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Incentive Plan (Restricted Stock Activity) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Shares      
Granted (in shares) 128,986 73,449 74,246
Market performance-based awards      
Weighted-Average Grant Date Fair Value Per Share      
Granted (in dollars per share) $ 119.29 $ 111.06 $ 82.90
Additional Information:      
Fair value of awards $ 21.8 $ 19.5 $ 21.2
Restricted stock      
Shares      
Unvested, beginning of period (in shares) 259,422    
Granted (in shares) 168,192    
Vested (in shares) (96,840)    
Cancelled or expired (in shares) (35,273)    
Unvested, end of period (in shares) 295,501 259,422  
Weighted-Average Grant Date Fair Value Per Share      
Unvested, beginning of period (in dollars per share) $ 90.54    
Granted (in dollars per share) 100.33 $ 108.65 $ 83.58
Vested (in dollars per share) 83.63    
Cancelled or expired (in dollars per share) 97.96    
Unvested, end of period (in dollars per share) $ 97.49 $ 90.54  
Additional Information:      
Award vesting period 4 years