Investments - Schedule of Available-for-sale Investments (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Investments	$ 419.0	$ 1,387.6
Certificates of deposit and time deposits
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Investments	169.6	1,005.3
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Investments	0.0	20.3
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Investments	158.4	199.4
Government securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Investments	65.8	116.8
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Investments	$ 25.2	$ 45.8