Basis of Presentation and Consolidation and Recently Issued Accounting Standards (Details) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income		$ 0.3
Payments Related to Tax Withholding for Share-based Compensation		$ 0.8	$ 0.0
Number of different types of foreign currencies		140
Number of accounts for customers company-wide		20,000
Non-Trading Technology and Support [Member]
Prior Period Reclassification Adjustment	$ 2.8		5.7
Communications
Prior Period Reclassification Adjustment	$ 1.2		$ 2.4