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Income Taxes - Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Unrecognized tax benefits [Roll Forward]      
Balance as of beginning of period $ 71.7 $ 69.8  
Additions based on tax positions related to current period 3.9 2.5  
Additions for tax positions of prior years 11.4 29.2  
Expiration of statutes of limitations 0.0 (5.0)  
Settlements of uncertain tax positions with tax authorities (2.5) (24.8)  
Balance as of end of period 84.5 71.7 $ 69.8
Unrecognized tax benefits   21.4 67.7
Income tax penalties and interest expense 0.4 1.6 33.5
Accrued interest and penalties $ 59.9 $ 52.3 $ 43.8