0.3	$ 0.4