Commitments and Contingencies - Contigent Consideration for Acquisitions (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014	Jan. 01, 2015
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	$ 0.8	$ 3.3
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	(0.4)	1.9	$ (1.8)
G.X. Clarke (INTL FCStone Partners) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	0.8
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	1.5			$ 0.1
Business Combination, Consideration, Including Contingent Consideration	27.5
Contingent Consideration [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	$ 0.4	$ 1.8	$ (2.3)