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Fair Value Measurements - Summary of Financial Assets and Liabilities Not Carried at Fair Value (Details) - Level 2 - USD ($)
$ in Millions
Mar. 31, 2016
Dec. 31, 2015
Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Senior notes $ 944.7 $ 944.6
Convertible securities 305.8 305.2
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Senior notes 954.6 966.3
Convertible securities $ 476.1 $ 483.6