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Deferred compensation (Phantom Stock Plan and Restricted Stock Unit Plan) (Details) - Predecessor - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Jan. 01, 2017
Phantom Stock Plan | Phantom Shares      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Award vesting period (in years) 5 years    
Days from vesting date to cash settlement 120 days    
Granted (in shares)   0  
Unvested and outstanding restricted stock units     0
Restricted Stock Unit Plan | Restricted Stock Units (RSU)      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Days from vesting date to cash settlement 120 days    
Granted (in shares)   0  
Unvested and outstanding restricted stock units     98,596
Restricted Stock Unit Plan | Restricted Stock Units (RSU) | Maximum      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Award vesting period (in years) 3 years    
Phantom Stock Plan and Restricted Stock Unit Plan | Phantom Stock and RSU      
Vest date fair value      
Estimated fair value per share at end of period ($ per share)     $ 0.00