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Goodwill and Intangible Assets - Goodwill Rollforward (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Goodwill [Roll Forward]    
Goodwill $ 765.9 $ 765.9
Accumulated impairment losses (674.9) (674.9)
Goodwill, net $ 91.0 $ 91.0