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Fair Value Measurements - Financial Assets and Liabilities Not Carried at Fair Value (Details) - Level 2 - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Senior notes $ 746.4 $ 746.2
Junior convertible securities 313.2 312.5
Junior subordinated notes 271.3 0.0
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Senior notes 760.6 747.5
Junior convertible securities 436.7 391.5
Junior subordinated notes $ 275.3 $ 0.0