Quarterly report pursuant to Section 13 or 15(d)

Asset Retirement Obligations (Reconciliation of Asset Retirement Obligations Liability) (Detail)

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Asset Retirement Obligations (Reconciliation of Asset Retirement Obligations Liability) (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2012
Dec. 31, 2011
Asset Retirement Obligations [Line Items]    
Beginning balance $ 393,880  
Liabilities settled (63,150)  
Accretion of discount 15,043  
Disposition of properties (3,993)  
Liabilities incurred 486  
Ending balance 334,249  
Less current portion 83,545 138,185
Long-term 250,704 255,695
Hurricane Ike [Member]
   
Asset Retirement Obligations [Line Items]    
Revisions of estimated liabilities 29,571  
All Other [Member]
   
Asset Retirement Obligations [Line Items]    
Revisions of estimated liabilities $ 21,554