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Debt Summary of Long-term Debt (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Sep. 30, 2017
Aug. 31, 2017
Dec. 31, 2016
Mar. 31, 2016
Nov. 30, 2010
Debt Instrument [Line Items]            
Long-term Debt $ 987.1     $ 990.5    
Notes 2018 Term | Senior Notes            
Debt Instrument [Line Items]            
Long-term Debt $ 0.0     $ 299.3    
Interest rate, stated percentage 5.95%   5.95%    
Notes 2022 Term | Senior Notes            
Debt Instrument [Line Items]            
Long-term Debt $ 297.9     $ 297.5    
Interest rate, stated percentage 3.625%     3.625%   3.625%
Notes 2026 Term | Senior Notes            
Debt Instrument [Line Items]            
Long-term Debt $ 394.4     $ 393.7    
Interest rate, stated percentage 3.35%     3.35% 3.35%  
Notes 2027 Term | Senior Notes            
Debt Instrument [Line Items]            
Long-term Debt $ 294.8          
Interest rate, stated percentage 3.15%   3.15%