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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, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]        
Change in fair value of coal derivatives and coal trading activities, net $ (15,138) $ (863) $ (11,724) $ (1,717)
Designated as Hedging Instrument        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income (Effective Portion) (12,757) 15 (8,068) 35
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) 0   0  
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) (15,462) 49 (10,231) 269
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) 0 0 0 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) 2,705 (34) 2,163 (234)
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) 0   0  
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 384 (836) 942 (1,494)
Unrealized Gain (Loss) on Derivatives (15,505) 0 (12,630) 26
Not Designated as Hedging Instrument | Coal Contract | Other operating (income) expense, net        
Derivative Instruments, Gain (Loss) [Line Items]        
Realized Gain (Loss) on Derivatives (1,649) 0 (2,680) 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 (17) (27) (36) (249)
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 $ 3,657 $ (1,147) $ 3,675 $ (4,725)