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Incentive Plan (Summary of Long-Term Incentive Units) (Details)
6 Months Ended
Jun. 30, 2018
$ / shares
shares
Units  
Granted (shares) 105,800
Long-term incentive units  
Units  
Unvested beginning of period (shares) 99,295
Granted (shares) 105,800
Vested (shares) (32,575)
Cancelled or expired (shares) (15,356)
Unvested end of period (shares) 157,164
Weighted-Average Grant Date Fair Value  
Unvested, beginning of period (in dollars per share) | $ / shares $ 90.59
Granted (in dollars per share) | $ / shares 99.71
Vested (in dollars per share) | $ / shares 97.49
Cancelled or expired (in dollars per share) | $ / shares 83.93
Unvested, end of period (in dollars per share) | $ / shares $ 95.76
Award vesting period 4 years