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Changes to the Company's Asset Retirement Obligations (Detail) (USD $)
In Thousands
9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Reconciliation of Changes in Asset Retirement Obligations [Line Items]  
Balance at beginning of year$ 3,063$ 2,955
Liabilities incurred223279
Liabilities settled(80)(331)
Accretion expense10683
Revision in estimated costs of plugging oil and natural gas wells(2) 
Asset retirement obligation at end of period$ 3,310$ 2,986