Investments - Investments Included Within Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total	$ 1,374.6	$ 1,387.6
Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total	1,100.5	1,124.6
Other Assets [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total	$ 274.1	$ 263.0