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Incentive Plan (Summary of Restricted Stock Activity) (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Units    
Granted (shares) 95,592  
Restricted stock    
Units    
Unvested beginning of period (shares) 259,422  
Granted (shares) 154,647  
Vested (shares) (80,614)  
Unvested end of period (shares) 333,455  
Weighted-Average Grant Date Fair Value    
Unvested, beginning of period (in dollars per share) $ 90.54  
Granted (in dollars per share) 99.34  
Vested (in dollars per share) 81.99  
Unvested, end of period (in dollars per share) $ 96.64  
Unearned compensation $ 30.5 $ 17.5
Unearned compensation, period of recognition (in years) 3 years 2 months 15 days  
Restricted stock | Maximum    
Weighted-Average Grant Date Fair Value    
Award vesting period 4 years