Investments (Details) - USD ($) $ in Millions	Jun. 30, 2017	Sep. 30, 2016
Investment [Line Items]
Total	$ 1,176.1	$ 902.8
Purchases of investments, amount outstanding	5.6
Certificates of deposit and time deposits [Member]
Investment [Line Items]
Total	960.0	902.8
Commercial paper [Member]
Investment [Line Items]
Total	17.7	0.0
Corporate debt securities [Member]
Investment [Line Items]
Total	95.3	0.0
Government securities [Member]
Investment [Line Items]
Total	$ 103.1	$ 0.0