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Asset Retirement Obligations - Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Asset Retirement Obligation [Abstract]    
Beginning of period $ 146,478 $ 84,810
Liabilities incurred 8,731 9,802
Liabilities settled (424) (3,649)
Disposition of wells (3,129) (1,457)
Accretion expense 10,778 8,793
Change in estimate 67,643 48,179
End of period 230,077 146,478
Less current portion (5,037) (2,470)
Long-term asset retirement obligations $ 225,040 $ 144,008