Commitments and Contingencies - Contigent Consideration for Acquisitions (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Sep. 30, 2016
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	$ 0.9			$ 0.8
Change in fair value of contingent consideration for acquisitions	0.0	$ 0.2
G.X. Clarke (INTL FCStone Partners) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	0.9
Present Value of Estimated Total Purchase Price	28.7
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	$ 0.1	$ 0.2	$ 0.2