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Asset Retirement Obligations (Details) (USD $)
3 Months Ended9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
Changes in Asset Retirement Obligations Detail [Table]    
Beginning balance$ 144,525,000$ 216,147,000$ 272,815,000$ 224,110,000
Activity in the period    
Accretion of liability631,0001,392,0003,789,0004,090,000
Retirement obligations incurred0000
Revisions in estimated cash flows14,311,00019,897,00040,120,00042,081,000
Settlement of retirement obligations(25,872,000)(35,201,000)(183,129,000)(68,046,000)
Ending balance133,595,000202,235,000133,595,000202,235,000
Aggregate amount of asset retirement obligations associated with sold properties$ 3,300,000 $ 122,000,000