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Stock Compensation Plans (Narrative) (Details)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Apr. 26, 2018
shares
Sep. 30, 2018
USD ($)
shares
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
$ / shares
shares
Sep. 30, 2017
USD ($)
$ / shares
Dec. 31, 2017
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Common stock, shares authorized (in shares)   150,000,000   150,000,000   150,000,000
Total stock compensation expense | $   $ 10.5 $ 8.7 $ 33.3 $ 31.5  
2006 Long Term Incentive Plan            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Common stock, shares authorized (in shares)   10,000,000   10,000,000    
Shares available for grant (in shares)   3,391,294   3,391,294    
Performance Awards            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Weighted average grant-date fair value of performance awards granted (in dollars per share) | $ / shares       $ 71.40 $ 61.94  
Shares awarded (in shares)       441,740    
Share-based compensation cost not yet recognized | $   $ 38.4   $ 38.4    
Share-based compensation, recognition period of unrecognized compensation cost, years       2 years    
Restricted Stock Unit Awards            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Weighted average grant-date fair value of performance awards granted (in dollars per share) | $ / shares       $ 63.99 $ 61.99  
Shares awarded (in shares)       247,404    
Share-based compensation cost not yet recognized | $   16.8   $ 16.8    
Share-based compensation, recognition period of unrecognized compensation cost, years       2 years    
Shares received for every restricted stock unit award given       1    
Requisite period       3 years    
Stock-Settled Appreciation Rights            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Share-based compensation cost not yet recognized | $   $ 4.4   $ 4.4    
Share-based compensation, recognition period of unrecognized compensation cost, years       2 years    
Restricted Stock            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares awarded (in shares) 17,226          
Shares awarded after shares with held for taxes (in shares) 12,629          
Total stock compensation expense | $       $ 1.1    
Vesting Year One | Restricted Stock Unit Awards            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percentage       33.33%    
Vesting Year Two | Restricted Stock Unit Awards            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percentage       33.33%    
Vesting Year Three | Restricted Stock Unit Awards            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percentage       33.33%