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Incentive Plan (Summary of Long-Term Incentive Units) (Details)
9 Months Ended
Sep. 30, 2018
$ / shares
shares
Units  
Granted (shares) 114,472
Long-term incentive units  
Units  
Unvested beginning of period (shares) 99,295
Granted (shares) 114,472
Vested (shares) (49,054)
Cancelled or expired (shares) (14,275)
Unvested end of period (shares) 150,438
Weighted-Average Grant Date Fair Value  
Unvested, beginning of period (in dollars per share) | $ / shares $ 90.59
Granted (in dollars per share) | $ / shares 101.26
Vested (in dollars per share) | $ / shares 88.97
Cancelled or expired (in dollars per share) | $ / shares 82.85
Unvested, end of period (in dollars per share) | $ / shares $ 99.49
Minimum | Long-term incentive units  
Weighted-Average Grant Date Fair Value  
Award vesting period 2 years
Maximum | Long-term incentive units  
Weighted-Average Grant Date Fair Value  
Award vesting period 4 years