Commitments and Contingencies - Contigent Consideration for Acquisitions (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2015	Mar. 31, 2017	Mar. 31, 2016	Sep. 30, 2016
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	$ 0.9		$ 0.9		$ 0.8
Change in fair value of contingent consideration for acquisitions	0.1		0.0	$ 0.3
G.X. Clarke (INTL FCStone Partners) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	0.9		0.9
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		$ 0.2	$ 0.1	$ 0.3