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Derivatives (Effects of Derivatives on Measures of Financial Performance) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]      
Change In Fair Value Of Coal Derivatives And Coal Trading Activities, Net $ 18,601 $ (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 18,601 (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) 9,018 (6,169) (1,185)
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) 9,481 (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 (1,013) 135 (2,047)
Coal risk management— unrealized 19,713 (9,530) (4,648)
Coal risk management — realized 487 (8,734)  
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 (99) 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 (2,291) (505) (1,057)
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) 10,249 (7,517) (2,127)
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) 10,167 (10,912) 0
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) (1,231) 1,348 942
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) $ (686) $ 2,707 $ 0