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Stock Compensation Plans (Narrative) (Details)
$ / shares in Units, $ in Millions
3 Months Ended
Apr. 27, 2017
shares
Mar. 31, 2018
USD ($)
$ / shares
shares
Mar. 31, 2017
USD ($)
$ / shares
Dec. 31, 2017
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Common stock, shares authorized   150,000,000   150,000,000
Stock compensation expense | $   $ 9.2 $ 12.0  
2006 Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Common stock, shares authorized   10,000,000    
Shares available for grant, shares   3,493,582    
Performance Shares [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weighted average grant-date fair value of performance awards granted, dollars per share | $ / shares   $ 71.40 $ 61.83  
Shares awarded   441,740    
Share-based compensation cost not yet recognized | $   $ 49.3    
Share-based compensation, recognition period of unrecognized compensation cost, years   2 years    
Restricted Stock Units (RSUs) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weighted average grant-date fair value of performance awards granted, dollars per share | $ / shares   $ 71.40 $ 61.83  
Shares awarded   110,404    
Share-based compensation cost not yet recognized | $   $ 12.9    
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 Appreciation Rights (SARs) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation cost not yet recognized | $   $ 5.7    
Share-based compensation, recognition period of unrecognized compensation cost, years   3 years    
Restricted Stock [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted stock issued, shares 17,226      
Restricted common stocks issued, shares 12,629      
Stock compensation expense | $   $ 1.1    
Vesting Year One [Member] | Restricted Stock Units (RSUs) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting percentage   33.33%    
Vesting Year Two [Member] | Restricted Stock Units (RSUs) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting percentage   33.33%    
Vesting Year Three [Member] | Restricted Stock Units (RSUs) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting percentage   33.33%