Fair Value Measurements - Financial Liabilities not Carried at Fair Value (Details) - Level 2 - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	$ 745.7	$ 945.1
Convertible securities	309.9	307.5
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	765.2	936.0
Convertible securities	$ 549.8	$ 466.9