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Acquired Sales Contracts Acquired Sales Contracts (Schedule of Acquired Sales Contracts) (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]    
Acquired fair value, Assets $ 131,819 $ 149,457
Acquired fair value, Liabilities 166,697 166,697
Accumulated amortization, Asset (123,776) (115,322)
Accumulated amortization, Liabilities (105,237) (69,699)
Total, Assets 8,043 34,135
Total, Liabilities 61,460 96,998
Net total, Liabilites (53,417) (62,863)
Coal Supply Agreement, Liabilities 14,038 38,441
Other Current Assets [Member]
   
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]    
Coal supply agreement, Assets 5,651 18,929
Accrued Liabilities [Member]
   
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]    
Coal Supply Agreement, Liabilities 14,038 38,441
Other Assets [Member]
   
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]    
Coal supply agreement, Assets 2,392 15,206
Other Noncurrent Liabilities [Member]
   
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]    
Coal Supply Agreement, Liabilities $ 47,422 $ 58,557