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Asset Retirement Obligations (Details) (USD $)
In Thousands, unless otherwise specified
6 Months Ended
Jun. 30, 2012
Information related to asset retirement obligation  
Balance at December 31, 2011 $ 180,662
Additions to asset retirement obligations 441
Accretion expense 5,066
Liabilities settled (2,833)
Balance at June 30, 2012 $ 183,336