Investments - Schedule of Available-for-Sale Investments (Details) - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017
Investment [Line Items]
Total	$ 734.0	$ 1,387.6
Certificates of deposit and time deposits [Member]
Investment [Line Items]
Total	477.9	1,005.3
Commercial paper [Member]
Investment [Line Items]
Total	0.0	20.3
Corporate debt securities [Member]
Investment [Line Items]
Total	160.3	199.4
Government securities [Member]
Investment [Line Items]
Total	65.8	116.8
Asset-backed securities [Member]
Investment [Line Items]
Total	$ 30.0	$ 45.8