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Asset Retirement Obligations (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Asset Retirement Obligations    
Beginning of period - continuing operations $ 60,693 $ 76,589
Liabilities incurred 3,265 1,495
Acquisitions-continuing operations   556
Liabilities settled (4,717) (2,331)
Disposition of wells (716) (12,891)
Accretion expense - continuing operations 5,488 5,137
Change in estimate 20,797 (7,862)
End of period - continuing operations 84,810 60,693
Less current portion (5,005) (4,020)
Long-term asset retirement obligations continuing operations 79,805 56,673
Asset retirement obligations-discontinued operations $ 0 $ 1,980