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Derivatives (Effects Of Derivatives On Measures Of Financial Performance) (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]        
Change in fair value of coal derivatives and coal trading activities, net $ (10,418) $ (1,028) $ (22,142) $ (2,745)
Designated as Hedging Instrument        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income (Effective Portion) (4,207) (17) (12,275) 18
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) (4,825)   (4,825)  
Designated as Hedging Instrument | Coal sales | Coal Contract | Coal sales        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income (Effective Portion) (4,631) (169) (14,862) 100
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) (6,996) 0 (6,996) 0
Designated as Hedging Instrument | Coal purchases | Coal Contract | Coal purchases        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income (Effective Portion) 424 152 2,587 (82)
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) 2,171   2,171  
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 (928) (696) 14 (2,190)
Unrealized Gain (Loss) on Derivatives (9,486) (212) (22,116) (186)
Not Designated as Hedging Instrument | Coal Contract | Other operating (income) expense, net        
Derivative Instruments, Gain (Loss) [Line Items]        
Realized Gain (Loss) on Derivatives (2,537) 0 (5,217) 0
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 (4) (120) (40) (369)
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 $ 719 $ 822 $ 4,394 $ (3,903)