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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, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]        
Change in fair value of coal derivatives and coal trading activities, net $ (1,530) $ (10,418) $ 19,851 $ (22,142)
Designated as Hedging Instrument        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income 191 (4,207) 8,532 (12,275)
Gains (Losses) Reclassified from Other Comprehensive Income into Income 2,963 (4,825) 5,064 (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 225 (4,631) 9,472 (14,862)
Gains (Losses) Reclassified from Other Comprehensive Income into Income 2,963 (6,996) 5,750 (6,996)
Designated as Hedging Instrument | Coal purchases | Coal Contract | Coal purchases        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income (34) 424 (940) 2,587
Gains (Losses) Reclassified from Other Comprehensive Income into Income 0 2,171 (686) 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 (127) (928) (1,228) 14
Unrealized Gain (Loss) on Derivatives (1,417) (9,486) 21,145 (22,116)
Not Designated as Hedging Instrument | Coal Contract | Other operating (income) expense, net        
Derivative Instruments, Gain (Loss) [Line Items]        
Realized Gain (Loss) on Derivatives 3,216 (2,537) (2,076) (5,217)
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 14 (4) (66) (40)
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 $ (1,670) $ 719 $ (2,402) $ 4,394