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Asset Retirement Obligaion (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Asset retirement obligation - beginning balance $ 128,709 $ 118,742
Additional liabilities accrued 2,105 4,757
Revisions in estimated cash outflows 5,888 (1,382)
Disposition of assets (1,323) (2,086)
Accretion expense 8,023 8,678
Asset retirement obligation - ending balance $ 143,402 $ 128,709