Annual report pursuant to Section 13 and 15(d)

Asset Retirement Obligations - Additional Information (Detail)

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Asset Retirement Obligations - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Asset Retirement Obligations [Line Items]    
Reduction in asset retirement obligations $ (112,827,000) $ (59,958,000)
Reduction in asset retirement obligations including plug and abandon wells and facilities damaged by hurricane 29,600,000 23,000,000
Liabilities assumed through acquisition 31,664,000 8,194,000
Hurricane Ike
   
Asset Retirement Obligations [Line Items]    
Revisions of estimated liabilities (20,616,000) 4,744,000
All Other
   
Asset Retirement Obligations [Line Items]    
Revisions of estimated liabilities $ 74,100,000 $ 19,200,000