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Share-based Compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Estimated share-based compensation expense $ 9.4   $ 9.4  
Share-based Compensation Expense [Abstract]        
Share-based Compensation Expense 1.8 $ 1.3 6.3 $ 5.5
Stock Options [Member]        
Share-based Compensation Expense [Abstract]        
Share-based Compensation Expense 0.3 0.2 $ 2.0 1.7
Number of options granted (in shares)     842,000  
Weighted-average grant-date fair value per share of options granted (dollars per share)     $ 3.51  
Number of options exercised (in shares)     33,700  
Restricted Stock [Member]        
Share-based Compensation Expense [Abstract]        
Share-based Compensation Expense 0.5 0.4 $ 2.4 2.3
Number of shares granted (in shares)     521,100  
Weighted average grant date fair value, granted (dollars per share)     $ 6.56  
Intrinsic value of shares vested/restrictions lapsed during the period     $ 3.1  
Restricted Stock Units Issued to Directors [Member]        
Share-based Compensation Expense [Abstract]        
Share-based Compensation Expense 0.3 0.3 0.6 0.7
Performance shares [Member]        
Share-based Compensation Expense [Abstract]        
Share-based Compensation Expense 0.6 0.4 $ 1.1 0.8
Number of shares granted (in shares)     366,500  
Weighted average grant date fair value, granted (dollars per share)     $ 8.05  
Long-term Performance Plan [Member]        
Share-based Compensation Expense [Abstract]        
Share-based Compensation Expense $ 0.1 $ 0.0 $ 0.2 $ 0.0
Share-based Compensation, Target Allocation of Equity Under Comprehensive Award (in hundredths)     30.00%  
Share Based Compensation Arrangement By Share Based Payment Award, Performance Period     three-year