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Asset Retirement Obligations - Reconciliation of Beginning and Ending Aggregate Carrying Amounts of Asset Retirement Obligation (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Dec. 31, 2012
Asset Retirement Obligation, Roll Forward Analysis          
Asset retirement obligations at December 31, 2012     $ 498,410    
Liability incurred upon acquiring and drilling wells     2,421 3,232  
Liability settled or disposed in current period     (75,060)    
Accretion 9,800 7,965 19,579 10,572  
Asset retirement obligations at June 30, 2013 445,350   445,350    
Less: current portion 79,953   79,953   118,504
Asset retirement obligations, net of current $ 365,397   $ 365,397   $ 379,906