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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, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Derivative Instruments, Gain (Loss) [Line Items]        
Change in fair value of coal derivatives and coal trading activities, net $ (2,649) $ (1,530) $ (3,263) $ 19,851
Designated as Hedging Instrument        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income   191 4 8,532
Gains (Losses) Reclassified from Other Comprehensive Income into Income (98) 2,963 (294) 5,064
Designated as Hedging Instrument | Coal sales | Coal Contract | Coal sales        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income (25) 225 574 9,472
Gains (Losses) Reclassified from Other Comprehensive Income into Income (569) 2,963 (1,471) 5,750
Designated as Hedging Instrument | Coal purchases | Coal Contract | Coal purchases        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income 25 (34) (570) (940)
Gains (Losses) Reclassified from Other Comprehensive Income into Income 471 0 1,177 (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 1 (127) 222 (1,228)
Unrealized Gain (Loss) on Derivatives (2,650) (1,417) (3,561) 21,145
Not Designated as Hedging Instrument | Coal Contract | Other operating (income) expense, net        
Derivative Instruments, Gain (Loss) [Line Items]        
Realized Gain (Loss) on Derivatives 2,593 3,216 6,950 (2,076)
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 76 (66)
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 $ (184) $ (1,670) $ (889) $ (2,402)