Investments (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Investment [Line Items]
Total	$ 1,387.6	$ 902.8
Certificates of deposit and time deposits [Member]
Investment [Line Items]
Total	1,005.3	902.8
Commercial Paper [Member]
Investment [Line Items]
Total	20.3	0.0
Corporate Debt Securities [Member]
Investment [Line Items]
Total	199.4	0.0
Government securities [Member]
Investment [Line Items]
Total	116.8	0.0
Asset-backed Securities [Member]
Investment [Line Items]
Total	$ 45.8	$ 0.0