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Share-based Compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Estimated share-based compensation expense $ 9.4  
Share-based Compensation Expense [Abstract]    
Share-based Compensation Expense 4.5 $ 4.2
Stock Options [Member]    
Share-based Compensation Expense [Abstract]    
Share-based Compensation Expense $ 1.7 1.5
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) 4,100  
Restricted Stock [Member]    
Share-based Compensation Expense [Abstract]    
Share-based Compensation Expense $ 1.9 1.9
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 0.4
Performance shares [Member]    
Share-based Compensation Expense [Abstract]    
Share-based Compensation Expense $ 0.5 0.4
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
Share-based Compensation, Target Allocation of Equity Under Comprehensive Award (in hundredths) 30.00%