Commitments and Contingencies - Contigent Consideration for Acquisitions (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Jan. 01, 2015
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	$ 1.0	$ 0.8
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	(0.2)	(0.4)	$ 1.9
G.X. Clarke (INTL FCStone Partners) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	1.0
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	$ 0.2	$ 0.4	$ 1.8