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Derivatives (Effects Of Derivatives On Measures Of Financial Performance) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Derivative Instruments, Gain (Loss) [Line Items]    
Change in fair value of coal derivatives and coal trading activities, net $ (528) $ (743)
Designated as Hedging Instrument    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) Recognized in Other Comprehensive Income   3
Designated as Hedging Instrument | Coal sales | Coal Contract | Coal sales    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) Recognized in Other Comprehensive Income   159
Designated as Hedging Instrument | Coal purchases | Coal Contract | Coal purchases    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) Recognized in Other Comprehensive Income 0 (156)
Gains (Losses) Reclassified from Other Comprehensive Income into Income 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   221
Unrealized Gain (Loss) on Derivatives (528) (1,040)
Not Designated as Hedging Instrument | Coal Contract | Other operating (income) expense, net    
Derivative Instruments, Gain (Loss) [Line Items]    
Realized Gain (Loss) on Derivatives $ 138 1,601
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   $ (1,033)