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Derivatives (Effects Of Derivatives On Measures Of Financial Performance) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]        
Change in fair value of coal derivatives and coal trading activities, net $ 8,400 $ (15,138) $ 21,381 $ (11,724)
Designated as Hedging Instrument        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income 3,670 (12,757) 8,341 (8,068)
Gains (Losses) Reclassified from Other Comprehensive Income into Income 1,743   2,101  
Designated as Hedging Instrument | Coal sales | Coal Contract | Coal sales        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income 4,010 (15,462) 9,247 (10,231)
Gains (Losses) Reclassified from Other Comprehensive Income into Income 1,743 0 2,787 0
Designated as Hedging Instrument | Coal purchases | Coal Contract | Coal purchases        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income (340) 2,705 (906) 2,163
Gains (Losses) Reclassified from Other Comprehensive Income into Income 0   (686)  
Not Designated as Hedging Instrument | Coal Contract | Coal and natural gas realized and unrealized        
Derivative Instruments, Gain (Loss) [Line Items]        
Realized and Unrealized Gain (Loss) on Derivatives (718) 384 (1,101) 942
Unrealized Gain (Loss) on Derivatives 9,137 (15,505) 22,562 (12,630)
Not Designated as Hedging Instrument | Coal Contract | Other operating (income) expense, net        
Derivative Instruments, Gain (Loss) [Line Items]        
Realized Gain (Loss) on Derivatives (881) (1,649) (5,292) (2,680)
Not Designated as Hedging Instrument | Natural gas | Coal and natural gas realized and unrealized        
Derivative Instruments, Gain (Loss) [Line Items]        
Realized and Unrealized Gain (Loss) on Derivatives (19) (17) (80) (36)
Not Designated as Hedging Instrument | Heating oil — diesel purchases | Other operating (income) expense, net        
Derivative Instruments, Gain (Loss) [Line Items]        
Heating oil realized and unrealized gains and losses $ (1,369) $ 3,657 $ (732) $ 3,675