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Equity-based compensation (Tables)
12 Months Ended
Dec. 31, 2018
Share-based Payment Arrangement [Abstract]  
Schedule of fair value assumptions

PICU fair values were calculated using the following assumptions:

 

     December 31, 2018     December 31, 2017  
(Percents)    Start     End     Start     End  

Risk-free interest rate

     2.20     2.51     1.93     2.20

Volatility

     91.00     80.00     101.03     91.00
Summary of stock option activity
Profit Interest Common Units activity summary    Number
of Units
     Weighted
Average Grant-
Date Fair Value
per Unit
 

Nonvested units at December 31, 2017

     959      $ —  

Granted

     332        442.30

Vested

     780        169.55

Forfeited

     30        442.30
  

 

 

    

 

 

 

Nonvested units at December 31, 2018

     481      $ 22.07