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Derivatives (Effects of Derivatives on Measures of Financial Performance) (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2016
Oct. 01, 2016
Dec. 31, 2018
Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]        
Change In Fair Value Of Coal Derivatives And Coal Trading Activities, Net $ (396)   $ (9,118) $ (7,222)
Derivatives Not Designated as Hedging Instruments        
Derivative Instruments, Gain (Loss) [Line Items]        
Change In Fair Value Of Coal Derivatives And Coal Trading Activities, Net (396)   (9,118) (7,222)
Coal contract | Derivatives Designated as Hedging Instruments        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income (Effective Portion) 0   (6,169) (1,185)
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) 0   (8,205) 0
Coal contract | Derivatives Not Designated as Hedging Instruments | Change in Fair Value of Coal Derivatives and Coal Trading Activity, Net        
Derivative Instruments, Gain (Loss) [Line Items]        
Trading - realized and unrealized (7)   135 (2,047)
Coal risk management— unrealized (408)   (9,530) (4,648)
Coal risk management — realized 116   (8,734) 0
Natural gas | Derivatives Not Designated as Hedging Instruments | Change in Fair Value of Coal Derivatives and Coal Trading Activity, Net        
Derivative Instruments, Gain (Loss) [Line Items]        
Trading - realized and unrealized 19   277 (527)
Heating oil -- diesel purchases | Derivatives Not Designated as Hedging Instruments | Other Operating Income (Expense)        
Derivative Instruments, Gain (Loss) [Line Items]        
Heating oil — diesel purchases 827   (505) (1,057)
Foreign currency        
Derivative Instruments, Gain (Loss) [Line Items]        
Foreign currency (9)     0
Foreign currency | Derivatives Not Designated as Hedging Instruments        
Derivative Instruments, Gain (Loss) [Line Items]        
Foreign currency     0  
Predecessor        
Derivative Instruments, Gain (Loss) [Line Items]        
Change In Fair Value Of Coal Derivatives And Coal Trading Activities, Net   $ (2,856)    
Predecessor | Derivatives Not Designated as Hedging Instruments        
Derivative Instruments, Gain (Loss) [Line Items]        
Change In Fair Value Of Coal Derivatives And Coal Trading Activities, Net   (2,856)    
Predecessor | Coal contract | Derivatives Designated as Hedging Instruments        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income (Effective Portion)   (136)    
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion)   397    
Predecessor | Coal contract | Derivatives Not Designated as Hedging Instruments | Change in Fair Value of Coal Derivatives and Coal Trading Activity, Net        
Derivative Instruments, Gain (Loss) [Line Items]        
Trading - realized and unrealized   (891)    
Coal risk management— unrealized   (1,662)    
Coal risk management — realized   (476)    
Predecessor | Natural gas | Derivatives Not Designated as Hedging Instruments | Change in Fair Value of Coal Derivatives and Coal Trading Activity, Net        
Derivative Instruments, Gain (Loss) [Line Items]        
Trading - realized and unrealized   (303)    
Predecessor | Heating oil -- diesel purchases | Derivatives Not Designated as Hedging Instruments | Other Operating Income (Expense)        
Derivative Instruments, Gain (Loss) [Line Items]        
Heating oil — diesel purchases   826    
Predecessor | Foreign currency        
Derivative Instruments, Gain (Loss) [Line Items]        
Foreign currency   (451)    
Short | Coal contract | Derivatives Designated as Hedging Instruments | Sales Revenue, net        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income (Effective Portion) 0   (7,517) (2,127)
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) 0   (10,912) 0
Short | Predecessor | Coal contract | Derivatives Designated as Hedging Instruments | Sales Revenue, net        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income (Effective Portion)   (672)    
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion)   1,634    
Long | Coal contract | Derivatives Designated as Hedging Instruments | Cost of Sales        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income (Effective Portion) 0   1,348 942
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) $ 0   $ 2,707 $ 0
Long | Predecessor | Coal contract | Derivatives Designated as Hedging Instruments | Cost of Sales        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income (Effective Portion)   536    
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion)   $ (1,237)