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Derivatives (Effects Of Derivatives On Measures Of Financial Performance) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Designated as Hedging Instrument [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) Recognized in Other Comprehensive Income(Effective Portion) $ 2 $ 5,527
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) 226 481
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) 13 10,265
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) 1,369 882
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) (11) (4,738)
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) (1,143) (401)
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 (1,115) (411)
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 163 1,091
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 (469) 159
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 (443) (2,365)
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 $ (171) $ 0