Fair Value Measurements - Summary of Financial Assets and Liabilities Not Carried at Fair Value (Details) - Level 2 - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	$ 944.9	$ 944.6
Convertible securities	306.3	305.2
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	982.4	966.3
Convertible securities	$ 470.7	$ 483.6