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Asset Retirement Obligations (Narrative) (Details) (USD $)
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Asset Retirement Obligations [Line Items]    
Liabilities settled $ (29,012,000) $ (2,839,000)
Surety bonds outstanding 263,000,000  
Asset Retirement Obligations [Member]
   
Asset Retirement Obligations [Line Items]    
Self-bonding obligations $ 420,500,000