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Asset Retirement Obligations - Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Asset Retirement Obligations [Line Items]    
Beginning of period $ 84,810 $ 60,693
Liabilities incurred 9,802 3,265
Liabilities settled (3,649) (4,717)
Disposition of wells (1,457) (716)
Accretion expense 8,793 5,488
Change in estimate 48,179 20,797
End of period 146,478 84,810
Less current portion (2,470) (5,005)
Long-term asset retirement obligations-continuing operations $ 144,008 $ 79,805