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Changes to Company's Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Reconciliation of Changes in Asset Retirement Obligations [Line Items]    
Balance at beginning of year $ 3,455 $ 3,063
Liabilities incurred 106 65
Liabilities settled (68) (15)
Accretion expense 37 35
Revision in estimated costs of plugging oil and natural gas wells 550 (2)
Asset retirement obligation at end of period $ 4,080 $ 3,146