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Incentive Plan (Long-Term Incentive Units) (Details) - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Shares      
Granted (in shares) 128,986 73,449 74,246
Long-term incentive units      
Shares      
Unvested, beginning of period (in shares) 99,295    
Granted (in shares) 128,986    
Vested (in shares) (51,820)    
Cancelled or expired (in shares) (15,356)    
Unvested, end of period (in shares) 161,105 99,295  
Weighted-Average Grant Date Fair Value Per Share      
Unvested, beginning of period (in dollars per share) $ 90.59    
Granted (in dollars per share) 101.86 $ 109.71 $ 87.18
Vested (in dollars per share) 88.88    
Cancelled (in dollars per share) 83.93    
Unvested, end of period (in dollars per share) $ 100.94 $ 90.59  
Minimum | Long-term incentive units      
Weighted-Average Grant Date Fair Value Per Share      
Award vesting period 2 years    
Maximum | Long-term incentive units      
Weighted-Average Grant Date Fair Value Per Share      
Award vesting period 4 years