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Net Income per Common Share - Basic and Diluted Net Loss per Share (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]        
Net income (loss) $ 46.6 $ (2.0) $ 52.6 $ (40.8)
Net income (loss) per common share:        
Basic (in USD per share) $ 0.53 $ (0.02) $ 0.60 $ (0.47)
Diluted (in USD per share) $ 0.52 $ (0.02) $ 0.59 $ (0.47)
Basic weighted average shares outstanding (in shares) 88.5 86.7 88.0 86.1
Diluted weighted average shares outstanding (in shares) 90.3 86.7 89.3 86.1
Effect of potentially dilutive stock options        
Net income (loss) per common share:        
Effect of potentially dilutive shares (in shares) 0.2 0.0 0.2 0.0
Effect of potentially dilutive share-based awards        
Net income (loss) per common share:        
Effect of potentially dilutive shares (in shares) 1.6 0.0 1.1 0.0