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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, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Derivative Instruments, Gain (Loss) [Line Items]        
Change in fair value of coal derivatives and coal trading activities, net $ (19,641) $ (2,649) $ (28,931) $ (3,263)
Designated as Hedging Instrument | Coal Contract        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income       4
Gains (Losses) Reclassified from Other Comprehensive Income into Income   (98)   (294)
Designated as Hedging Instrument | Coal sales. | Coal Contract | Coal sales        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income   (25)   574
Gains (Losses) Reclassified from Other Comprehensive Income into Income   (569)   (1,471)
Designated as Hedging Instrument | Coal purchases. | Coal Contract | Coal purchases        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income 0 25   (570)
Gains (Losses) Reclassified from Other Comprehensive Income into Income 0 471   1,177
Not Designated as Hedging Instrument        
Derivative Instruments, Gain (Loss) [Line Items]        
Change in fair value of coal derivatives and coal trading activities, net     (28,931) (3,263)
Not Designated as Hedging Instrument | Coal Contract        
Derivative Instruments, Gain (Loss) [Line Items]        
Change in fair value of coal derivatives and coal trading activities, net (19,641) (2,649)    
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   222
Unrealized Gain (Loss) on Derivatives (19,641) (2,650) (28,931) (3,561)
Not Designated as Hedging Instrument | Coal Contract | Other operating (income) expense, net        
Derivative Instruments, Gain (Loss) [Line Items]        
Realized Gain (Loss) on Derivatives $ (6,495) 2,593 $ (7,008) 6,950
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       76
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)   $ (889)