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Derivatives (Effects Of Derivatives On Measures Of Financial Performance) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Designated as Hedging Instrument [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income(Effective Portion) $ (67) $ (476) $ (65) $ 5,051
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) 96 2,727 321 3,208
Designated as Hedging Instrument [Member] | Coal Contract [Member] | Coal sales [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income(Effective Portion) (73) (1,163) (60) 9,102
Designated as Hedging Instrument [Member] | Coal Contract [Member] | Coal sales [Member] | Sales Revenue, Net [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) 157 4,990 1,526 5,872
Designated as Hedging Instrument [Member] | Coal Contract [Member] | Coal purchases [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Other Comprehensive Income(Effective Portion) 6 687 (5) (4,051)
Designated as Hedging Instrument [Member] | Coal Contract [Member] | Coal purchases [Member] | Cost of Sales [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) (61) (2,263) (1,205) (2,664)
Not Designated as Hedging Instrument [Member] | Coal Contract [Member] | Change in Fair Value of Coal Derivatives and Coal Trading Activity, Net [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Unrealized Gain (Loss) on Derivatives 19 (875) (1,096) (1,286)
Not Designated as Hedging Instrument [Member] | Coal Contract [Member] | Other Operating Income (Expense) [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Realized Gain (Loss) on Derivatives (180) 826 (343) 1,917
Not Designated as Hedging Instrument [Member] | Natural Gas [Member] | Change in Fair Value of Coal Derivatives and Coal Trading Activity, Net [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Unrealized Gain (Loss) on Derivatives 235 (221) (384) (62)
Not Designated as Hedging Instrument [Member] | Heating Oil-Diesel Purchases [Member] | Other Operating Income (Expense) [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Heating oil realized and unrealized gains and losses 2,039 628 1,596 (1,737)
Not Designated as Hedging Instrument [Member] | Foreign Currency [Member] | Other Operating Income (Expense) [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Heating oil realized and unrealized gains and losses $ 34 $ 0 $ (137) $ 0