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Derivatives (Effects Of Derivatives On Measures Of Financial Performance) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]    
Change in fair value of coal derivatives and coal trading activities, net $ 12,981 $ 3,414
Designated as Hedging Instrument    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) Recognized in Other Comprehensive Income (Effective Portion) 4,671 4,689
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) 361  
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) 5,237 5,231
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) 1,044 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) (566) (542)
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) (683)  
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 (383) 558
Unrealized Gain (Loss) on Derivatives 13,425 2,875
Not Designated as Hedging Instrument | Coal Contract | Other operating (income) expense, net    
Derivative Instruments, Gain (Loss) [Line Items]    
Realized Gain (Loss) on Derivatives (4,411) (1,031)
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 (61) (19)
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 $ 637 $ 18