Investments - Schedule of Investments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017
Investment [Line Items]
Total	$ 826.3	$ 1,387.6
Certificates of deposit and time deposits [Member]
Investment [Line Items]
Total	539.9	1,005.3
Commercial paper [Member]
Investment [Line Items]
Total	4.5	20.3
Corporate debt securities [Member]
Investment [Line Items]
Total	168.1	199.4
Government securities [Member]
Investment [Line Items]
Total	78.5	116.8
Asset-backed Securities [Member]
Investment [Line Items]
Total	$ 35.3	$ 45.8